WO2018158591A1 - Radiation detection apparatus - Google Patents

Radiation detection apparatus Download PDF

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Publication number
WO2018158591A1
WO2018158591A1 PCT/GB2018/050546 GB2018050546W WO2018158591A1 WO 2018158591 A1 WO2018158591 A1 WO 2018158591A1 GB 2018050546 W GB2018050546 W GB 2018050546W WO 2018158591 A1 WO2018158591 A1 WO 2018158591A1
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WO
WIPO (PCT)
Prior art keywords
radiation
cable
detector
diamond
detection apparatus
Prior art date
Application number
PCT/GB2018/050546
Other languages
French (fr)
Inventor
Liam PAYNE
Chris HUTSON
Tom Scott
Original Assignee
The University Of Bristol
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Priority to GB1703496.8 priority Critical
Priority to GBGB1703496.8A priority patent/GB201703496D0/en
Application filed by The University Of Bristol filed Critical The University Of Bristol
Publication of WO2018158591A1 publication Critical patent/WO2018158591A1/en

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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01TMEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16Measuring radiation intensity
    • G01T1/26Measuring radiation intensity with resistance detectors

Abstract

In an example, a radiation detection apparatus (100) comprises a diamond radiation detector (102) and a cable (104). A first end of the cable (104) is arranged to be connected to a voltage source (108) and an ammeter (110) and a second end of the cable (104) is arranged to be connected to the diamond radiation detector (102). The cable (104) is arranged to connect the voltage source (108) to the diamond radiation detector (102) to provide a bias voltage and to connect the ammeter (110) to measure a current generated by radiation passing through the diamond radiation detector (102).

Description

RADIATION DETECTION APPARATUS
This invention relates to methods and apparatus for detection, and in particular but not exclusively to detection of radiation using a diamond radiation detector and via a signal cable.
Radiation detection is carried out in a number of fields. Radiation detection apparatus, including Geiger counters, scintillator counters, gaseous ionisation chambers and the like are used for a range of applications such as in geological surveys, smoke detectors, particle research, and safety apparatus.
For example, a radiation detector may be used as a 'dosimeter' to determine if an area may be hazardous to health, and/or to monitor dosage in a radiation based medical treatment. Such apparatus may for example comprise a film badge dosimeter or an Alanine dosimeter. However such apparatus requires a subsequent development step to reveal the dose of radiation received thereby. In addition, semiconductor-based detectors may become saturated or damaged in high radiation environments.
Diamond radiation detectors which are capable of detecting radiation such as gamma radiation have been proposed. Such detectors have proved to be robust in a high radiation environment and can be fabricated as compact, responsive devices. In such devices, an ionising particle passes through a single crystal diamond and causes formation of an electron-hole pair, which are caused to drift towards separate electrodes by an applied electric field.
Diamond detectors have been used for particle tracking and beam loss monitoring for example in high energy physics applications such as in the ATLAS experiment at CERN. Detectors are used in a mode which allows a low flux of particles to be measured as individual charge events (pulse counting), which are then collected and used to make inferences about the state of a particle beam. Similar detectors have been used to measure dosage in medical therapies, for example by pulse counting. Diamond detectors have proved to be highly radiation tolerant and operate without cooling (in contrast to, for example, a Gallium Arsenide detector which may require cooling). Waste products from the energy industry which produce ionising radiation (e.g. the nuclear energy industry) may be stored for many years. As some point, the storage facilities for such waste may be decommissioned, for example by performing a post operational clean out. In the interests of the safety of the public and personnel in the vicinity, it may be desirable to determine the level of ionising radiation in the vicinity of, and/or inside, the storage facility. However, such facilities are hostile operating environments, which may render electronic monitoring equipment ineffective. For this reason, passive sensors may be used, for example film badge dosimeters or Alanine dosimeters as described above. However, the subsequent processing required means that these methods are inconvenient. Moreover, it can be difficult to place the detection apparatus in the vicinity of the waste, to which access is usually limited to small inspection channels (for example, around 15-20cm) to reduce risk to personnel.
The charge generation process in a diamond detector differs according to the use case. At low to medium photon energies (such as with use in the nuclear industries), Compton scattering dominates whereas in medical applications, which occur at higher photon energies, pair production is the dominant process.
According to a first aspect of the invention, there is provided a radiation detection apparatus comprising a diamond radiation detector and a cable, wherein a first end of the cable is arranged to be connected to a voltage source and an ammeter and a second end of the cable is arranged to be connected to the diamond radiation detector, and the cable is arranged to connect the voltage source to the diamond radiation detector to provide a bias voltage and to connect the ammeter to measure a current generated by radiation passing through the diamond radiation detector.
Radiation incident on a diamond radiation detector generates charged particles (electron-hole pairs) through Compton scattering and the photoelectric effect. The charged particles will travel through a diamond crystal of the detector due to an applied bias voltage from a connected voltage source. An ammeter may be used to measure the resultant current due to these charged particles travelling through the detector, the measured current being indicative of the 'dose rate' of the radiation that is incident on the detector. In some examples, the apparatus may be configured for use in detecting levels of ionising radiation emitted by waste products from the nuclear energy industry. The ionising radiation which is detected may be (at least primarily) gamma radiation (although other types of radiation may also be detected).
In some examples, the cable may be detachably connected to the diamond radiation detector, and/or connected thereto in use of the radiation detection apparatus. In some examples, the radiation detection apparatus may be provided with a plurality of cables, and in some such examples, the cables may have different physical characteristics. For example, one cable may be physically stiffer than another, more flexible, cable. This allows a choice of cable to be made for a particular use case.
As has been noted above, diamond radiation detectors are highly radiation tolerant (in particular when compared to silicon or gallium arsenide detector apparatus) and do not require cooling. Moreover they may be formed to have a small size and are chemically inert, which increases the versatility of use. Such detectors also exhibit a relatively low noise-to-signal ratio due to a large band gap and are intrinsically simple. The present inventors have found that they may be reliably used to detect radiation over a wide range, for example from 0.1 or 0.5 Grays per hour (Gy/hr) to 3600Gy/hr without becoming saturated. This may exceed at least some typical operational ranges.
In some examples the cable is at least 10 metres in length. In other examples the cable is around 20, 30, 40 or 50 metres in length, or longer. This allows the detector to be located remotely from the voltage source and the ammeter. Therefore the detector may be exposed to radiation, while the other components of the apparatus such as the voltage source and ammeter are remote from the radiation source, and therefore protected from high doses of radiation. High doses of radiation can damage sensitive electronic components, therefore locating these components away from a radiation source improves the reliability and accuracy of the apparatus as damage caused by radiation is reduced. In some examples there may be a human operator of such components, who may also be able to detect radiation at a safe distance via the cable. The use of a relatively long cable therefore allows the diamond detector to be safely used in a high activity environment. In some examples, the cable comprises a first conductor arranged to apply the bias voltage and to carry the current generated by radiation passing through the diamond radiation detector; a second conductor to act as a reference for the current measurement and an insulator to provide galvanic isolation between the first and second conductor. This allows each conductor to be tailored to perform a specific purpose. The first and second conductor may be arranged in a co-axial manner.
In some examples, the cable comprises a third conductor arranged to provide an electromagnetic shield. By providing electromagnetic shielding to the first and second conductors, noise introduced to the system may be reduced. This is particularly advantageous for longer cables, as the potential for introducing noise increases as the length of the cable increases. The first, second and third connectors may be arranged concentrically to form a tri-axial cable. In some examples the cable comprises a stiffening member to provide longitudinal stiffness to the cable. A cable with a stiffening member may advantageously be used to manipulate the detector. By moving the cable, the detector's position and orientation can be manipulated. For example the detector may be inserted into a pipe and its position along the length of the pipe may be varied by controlling the length of cable inserted into the pipe. A relatively stiff cable may act as a 'push rod', moving the detector in a horizontal direction, which increases the number of situations in which the detector may be used beyond those which a more flexible cable may access. Therefore, in some examples, the stiffness may be at least a longitudinal resistance to compressibility or deformation (e.g. crumpling or folding) of the cable.
In addition, a relatively stiff cable may be used to rotate a detector, which may be advantageous as the detection sensitivity may change with angle. This may for example be used to find a rotational position of increased, or even maximum sensitivity. In some examples, there may instead or in addition be provided a separate relatively stiff positioning rod, and/or the detector may be positioned using a remotely controlled or robotic conveying means such as a 'pipe crawler' vehicle, or lowered into position.
For example, it may be the case that a detector demonstrates an orientation dependency, i.e. the amount of charge generated in the diamond crystal may change as a function of incident photon angle. In some examples of apparatus suitable for use as a detector of this invention, this variation has been quantified as 13.5% maximum difference across the full orientation range. The orientation dependency of the sensitivity is surprising, in particular as it does not depend on the thickness of diamond crystal being traversed by the ionising radiation (e.g. a gamma photon), even though the thickness does vary with orientation. Rather, the effect appears to be caused by the detector electrode metals and the detector housing, which are higher in atomic number than diamond. These higher atomic number metals interact more strongly than diamond, producing a shower of secondary beta particles from a gamma interaction, and this causes a proportion of (which may be a significant proportion, or the majority of) the detected signal in the diamond.
Where the orientation cannot be accurately controlled, the variation contributes to the uncertainty in the measurement.
However, in some examples, the calibration of response as a function of orientation may be carried out, and orientation control may be applied in use of the apparatus. In another example, the detector head may be designed to be substantially rotationally symmetric with respect to the electrodes and housing. For example, these may be formed to have a cylindrical or partially cylindrical shape.
In some examples the stiffening member is conductive. A conductive stiffening member can (and may be configured to) provide electromagnetic shielding to the first and second (and where provided, third) conductors, thereby reducing noise introduced to the signal. In some examples, the stiffening member comprises a steel coil which encloses the first and second (and where provided, third) conductors. In some examples, the cable comprises a removable sheath, which may comprise the outer coating of the cable. The cable may be used in locations with harsh environmental conditions and as such may become damaged or contaminated. For example the cable may experience high doses of ionising radiation, may be contaminated with radioactive material, may be exposed to various chemical compositions and/or may experience mechanical wear and tear. Therefore it is advantageous for the sheath to be removable to facilitate replacement in the event of it becoming contaminated or damaged. In some examples, as only the sheath is contaminated, this reduces the amount of contaminated waste to be disposed of, which in turn reduces the practical burden of disposal.
In some examples, the diamond radiation detector is a detector having a dark current which is within a predetermined range over a range of bias voltages. For example, the dark current may vary within a range of less than 100 nanoamps (nA), or less than 50nA, or less than 20nA (in some examples on the order of 0 to 20 nA) over a range of bias voltages which may vary between around -1000V to 1000V, or between -500V and 500V, or some other reasonably large range (e.g. several hundred volts). This may be indicative of a radiation detector which is likely to perform well in a detection environment. In some examples, the radiation detection apparatus further comprises a monitoring apparatus, the monitoring apparatus comprising the voltage source and the ammeter. The monitoring apparatus allows data collection from the diamond radiation detector, by applying a voltage and measuring the current across (or through) the detector. The cable may be detachably connected to the monitoring apparatus, and may be connected thereto in use. The voltage source may be arranged to provide a stable bias voltage.
In some examples, the monitoring apparatus further comprises a battery connection arranged to connect the monitoring apparatus to at least one battery. Powering the apparatus by at least one battery may have advantages over using a mains supply. A battery can provide a stable, reliable electricity supply. In comparison, in locations where such an apparatus may be used, the mains electricity supply may be unreliable or may introduce interference potentially affecting measurements. Furthermore, a battery supply allows the apparatus to be used remotely from any fixed mains power supply or during emergency situations when a mains power supply is not operational. A battery may also remove the need to recalibrate the apparatus based on the local conditions of the local mains power supply. However, in other examples the monitoring apparatus may comprise a mains connection. In some such examples, the monitoring apparatus may further comprise filtering electronics to smooth the mains supply. In some examples, the monitoring apparatus further comprises an interface, which may comprise a display screen. The display screen provides ease of use, for example so that a user can see real time information about the operation of the apparatus. For example, this may comprise an instant and/or updating plot of dose rate measurements, which may be determined from the current measurements. In other examples, it may comprise an alphanumeric output. In some examples, the display screen may comprise a touch screen and provide an input of the monitoring apparatus, and/or other input(s) such as at least one switch, keyboard, touch pad, mouse or the like to interact with the monitoring apparatus may be provided as part of the monitoring apparatus.
In some examples, the ammeter and voltage source are rigidly fixed to one another. This may improve the quality of the signal by reducing noise. If the voltage source and ammeter were not rigidly fixed to one another their relative motion could introduce noise to the signal, reducing the accuracy of the measurement. In some examples, the ammeter and voltage source may be effectively combined, such that one is integral to the other.
In some examples the ammeter and voltage source are mounted in a common shielding. A common shielding reduces the amount of the apparatus which is unshielded and therefore exposed to environmental noise, and therefore the quality of the signal may be improved.
In some examples the monitoring apparatus comprises a memory, the memory comprising at least one conversion coefficient associated with at least one combination of a cable and the detector of the apparatus. Such conversion coefficient(s) may be used in converting a current measurement into an estimate of incident radiation, i.e. a dose rate. In order to determine the conversion coefficient(s), a calibration may be performed (for example using a source of known activity, which delivers a known dose rate, which is preferably varied in a known manner) for each specific detector and cable combination resulting in conversion coefficients specific to each detector and cable combination. Providing each detector and cable combination with bespoke conversion coefficients based on the calibration measurements collected improves the accuracy of the measurements as the effects of physical variations in the manufacture of each apparatus is reduced. In some examples, the coefficient(s) may be specific to the monitoring apparatus used. In examples where a plurality of cables are provided, there may be a calibration of each combination of a given cable with a particular detector, and conversion coefficients for each combination are stored in the memory. The combination in use may for example be detected automatically or specified by a user.
Calibration may for example be carried out using a high activity Cobalt-60 or Caesium- 137 source. Such sources have been shown to result in a similar detector response as nuclear energy waste material and are thus suited to calibrating the detector for use in such fields.
The use of a high activity source may be advantageous as it accounts for the effect of the ionisation of the cable by ionising the cable as well as the detector. In use of the monitoring apparatus in the field, the signal cable may contribute a current. This is because the cable dielectric may undergo partial ionisation, and thereby contribute an amount- which may be a relatively small amount- to the measured detector current.
By carrying out a calibration with a high activity source such as cobalt 60, a detector response which is similar to (indeed, in some measured examples, functionally identical to) the response of the detector to radiation emitted by nuclear energy waste material, for example when placed within a cell containing such waste. The calibration may be carried out in a container into which at least a portion of the cable is introduced. Such a calibration method takes account of any contribution to the current that an ionised cable may make. Thus, in some examples, at least a portion of the cable may be exposed to the ionising radiation in calibration. The portion may be selected according to the proportion of the cable which is likely to be exposed to the radiation in use. In some examples, different calibration exercises may be carried out with different portions and/or lengths of the cable exposed to radiation, or calibration/conversion coefficients may be interpolated or inferred for different lengths/portions. In use of the apparatus, an appropriate calibration/conversion coefficient may be selected based on the length/portion of cable exposed to radiation in that instance. In some examples, a conversion from the current detected by an ammeter to a measure of radiation may use a power law conversion, where at least one coefficient of the power law conversion is specific to at least one component of the apparatus used to perform the measurement and is stored in the memory. In some examples, the monitoring apparatus may comprise processing circuitry arranged to perform the conversion. In some examples, which may include examples where the range in detected radiation is expected to be relatively small, a linear conversion may be used. In some examples, the calibration data may be used to provide coefficients of a look-up table associating measured currents with radiation dose rates. In some examples, processing circuitry may be provided as, or comprise, a tablet computer, or other relatively small format, computing device. Such devices are compact, familiar to users and may provide additional functionality, such a display screens (which may also function as inputs), input buttons and the like. In some examples, the memory may be used to store measurement data as it is collected. In some examples, the memory and/or associated processing circuitry may also be used to perform statistical analysis of the data in real time, which may increase the accuracy of the measurements (for example by collecting a plurality of measurements at a single location).
In some examples, the monitoring apparatus may be arranged in a case. In some such examples, a user interface and a battery connection or other power supply connection may be arranged in a first portion of the case, which is exposed to a user on opening the case, and the remaining components may be disposed within a separate section of the case, which may be generally obscured from the user.
In some examples the diamond radiation detector is electrically passive. An electrically passive detector may be more robust to radiation. Therefore the detector can be exposed to high doses of ionising radiation without degradation of its performance. In some examples, any and all components which are intended to be introduced into a region in which a high radiation level is expected (i.e. any components attached, in use, to second end of the cable) are electrically passive (i.e. do not themselves contribute to the energy in the circuit in the way of amplifiers or power supplies such as batteries). In some examples, any components attached, in use, to second end of the cable are arranged to exhibit a high level of radiation hardness. In some examples, the active volume of the detector may be a single crystal of intrinsic diamond.
According to a second aspect of the invention, there is provided a method of detecting ionising radiation in a region of interest comprising (i) introducing a diamond radiation detector into the region of interest, the diamond radiation detector being connected to a signal cable, (ii) arranging a monitoring apparatus outside the region of interest, the monitoring apparatus being connected to the signal cable, (iii) applying a bias voltage to the diamond radiation detector from a voltage source of the monitoring apparatus, (iv) measuring a current returned via the signal cable, and (v) determining a measurement of ionising radiation in the region of interest based on the current.
Such a method allows a measurement of radiation to be made in a region of interest, without a user or sensitive electronic equipment entering the region of interest, which may be a region of high radiation. In some examples, the region of interest may be defined as a region in which a dose rate actually or potentially exceeds a threshold, wherein the threshold may be related to safety and/or electronic equipment operation radiation limits. High doses of radiation may be damaging to both electronic equipment and a user of said equipment. Therefore it is advantageous to allow a measurement of a potentially radioactive region of interest without entering said region. The region of interest may be defined by a physical barrier, for example a radiation shielding barrier such as a concrete wall or the like. Therefore outside the region of interest may be the other side of the barrier to a radiation source, and introducing a diamond radiation detector into the region of interest may comprise introducing the detector through a radiation shielding barrier. In some examples, the region of interest contains waste products from the energy industry which produce ionising radiation (e.g. nuclear energy waste products).
In some examples, measuring the current comprises measuring the current while the diamond radiation detector is moved through the region of interest. By measuring the current as the detector is moved, measurements may be performed at different locations within the region of interest providing a measurement of the distribution of ionising radiation within the region of interest. For example, measurements may be made every 10ms, 20ms, 50ms, 100ms, second or the like. In some examples, statistical analysis may be used to improve the accuracy of the readings (for example, by obtaining a number of measurements while the detector is at least substantially stationary). Alternatively the relatively fast acquisition rate can be used to gather readings while the detector is moved continuously. In some examples, the method comprises adjusting at least one of a rotational position or a horizontal position of the diamond radiation detector using the signal cable. A cable of sufficient stiffness can provide a means for controlling the position of the detector. The cable may be rotated to change the orientation of the detector (for example to change the sensitivity of detection) or may for example be inserted into a pipe to control a horizontal position of the detector along the pipe. In some examples, introducing the diamond radiation detector into the region of interest may comprise introducing the detector on a cable having such a stiffness. Controlling the horizontal position may comprise moving the detector at any angle to the vertical. In some examples, the method further comprises displaying an indication of the current and/or an indication of an ionising radiation level (i.e. dose rate) in at least substantially real-time on a display screen of the monitoring apparatus. For example, this may comprise a substantially instantaneous and/or updating plot of dose rate measurements, which may update while the detector is moved through the zone of interest. A real-time display provides a user with feedback relating to the local environment of the sensor at a given time. This may for example allow a user to manipulate the position of the sensor in response to the real time feedback. For example a user may identify a particular region of increased interest based on the real time display and may manipulate the position of the detector to take additional measurements in this region. Similarly a user may conclude a region is of less interest and take fewer measurements allowing the user to collect more relevant and more useful data. In another example, a user may rotate the detector to change a sensitivity based on real-time information. In some examples determining the measurement of ionising radiation in the region of interest based on the current comprises using at least one conversion coefficient associated with the particular cable and diamond radiation detector used in carrying out the method. Such coefficient(s) may be use in a power law conversion and/or stored in a look up table or the like. At least one pre- or post-determined conversion coefficient may be applied to the measured current. The coefficient(s) may be uniquely determined for each unique apparatus, thereby improving the accuracy of the measurements. A post-determined conversion coefficient(s) may be used for example when an uncalibrated combination of a cable and detector is used to perform the measurements. In some examples determining the measurement of ionising radiation in the region of interest based on the current comprises determining the measurement using processing circuitry of the monitoring apparatus.
Features described in relation to one aspect of the invention may be used in another aspect of the invention.
Examples of the invention are now described with reference to the accompanying Figures in which:
Figure 1 is an example of a radiation detection apparatus;
Figure 2 is an example of a cable shown in cross section;
Figure 3 is an example of a monitoring apparatus, which may comprise part of the radiation detection apparatus of Figure 1 ;
Figure 4 is an example of data which may be collected by the monitoring apparatus; and
Figure 5 is an example plot of measured ammeter response and dose rate for a particular detector apparatus.
Figure 1 shows a radiation detection apparatus 100 comprising a diamond radiation detector 102, a cable 104 and monitoring apparatus 106. In this example, the monitoring apparatus 106 comprises a voltage source 108 and an ammeter 110.
A first end of the cable 104 is connected to the monitoring apparatus 106, and connects both the voltage source 108 and the ammeter 1 10 to the cable 104. A second end of the cable 104 is connected to the diamond radiation detector 102. The cable 104 is arranged to connect the voltage source 108 to the diamond radiation detector 102 to provide a bias voltage and to connect the ammeter 110 to measure a current generated by radiation passing through the diamond radiation detector.
The detector 102 in this example comprises a rectangular metallic housing 112 enclosing a chemical vapour deposition (CVD) diamond 114 with dimensions of a few millimetres (for example, on the order of 4.5mm x 4.5mm x 0.5mm). The small size allows the detector 102 to pass through small bore pipework. In some examples, the cross section of the detector 102 is similar to that of the cable 104. In some examples, the dimensions may be selected for a particular application. The detector 102 is electrically passive. In some examples, any and all components which are intended to be introduced into a region in which a high radiation level is expected (i.e. any components attached to the second end of the cable 104) may be electrically passive. The cable 104 itself may also be electrically passive. The diamond 114 has metallic contacts 116 (for example, titanium/gold metallic contacts) on opposed faces which are gold bonded to contacts that lead to a connection with the cable 104. In one example, the connection may be provided by a detachable connection, for example a SubMiniature version A (SMA) co-axial cable plug, although other connectors are available.
The detector 102 may be designed (and/or selected and/or tested) to have little variation in 'dark' current with a varying bias voltage, as this is indicative of a good metal electrical contact and is in turn indicative of a reliable detector. Dark current may be attributed to the (usually small) electric current that flows through a detector device when no incident ionising particles are arriving at the device. In some examples, therefore, a detector may be tested by applying a swept bias voltage over a relatively large range of voltages (for example, 100s of volts, or at least over 1000V, or between - 1000V to 1000V) while in a test environment having no ionising radiation source. A low variation in dark current may be a variation of less than around 100nA, or less than around 50nA, or less than around 20nA, or around 10nA. A good metal contact is likely to limit polarisation effects and result in good mechanical adhesion.
As is set out in greater detail below, when a bias voltage (which may for example be a value of between around 0V to 1000V DC) is applied and ionising radiation interacts with the diamond, a current is generated that can be measured and correlated to the radiation dose rate. Viewed another way, the resistance decreases with radiation in a repeatable manner. A bias voltage may be selected as a compromise between signal level (which generally increases with bias voltage magnitude) and dielectric breakdown in the cable 104, which depends on the cable 104 used.
In some examples, the cable 104 is a coaxial or tri-axial cable.
Figure 2 shows a cross section of an example of a cable 200, which may provide the cable of Figure 1.
The cable 200 in this example is a tri-axial cable with detachable connections at each end. In some examples, at least one connection comprises a RG59 co-axial (BNC) termination. The detachable connections allow the cable 200 to be used with a plurality of detectors 102 and monitoring apparatus 106, and vice versa.
The cable comprises a first conductor 202 which in this example is an inner core of the cable 200, and which is to apply a bias voltage to the detector 102 so as to generate an electric field through a diamond crystal of a detector 102. The first conductor 202 also carries current generated in the detector 102 in the presence of a bias voltage and ionising radiation. This is surrounded by a first insulator layer 204, which is a dielectric insulator and which galvanically separates the first conductor 202 from a second conductor 206. The second conductor 206 may be termed the 'reference conductor' and may in use be held at ground or 0V. This provides a reference for determining the current.
The second conductor 206 is surrounded by a second concentric insulator layer 208, which separates the second conductor 206 from a third conductor, which is an electromagnetic shield conductor 210. The electromagnetic shield conductor 210 reduces interference, allowing more sensitive measurements. In addition, it allows movement of cable 200 without inducing extra noise from a range of stray electromagnetic fields (machinery, humans walking nearby, etc.). As the measurement signal is low (of the order of pico-amps to milliamps), noise could obscure the measurement. This is surrounded by a cable sheath 212, which provides additional insulation. The cable sheath 212 is disposed within a conduit 214, in this example a steel conduit, which acts as a stiffening member to provide longitudinal stiffness to the cable. For example, this may be a resistance to compression of the cable which may allow the cable to push the detector horizontally or at an angle to the vertical along a conduit (whereas a more flexible cable may 'crumple' in such a scenario) and may/or allow controlled rotation of the detector 102. This stiffness also reduces variations in capacitance (which in turn inhibits noise from cable movement). Moreover, the steel conduit 214 provides mechanical protection, meaning that risk of damage to the cable 200 is reduced, and provides additional radiation shielding for the cable 200 (which could otherwise induce additional dielectric ionisation).
In other examples, a more flexible cable may alternatively or additionally be provided. For example, a particular cable having particular physical characteristics may be selected for use from a plurality of available cables depending on the deployment scenario. The cables may be interchanged (although in some examples this may require selection of different conversion coefficients when converting a current to a dose rate).
The conduit 214 is encased in an outer sheath 216, which may for example comprise a plastic material or the like, and/or may be wipeable to allow for decontamination and increase the reusability of the cable 200 without producing undue contaminated waste. In some examples, the outer sheath 216 is a replaceable sheath, which may be replaced when otherwise the cable 200 itself may be replaced, saving money and resources.
In some examples, the first conductor 202 is a solid copper wire and the second conductor 206 and electromagnetic shield conductor 210 are copper braiding. The electromagnetic shield conductor 210 and the conduit 214 may be connected to earth to reduce interference noise. In other examples other conductor types may be used, for example aluminium conductors and the like.
In some examples such a cable 200 may be deployed from a cylindrical drum on a dispensing stand. In some examples, the cable 200 is at least 10 meters in length, and may be longer. In other examples, the cable may be around 50m in length. Careful shielding and control of apparatus variables means a relatively long cable may be used without the measurements being unduly obscured by noise.
Although in this example a concentric cable design is shown, this need not be the case in all examples. For example, a 'twisted pair' cable may be used.
Figure 3 shows an example of a monitoring apparatus 300, which may be an example of a monitoring apparatus 106 as described in relation to Figure 1.
The monitoring apparatus 300 comprises a voltage source 302 and an ammeter 304.
The ammeter 304 is selected so as to be capable of measuring in the pico-amp to milliamp range, or a subrange thereof, which may be determined based on an intended use. A pico-amp to milliamp range generally corresponds to an operating range of less than 1 Grays per hour (Gy/hr) (e.g. 0.1 Gy/hr) to above 3600 Gy/hr. The range of the ammeter 304 may be selected bearing in mind an intended use. The voltage source 302 in this example is arranged to provide a stable bias voltage, for example of around 300V, which provides sufficient electric field magnitude without breaking down the cable dielectric. The voltage source 302 is associated with a microprocessor 306 which provides a stable regulated voltage reference and assists in regulating the bias voltage to a fixed value. Providing a bias voltage at a reliably fixed value assists in ensuring the validity of a conversion between the measured current and a radiation level. The regulated voltage reference may be the nominal voltage of a battery, as is described below, for example being 5 or 12V. The voltage source 302 may for example operate using a 12V supply and be operative to convert a 5V input into 300V. Both the 5V and the 12V supply may be derived from a common battery source or may be derived from different batteries. Such apparatus facilitates providing a relatively high bias voltage with a relativly low voltage power source.
The microprocessor 306 in this example controls the output of the reference voltage and thereby the higher bias voltage using software, which may improve safety as the microprocessor software must be operational for the 300V bias to be applied. This software may for example comprise software to recognise a state of a physical control switch for application of the bias voltage.
The ammeter 304 and voltage source 302 in this example are provided within a common housing 308, which also provides electromagnetic shielding thereof. In some examples, the voltage source 302 may be integral to the ammeter 304, or they may be otherwise rigidly fixed to one another. In this example, the housing 308 is provided with a bulkhead connector 310, which provides a fixed connection to a cable, and assists in minimising any unshielded cable length (which in turn may reduce electromagnetic noise). The bulkhead connector 310 in this example also provides ground connections, which may be connected to any electromagnetic shielding elements of a cable.
The monitoring apparatus 300 further comprises a USB hub 312, which allows communication to several feedback systems through a single port on a tablet computer 314. This reduces external footprint and guards against the accidental removal of critical components, while reducing the potential for contamination. In other examples, other connectors may be provided to allow communication with the tablet computer 314. In this example, the ammeter 304 is controllable by the tablet computer 314 via the hub 312. In other examples, alternative communications interfaces may be provided.
The tablet computer 314 provides an interface in the form of a touch sensitive display screen (which therefore provides an input device) and a memory (although in other examples, these may be provided separately, or may be supplemented, for example by a keyboard, a removable memory or the like). In this example, the tablet computer 314 controls system components using software and user input. The tablet computer 314 allows for the collection and real-time display of data. The memory therein may hold collected data and/or conversion coefficients allowing conversion of collected current data to infer radiation levels. In some examples, the tablet computer 314 may also allow for export of data, for example via a wired (e.g. USB) or wireless connection. A wired connection may be useful in operating environments in which a wireless connection may be absent or prohibited.
The monitoring apparatus 300 further comprises a battery connection 316 arranged to receive at least one battery. This may for example comprise one or more Lithium Polymer (LiPo) battery or the like, which provide high power density and are likely to run for longer than the working day on a single charge in the monitoring apparatus 300. The battery or batteries may be selected so as to provide a plurality (e.g. 12V and 5V) of regulated outputs. Regulated outputs provide stable power to system components without the need for mains electricity, which may vary in consistency. In some examples, the battery connection 316 may provide a permanent reference ground or 'pseudo ground' (also referred to as a virtual ground), and therefore does not rely on a local ground such as a main grounding. In some examples, the tablet computer 314 may provide an indication of the charge level of a battery.
The microprocessor 306 may monitor the battery charge level, for example alerting a user via the tablet computer 314 of a need to change or recharge a battery, and/or automatically halting measurements and ensuring data is saved. In this example, in general, the tablet computer 314 performs the processing of collected signals and the conversion of a measured current to a radiation dose and the microprocessor 306 provides and/or monitors voltages. While particular control actions have been ascribed to the microprocessor 306 and the tablet computer 314, it will be appreciated that each of these devices provides processing circuitry and as such processing may be carried out in either apparatus and/or the functions thereof provided by alternative processing circuitry. Therefore, at least one of the microprocessor 306 and the tablet computer 314 may be absent in alternative embodiments.
In some examples, there is also provided a physical bias control switch, which provides a safety interlock in that the user will have to selectively operate the switch, and also provides a physical representation of the status of the apparatus 300, in particular as to whether the relatively high bias voltage is being applied. As described above, the status of the switch may be detected by software running on the microprocessor 306. Figure 4 shows an example of data which may be collected. In an example, data may be collected by introducing a diamond radiation detector into the region of interest, the diamond radiation detector being connected to a signal cable (e.g. the signal cable 104, 200) while leaving a monitoring apparatus which is also connected to the signal cable (e.g. monitoring apparatus 106, 300) outside the region of interest. In this example, a detector apparatus is used to monitor the activity of highly active material and the detector may be introduced into a zone of interest via a relatively small inspection port. Such a port may for example be formed through a physical barrier comprising a shield material, which may for example comprise a thick concrete wall of perhaps 1 or 2 meters thickness, which encloses a highly active cell (e.g. an enclosed area containing ionising/radioactive waste products from the energy industry). The port may be a vertical port such that the detector may be lowered into the cell, which provides the region of interest, or a port at an angle to the vertical (for example a horizontal port). In this example, a shielding tube is introduced through the port such that the cell remains substantially sealed, and the detector is introduced inside the shielding tube.
While this provides one example of use of the detection apparatus, the apparatus may be used in other examples, including those where the distribution of ionising radiation sources may be unknown.
In this example, measurement begins as the detector moves through the shielding material (zone 402). Therefore, a bias voltage is applied to the diamond radiation detector from a voltage source of the monitoring apparatus and the returned current is measured as the diamond radiation detector is moved first into, then through, the region of interest (zone 404).
The radiation level builds with proximity to the active material within the cell, which in this example is located in the lower regions of the cell. The distance into the cell may be determined based on the length of cable extended.
In zone 404, two features 406a, 406b can be seen. These features 406a, 406b relate to the shielding tube used: the tube is formed of joined sections, and there is a region of overlap in the join, which provides additional shielding and a drop in current. Such features may be used to assist in locating the detector in the zone of interest and/or may be compensated for when determining a dose rate.
It will be appreciated that the radiation measured is indicative of an activity level, but that some ionising radiation will be absorbed by containment vessels and the like. Such absorption may be modelled to derive the activity of material within a vessel in some examples.
In some examples, the method may be carried out before and after purging and/or cleaning of such vessels. In this way, it may be determined if purging/cleaning has had an anticipated affect.
Dose rate measurements, which may in some examples be determined and viewed in real time, may thereby be made remotely from inside difficult to access areas. The apparatus described herein may be used to create a "map" or distribution of radioactivity in facilities which have small access ports, contain complex networks of pipes and vessels, and/or have a high radiation hazard.
Figure 5 shows an example associating the measured ammeter response and an activity level, or dose rate, for a particular detector apparatus, which may for example be determined during calibration. Over this range, the relationship appears to be substantially linear. However, over a larger range, it may be revealed that a power law applies. The conversion coefficients for each apparatus may be modelled and associated with the apparatus (for example, the combination of a cable and a detector used), and may be stored in a memory of the monitoring apparatus. It may be noted that, while standard conversion coefficients may be used, the signal is a low level signal and therefore subject to disruption in the presence of noise. Therefore, providing conversion coefficients and using these to convert the current readings for a particular combination of components may increase accuracy of conversion.
Conversion coefficients may for example be determined using a cobalt-60 source in an irradiation chamber allowing for exposure to a range of dose rates. In some examples, this range may be a large range, for example from approximately 0.1 Gray/hour up to >4000 Gray/hour), although the range of calibration may be tailored to an intended use case. At the time of calibration, a diamond detector may be located on the end of a cable with which it is intended to be used. Data may be recorded at regular intervals, for example of the order of every 10, 20, 50 or 100ms. In some examples, a background radiation level may be recorded before and after a detector is exposed to the radiation source (for example, over a 30 second period). The detector may be exposed to the radiation source for an exposure period (for example, around 60 seconds or so) and a mean and standard deviation of the current recorded calculated over the exposure period.
This may be repeated at different radiation exposures (for example, different distances to the source) to calibrate the detector over a range. This calibration may provide conversion coefficients for a power law, linear or look up table conversion of current measurements detected in use of the apparatus.
In some examples, the orientation of the detector may be varied during calibration, for example to characterise the orientation dependence of the detector. In some examples, the length and/or portion of cable may be varied during calibration (e.g. the length/portion exposed to the radiation source), for example, to characterise the effect of ionisation of the cable. This may be used to provide conversion coefficients relating to orientation and/or exposed cable length/portion.
In some examples, detectors which exhibit a low dark current variation over a range of bias voltage, a low standard deviation in leakage current measurement in an exposure period and/or a consistent variation in leakage current with dose rate may be selected for use in detection.
While the methods, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims. Features described in relation to one example may be combined with features of another example.
The word "comprising" does not exclude the presence of elements other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.
Any operating parameters mentioned above are purely by way of example and may alter depending on operating conditions and use cases.

Claims

1 A radiation detection apparatus comprising:
a diamond radiation detector and a cable;
wherein:
a first end of the cable is arranged to be connected to a voltage source and an ammeter and a second end of the cable is arranged to be connected to the diamond radiation detector; and
the cable is arranged to connect the voltage source to the diamond radiation detector to provide a bias voltage and to connect the ammeter to measure a current generated by radiation passing through the diamond radiation detector.
2. A radiation detection apparatus according to claim 1 wherein the cable is at least 10 meters in length.
3. A radiation detection apparatus according to claim 1 which is configured to detect radiation emitted by waste products of the nuclear energy industry.
4. A radiation detection apparatus according to any preceding claim wherein the cable comprises:
a first conductor arranged to apply the bias voltage and to carry the current generated by radiation passing through the diamond radiation detector;
a second connector to act as a reference for the current measurement; and an insulator to provide galvanic isolation between the first and second conductor.
5. A radiation detection apparatus according to claim 4 wherein the cable comprises a third conductor arranged to provide an electromagnetic shield.
6. A radiation detection apparatus according to any preceding claim wherein the cable comprises a stiffening member to provide longitudinal stiffness to the cable.
7. A radiation detection apparatus according to claim 6 wherein the stiffening member is conductive.
8. A radiation detection apparatus according to any preceding claim wherein the cable comprises a removable sheath.
9. A radiation detection apparatus according to any preceding claim further comprising a monitoring apparatus, the monitoring apparatus comprising the voltage source and the ammeter.
10. A radiation detection apparatus according to claim 9 in which the monitoring apparatus further comprises a battery connection arranged to connect to at least one battery.
1 1. A radiation detection apparatus according to claim 9 or claim 10 in which the monitoring apparatus further comprises an interface, the interface comprising a display screen.
12. A radiation detection apparatus according to any of claims 9 to 1 1 in which the ammeter and voltage source are rigidly fixed to one another.
13. A radiation detection apparatus according to any of claims 9 to 12 in which the ammeter and voltage source are located in a common shielding.
14. A radiation detection apparatus according to any of claims 9 to 13 in which the monitoring apparatus comprises a memory, the memory comprising at least one conversion coefficient associated with at least one combination of a cable and a detector of the apparatus.
15. A radiation detection apparatus according to any preceding claim in which the diamond radiation detector exhibits a dark current which varies within a range of less than 100nA over a bias voltage range of at least 1000V.
16. A radiation detection apparatus according to any preceding claim in which the ammeter is configured to detect a current in a pico-amp to milliamp range.
17. A method of detecting ionising radiation in a region of interest comprising: introducing a diamond radiation detector into the region of interest, the diamond radiation detector being connected to a signal cable,
arranging a monitoring apparatus outside the region of interest, the monitoring apparatus being connected to the signal cable;
applying a bias voltage to the diamond radiation detector from a voltage source of the monitoring apparatus;
measuring a current returned via the signal cable; and
determining a measurement of ionising radiation in the region of interest based on the current.
18. A method according to claim 17 wherein measuring the current comprises measuring the current while the diamond radiation detector is moved through the region of interest.
19. A method according to claim 17 or 18 comprising adjusting at least one of a rotational position or a horizontal position of the diamond radiation detector using the signal cable.
20. A method according to any of claims 17 to 19 further comprising displaying an indication of the current and/or an indication of an ionising radiation level in real time on a display screen of the monitoring apparatus.
21. A method according to any of claims 17 to 20 in which determining the measurement of ionising radiation in the region of interest based on the current comprises using a conversion coefficient associated with the cable and the diamond radiation detector used in carrying out the method.
PCT/GB2018/050546 2017-03-03 2018-03-02 Radiation detection apparatus WO2018158591A1 (en)

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