GB2426325A

GB2426325A - Beta radiation detector

Info

Publication number: GB2426325A
Application number: GB0510402A
Authority: GB
Inventors: Michael Gooding; Duncan Housden; Stuart M Vincent
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 2005-05-21
Filing date: 2005-05-21
Publication date: 2006-11-22
Also published as: GB0510402D0

Abstract

A detection device that utilizes the CERENKOV effect for high sensitivity 'in-situ detection of certain radionuclides present within large bodies of water or other liquids comprises a sampling chamber 1 viewed by photo-electric devices 2, 3. The sampling chamber 1 is preferably several litres in volume and it's internal surfaces are provided with a reflective coating to enhance light reflection. The sampling chamber 1 is provided with liquid inlet valves 4 and liquid outlet valves 5 to enable temporary sample isolation such that 'real-time' detection is possible.

Description

Beta Radiation Detector This invention relates to apparatus for detecting

certain beta emitting radionuclides in-situ' in large bodies of water or other liquids, for example at depth within seawater, lakes or rivers. It is applicable to the measurement of radionuclides such as' potassium-40, protactinium-234m, and bismuth-21 4.

Conventional methods for the direct in-situ' detection of radionuclides within large bodies of water (such as the sea or lakes) involve measurement of the gamma emissions associated with certain radionuclides, for example through use of sodium iodide scintillation detectors. This is primarily because gamma emissions generally have much greater penetrating power than any alpha or beta emissions that may also be present. Thus, a gamma detector would normally have a greater range' in water than either a beta or alpha detector, and would therefore be expected to provide superior detection sensitivity.

However, gamma detection is not suitable in a confined environment because considerable unwanted background emissions from the surroundings would render such a probe very insensitive. Therefore, an alternative detection technique is required for confined environments.

It is well known that certain radionuclides emit beta particles, and that when these beta particles pass though a medium such as water, a form of light radiation, known as CERENKOV radiation, is produced. CERENKOV radiation emissions (photons) can be measured using photo-electric devices such as photo-multipliers, and the intensity of the emissions can be used to ascertain the original energy of the beta particle giving rise to the CERENKOV interaction. Only those beta particles having energies above a threshold, approximately 0.3 MeV in the case of water, can produce CERENKOV emissions. Accordingly, in the case of a CERENKOV detector using water as the detection medium, characterisation of beta emissions by the CERENKOV effect is limited in application to those radionuclides having beta emissions with energies in excess of approximately 0.3 MeV.

For beta emissions with energies greater than approximately 0.8 MeV, the yield of CERENKOV light photons associated with each beta interaction has approximately direct proportionality to the initial beta energy. Accordingly, if the CERENKOV photon emissions are efficiently detected, the magnitude of pulses produced in photo-electric devices will have near direct proportionality to the initial beta energy. This phenomenon may be used to perform spectroscopy to characterise the emissions of particular radionuclides, such as pottassium-40, protactinium-234m, and bismuth-21 4.

However, the CERENKOV effect is weak and is traditionally used to detect beta emissions in confined environments such as in the laboratory or an industrial environment. An example of one such detection device is described in GB 1022566 (Commissariat A L'Energie Atomique), in which a water sample is contained in an open-topped receptacle of approximately 1 litre volume. A small-scale in-situ' CERENKOV device is described in US 5229604 (Larson et al). This device is designed for use in limited-access areas such as well-logging boreholes, which may be only a few centimetres in diameter.

For large-scale in-situ' detection, gamma detection is traditionally used as described above.

It is an object of the present invention to provide a detection device that utilises the CERENKOV effect for high sensitivity in-situ' detection of certain radionuclides present within large bodies of water or other liquids.

Furthermore, the device can be used to undertake spectroscopic analysis for the purpose of identifying the radionuclides present. The present invention offers the possibility of real-time' detection of beta emitters at high sensitivity, without the need for removal and transfer of samples to a laboratory for analysis.

According to the present invention there is provided a detection device for determining the presence of beta emitting radionuclides in situ' in a large body of liquid by application of the CERENKOV effect, comprising a sampling chamber, at least one photo-electric device mounted to view the sampling chamber and means for detecting the electrical output from the at least one photo-electric device, wherein the sampling chamber is a substantially closed vessel having a volume of at least one litre and wherein the internal surfaces of the sampling chamber are adapted to provide efficient light reflection.

In order for the detection device to yield a sensitivity comparable to, or greater than, that of a gamma sensor in a bulk liquid environment, it is necessary for the device to have a much larger sampling chamber than previously found in CERENKOV detection devices. The maximum volume of the sampling chamber is likely to be dictated by the photon detection efficiency of the system. However, using design features described in this application, a sampling chamber of several litres volume is possible.

The construction of the sampling chamber is such that the liquid that is to be analysed is able to pass into the sampling chamber through a liquid inlet aperture and out through a liquid outlet aperture. The flow of liquid through the sampling chamber may be either by natural motion, if for example the detector is being towed horizontally or hoisted vertically within the liquid, or effected using one or more pumps. In addition, the sampling chamber may incorporate a system of valves, to isolate the sample for counting over a defined period, for example a few minutes.

In order to enhance sensitivity, it is beneficial to operate the detection device at depth to avoid the influences of cosmic radiation background. For this reason the sampling chamber preferably comprises a pressure resistant vessel and may incorporate one or more pressure release valves.

The efficiency of the detection device can be maximised by selecting the photo-electric device(s) to match the wavelength of CERENKOV emissions.

Furthermore, the internal surfaces of the sampling chamber are preferably provided with a coating of a reflective material selected to provide optimal reflection of photons with emission spectra characteristic of CERENKOV emissions.

The detection device may incorporate at least two photo-electric devices mounted to view the sampling chamber, in which case coincidence circuitry may be provided to facilitate background suppression by rejection of pulses other than those occurring simultaneously, or within a defined resolving time, in the photo-electric devices. Performance may also be enhanced by the use of electronic circuitry to perform pulse shape discrimination and/or pulse timing discrimination to suppress background effects.

If the detection device further comprises a Multi-Channel Analyser (MCA) it is possible to characterise pulse amplitude in order to effect spectroscopy of beta emissions and hence radio-isotopic analysis. Such real-time' radio- isotopic analysis is not possible with conventional gamma detection methods for in-situ' detection of radionuclides within large bodies of liquid.

The invention is notable in that the CERENKOV measurement system described is specifically designed as a deployable device for in-situ' detection and spectroscopy of beta emitting radionuclides in large bodies of water (or other liquids), such as the sea, lakes, or rivers. The components of the device are such that there is no need for collection and removal of samples and transport of those samples for analysis at a remote laboratory, as would conventionally be required for detection and spectroscopy of beta emitting radionuclides in such large bodies of liquids. Furthermore, a large volume flow-though' device, linked by a supply and signal cable to a vessel such as a ship, allows real-time' beta radiation detection and analysis with favourable limits of detection.

In use the detection device is submerged in a large body of liquid and the real-time' output from the photo-electric device(s) is monitored.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of part of a detection device according to the invention; and Figure 2 is a schematic representation of a detection device including associated circuitry.

The detection device shown in Figure 1 comprises a sampling chamber 1 which is viewed by two photo-electric devices 2, 3. Means for detecting the electrical output from the photo-electric devices is not shown in Figure 1. To maximise the light collection efficiency of the device, the internal surtace of the sampling chamber 1 is coated with a reflective material and the photo- electric devices 2, 3 are photo-multipliers which have been selected so as to be of a type optimised to the wavelength of CERENKOV emissions. Those skilled in the art will understand that it is desirable to select photo-multipliers which have high gain, large area end-windows of a type transparent to CERENKOV emissions, good (i.e. fast) pulse timing properties and low concentrations of naturally occurring radioactive material. Photo-multiplier end-windows of quartz, fused silica, sapphire, magnesium fluoride or Perspex may be appropriate for applications where low-level' sampling is required.

In order to facilitate use of the detection device at depth, the sampling chamber 1 is a submersible substantially closed vessel having one or more liquid inlet valves 4 located within liquid inlet apertures and one or more liquid outlet valves 5 located within liquid outlet apertures. The liquid inlet valves 4 may incorporate a pump (not shown).

The photo-multipliers 2, 3 and ancillary electronic components may be housed in an appropriate pressure resistant container 6, 7. Additionally, if the chamber is to be raised or lowered when the inlet and outlet valves are closed, the sampling chamber housing may be required to be pressure resistant. Furthermore, it may be appropriate to incorporate one or more pressure release valve(s) (not shown) to prevent overpressurising (either from the outside or inside) in the event that the normal inlet/outlet is jammed shut.

A multi-functional cable 8 may be utilised to tow the detection device through the water or other liquid. The cable 8 may also be used to provide the power supply to the detection device and to return the electrical output from the photo-multipliers to a remote monitoring station such as on board a ship.

Figure 2 illustrates the electronic components which may be incorporated into the detection device. The photo-multipliers 2, 3 are operably connected to a high voltage supply 10. The output pulses from each of the photo-multipliers 2, 3 are amplified in pre-amplifiers and linear amplifiers 11, 12. Coincidence circuitry, comprising timing single channel analysers 13, 14 and a fast coincidence analyser 15 is used to remove pulses that are yielded by one photo-multiplier only, as opposed to pulses yielded by both photo-multipliers simultaneously. The pulses of those coincident events remaining are summed, by means of pulse addition circuitry 16, to yield a pulse, the amplitude of which is characterised in a Multi-Channel Analyser (MCA) 17.

The results can then be interpreted and presented with the aid of a computer 18. As stated previously, the number of CERENKOV photons emitted during a specific beta decay event will be proportional to the initial beta energy.

Therefore, the amplitude and corresponding channel for pulses within the MCA 17 will be representative of this beta energy, allowing the system to be used for beta spectroscopy.

Where favourable limits of detection are required, a number of additional features (not shown) may be appropriate. In order to ensure optimised performance, the introduction of fixed and variable timing delays within the circuitry is likely to be appropriate. Furthermore, pulse shape discrimination and analysis techniques may be used to enhance limits of detection through rejection of pulses with timing and/or amplitude properties uncharacteristic of CERENKOV events.

In use, for example at sea, the sampling chamber is submerged to the required depth, water enters the sampling chamber 1 via the inlet valves 4.

Any beta emitting radionuclide having emissions in excess of the CERENKOV threshold energy of approximately 0.3 MeV and present within the sampling chamber 1 will give rise to emission of CERENKOV photons which are detected by the photo-multipliers 2, 3. The output from the photomultipliers 2, 3 is processed to provide real-time' detection indicators and/or spectroscopy results to an operator via the computer 18.

Where sample isolation is required, it may be beneficial to add small amounts (of the order of 0.lg/litre) of an appropriate wavelength shifter' substance, such as the sodium potassium salt of 2-naphthylamine-6,8disulphonic acid, to the sampling chamber in order to enhance detection efficiency.

The detection device may be towed by, or attached to, a vessel, in which case it is envisaged that certain components of the system, as described in Figure 2, would be located on that vessel. Alternatively, certain components may be housed in a remotely operated vehicle (ROy), which is deployed with the detection device. Where appropriate, any towing lines employed will be required to be sufficiently long and robust such as to carry the measurement device to the required depth. Additionally, the power supply and signal cabling will require construction such as to ensure that losses and interference effects do not become significant.

Claims

Claims 1. A detection device for determining the presence of beta emitting

radionuclides in situ' in a large body of liquid by application of the CERENKOV effect, comprising a sampling chamber, at least one photoelectric device mounted to view the sampling chamber and means for detecting the electrical output from the at least one photo-electric device, wherein the sampling chamber is a substantially closed vessel having a volume of at least one litre and wherein the internal surfaces of the sampling chamber are adapted to provide efficient light reflection.
2. A detection device as claimed in claim 1 wherein the sampling chamber comprises a liquid inlet aperture and a liquid outlet aperture.
3. A detection device as claimed in claim 1 or claim 2 wherein the sampling chamber is provided with one or more pumps to provide liquid flow through the vessel.
4. A detection device as claimed in any preceding claim wherein the sampling chamber comprises a system of valves to enable sample isolation.
5. A detection device as claimed in any preceding claim wherein the sampling chamber comprises a pressure resistant vessel.
6. A detection device as claimed in any preceding claim wherein the sampling chamber incorporates one or more pressure release valves.
7. A detection device as claimed in any preceding claim wherein the at least one photo-electric device is matched to the wavelength of CERENKOV emissions.
8. A detection device as claimed in any preceding claim wherein the internal surfaces of the sampling chamber are provided with a coating of a reflective material selected to provide optimal reflection of photons with emission spectra characteristic of CERENKOV emissions.
9. A detection device as claimed in any preceding claim wherein at least two photo-electric devices are mounted to view the sampling chamber and coincidence circuitry is provided to facilitate background suppression by rejection of pulses other than those occurring simultaneously, or within a defined resolving time, in the photo-electric devices.
10. A detection device as claimed in any preceding claim wherein pulse shape discrimination is used to suppress background effects.
11. A detection device as claimed in any preceding claim wherein pulse timing discrimination is used to suppress background effects.
12. A detection device as claimed in any preceding claim wherein the device further comprises a Multi-Channel Analyser (MCA) to enable characterization of pulse amplitude in order to effect spectroscopy of beta emissions and hence radio-isotopic analysis.
13. A method for determining the presence of beta emitting radionuclides in situ' in a large body of liquid by application of the CERENKOV effect, comprising the steps of submerging the sampling chamber of a detection device as claimed in any preceding claim in a large body of liquid and monitoring the real-time' output from the at least one photo-electric device.
14. A method for determining the presence of beta emitting radionuclides as claimed in claim 13 wherein a wavelength shifter' substance is added to the sampling chamber in order to enhance detection efficiency.
15. A detection device substantially as hereinbefore described with reference to Figures 1 to 2 of the accompanying drawings.