SE515336C2 - Apparatus for detecting particles from radioactive decay, as well as a method for measuring the content of radioactivity - Google Patents

Abstract

An apparatus for detecting particles from radioactive decay, such as alpha particles, has a first detector circuit (101) and at least a second detector circuit (102) adapted to generate a respective electric signal on a respective output (111, 112) for indicating a hit by a radioactive particle. A controller (140) is operatively coupled to the output of the first and second detector circuit, respectively. The controller provides a result output indicative of a number of radioactive particle hits as indicated by the first and second detector circuits. Moreover, the controller ignores any second particle hit, which follows after a first particle hit, if the first and second particle hits are indicated within a time period, which is shorter than a predetermined time period.

Description

20 25 30 35 §gs zs6 2 2000-12-13 E: \ PUBLIC \ DOC \ P \ 4l40002 Sv óveríLdoc. . . -. . .. * '' ''. | o ~. ~ = «| .. in addition, derived from water. If such houses are not adequately ventilated, the presence of radon gas is a potentially serious health problem. Scientists have estimated that 1,000 new cases of lung cancer are discovered each year in Sweden alone due to exposure to radon gas. It would therefore be desirable for owners and residents of houses, that a cheap, compact and easy-to-use device was available to measure the content of radon at home or at work.

Radon is a noble gas and is usually monoatomic, ie a gas molecule consists of only one atom (unlike, for example, nitrogen or oxygen, which is usually diatomic and comprises two atoms per molecule).

Radon is formed when radium decays. Radon occurs 222 in several different isotopes, of which Rn is the most common in the ground and indoor air. The soil also contains large amounts of the isotope 2 ”Rn, which is usually called toron and occurs where the element thorium is found.

The isotope ”2Rn is unstable and decomposes to a radon daughter” 8Po, whereby alpha particles are generated (helium nuclei). The radioactive alpha particles can travel about 4.5 cm in air. The radon content is measured in Bq / mi, ie the number of decays per second and cubic meters. The radon daughters, which are formed when radon decomposes, are also radioactive and give rise to alpha particles and beta particles. Unlike radon, however, the radon daughters are not gaseous and will therefore, usually, remain at the place where they originated. The radon daughters attach effectively to electrically charged dust, water or soot particles, or to air molecules. The daughters continue to attach to walls and protruding parts in a room.

Alpha radiation and beta radiation, which occur when radon and radon daughters decay, are dangerous to the human body, as energy will be transferred from the alpha particles and beta particles to the cells in the body.

It is well known per se that such radiation can cause 10 15 20 25 30 515 sssgßggg 3 2000-12-13 E: \ PUBLIC \ DOC \ P \ 4] .40002 EN ÖversS.d0C 13A /. | »». . - A. - =. abnormal cells such as cancer cells. In particular, the cells in the lungs are very sensitive to radiation caused by radon gas and radon daughters inside the lungs.

Several different types of detectors for radon and radon daughters are previously known. Such detectors are essentially either of the instantaneous / continuous measuring type or of the integrating type. The first type of detectors momentarily detects the individual alpha particles by an electrical pulse, which is generated by a sensor when it is struck by the alpha particle, and which is detected by appropriate electronics. The second type registers the radioactivity over a long period of time, for example for several days or even several months.

Integrating radon gas detectors include, for example, trace film with or without filters, activated carbon spectrometry and thermoluminescence dosimeters.

The apparatus for detecting radioactive particles according to the invention belongs to the instantaneous / continuous type of detectors. Such detectors constantly provide an instantaneous value of the radon content in a given environment. In order to function correctly, the air volume where the measurements take place must be regularly replaced via a filter, for example by diffusion or by means of a fan or pump.

An ion chamber is perhaps the simplest form of instantaneous / continuous particle detectors. A high voltage electric field is generated between two electrode plates. As alpha particles pass between the electrode plates, the air will be ionized and cause a discharge between the electrodes, which can be detected with appropriate electronics.

Ion chambers have a disadvantage in that they require high voltage equipment and often have a large format. Ion chambers are also sensitive to virtually all types of ionizing radiation, which makes an ion chamber unsuitable if 10 15 20 25 30 35 515 336 4 2000-12-13 E: \ PUBLIC \ DOC \ P \ 4] .40002 Sv Ö 'n. n is only interested in one type of radiation, such as alpha radiation.

Other types of instantaneous / continuous particle detectors are Teflon electret and Lucas cells.

The present invention belongs to a fourth type of particle detectors; surface barrier detectors. Surface barrier detectors include a semiconductor sensor. When an alpha particle passes through a p-n junction in silicon, an electrical pulse is generated, which is recorded and used to keep track of the number of detected alpha particles.

Commercially available radon gas meters are, for example, ATMOS 12 from Gammadata Mätteknik AB, Uppsala; the radon meter RM3-B from Studsvik Instrument, Nyköping; and the AlphaGUARD PQ 2000 / MCSO detector from ScandnoraX AB, Vallentuna. Large device format, heavy weight and high cost are common to these products.

SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus for detecting particles from radioactive decay, which apparatus is simpler, cheaper and more compact than the commercially available products mentioned above.

This object has been achieved by an apparatus which is equipped with at least two separate and parallel semiconductor detector channels, each of which is arranged to detect a radioactive particle and as a result generate an electrical signal. The output signals from the semiconductor detector channels are independently connected to a control unit, which keeps track of a total number of the particle hits that have been detected by the independent semiconductor channels. In order to reduce the effect of interference, and consequently to achieve improved accuracy, the control unit is further arranged to ignore a second particle hit, reported by one of the independent semiconductor detector channels, on the 10 15 20 25 30 .n i. 15 336 "'' '. 2000-12-13 a; \ PusL1c \ Doc \ P \ 414ooo2 en oversnoc may follow within a predetermined time after a first particle hit.Because "real" radioactive particles essentially always occur one by one, simultaneous particle hits, which have been reported from more than one of the semiconductor channels at the same time or only for a short time in succession, will obviously have been caused by disturbances and must therefore be ignored by the control unit.

The object of the present invention is also achieved by a method for measuring the content of radioactivity, wherein a counter is allowed to represent a number of detected radioactive particles after a given starting moment; a first detection signal is received as an indication of a first radioactive particle; a second detection signal is received as an indication of a second radioactive particle; a period of time is determined between the reception of the first and second detection signals; determining whether this time period is shorter than a predetermined limit value; and, if so, the first and second detection signals are ignored; or the counter is otherwise enumerated.

Other objects, advantages and features of the present invention will become apparent from the following detailed description of a preferred embodiment, from the drawings and also from the appended subclaims.

Brief Description of the Drawings A preferred embodiment of the present invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 is a schematic block diagram of an apparatus for detecting radioactive particles according to the preferred embodiment, Fig. 2 is a schematic circuit diagram of an independent particle detector circuit in the embodiment shown in FIG. 1, and v ~ «; 0 vn | I I. 1. . . » .. - nu nu. . ~ I s. . 1 ne 1 »-. | n, 1; -4 .xu -1-0 6 2000-12-13 E: \ PUBLIC \ DOC \ P \ 414000Z En Over5.d0C ÉA / LJ FIG. 3 is a flow chart of a working principle of a control unit, which is used in the one in FIG. showed the embodiment.

Detailed Description of a Preferred Embodiment Fig. 1 illustrates an apparatus for detecting radioactive particles of the surface barrier detector type described above. The apparatus comprises four independent particle channels 101-104, comprises a semiconductor detector for detecting a detector circuit or which are each incoming alpha particle and consequently generating a voltage 112, 113, 114, which will be described in more detail with reference to FIG 2.

A comparator circuit 120 is connected to the pulse outputs at a respective output 111, 111-114 of the particle detector circuits 101-104. In the preferred embodiment, the comparator circuit 120 is an LM339 comparator, which is commercially available from, for example, Motorola Inc. and National Semiconductor Inc., USA. The comparator circuit 120 also has a reference input which is connected to a voltage reference source 122. The comparator circuit is arranged to compare the voltage pulses arriving from one of the particle detector circuits 101-104 when struck by an alpha particle with a fixed reference voltage Vref from the voltage source 122. such an incoming voltage pulse exceeds the fixed reference voltage Vref, the comparator circuit 120 will emit a logic zero at a corresponding output among a set of outputs 141, which are connected to a corresponding number of inputs of a microcontroller 140. Otherwise, the outputs 141 are usually constantly high .

The microcontroller 140 in the preferred embodiment is a PIC16C64A, Microchip Technology Inc., USA. The microcontroller 140 can, which is commercially available from 10 15 20 25 30 515 536 §; §¿; ¿yv 2000-12-13 E: \ PuBLIc \ Doc \ P \ 414oooz en oversnoc BA / LJ m ... but be implemented by someone another commercially available microcontroller in the PIC16Cxx family, such as PIC16C73, or by PIC16688 (which is also available from Microchip Technology Inc.), or by any other commercially available microcontroller.

The microcontroller 140 has a second input 142, which is connected to a 12-position switch 130, by means of which the user of the apparatus can set a desired integration time. In the preferred embodiment, the different positions of the switch 130 represent 1 hour, 4 hours, 8 hours, 12 hours, 30 days and 60 days, as well as a position for continuous measurements. 1 day, 2 days, 7 days, 14 days, If this latter position is selected, the device will count all electrical pulses from the detector circuits 101-104 from a given start moment. The value of the radon content will then be updated once per minute or whenever a new pulse arrives, whereby the device will present an average value of the content during the entire measurement period.

The values for full days will be updated once an hour.

The microcontroller 140 also receives two different clock signals (which are not illustrated in FIG. 1), the first of which indicates the processor frequency of the microcontroller 140 and is set to 12 MHz in the preferred embodiment. The second clock signal is applied from a real-time clock 170 over an IC bus 143, which is received at a third input of the microcontroller 140.

The microcontroller 140 has a first output 144, which is connected to a character display 150, through which the user can read the measured radon content as well as an error margin (accuracy). The display 150 is commercially available from Hitachi Ltd, Japan, under the model name LM020XMBL. The real-time clock 170 is manufactured by Dallas Semiconductor Inc., USA, under the name DS1307. 10 15 20 25 30 35 ... fv- 515 336 "" "1 i 1 noo. I. 4 1 ß. Vv ~ uv; n 2000-12-13 E; \ PUBLIC \ DOC \ P \ 4l40002 Sv Övers. d0c BA / LJ: II i ”. Ip: 2 1 j: 'uan. ..» - The microcontroller is further operatively connected via the IC bus 143 to a serial memory 160 of 8K, which is commercially available from Microchip Technology Inc. under The serial memory 160 is used to store temporary variables, instantaneous counter results, etc., which are used by the microcontroller 140 during its operation.The memory 160 may also store program instructions to be executed by the microcontroller 140. In the preferred embodiment, however, such programs gram instructions internally in an EPROM program memory in the microcontroller 140.

The microcontroller 140 may also provide an output signal to be used to control a fan (not shown in FIG. 1), which is located inside the housing of the apparatus for detecting radioactive particles and which has the task of exchanging the air inside the apparatus. The fan can either run constantly or be operated regularly, for example for one minute after every ten minutes. Alternatively, the air inside the apparatus can be replaced by means of a pump, preferably provided with a filter.

Finally, the microcontroller 140 is connected, via an input 145, to a serial RS232 interface 190, which can be used to connect external devices (for example a laptop) to the apparatus for detecting radioactive particles. In this way, measurement results can be transmitted electronically from the apparatus for detecting radioactive particles to an external computer, where the results can be processed and further analyzed.

Referring to FIG. 2, there is illustrated a circuit diagram of the first particle detector circuit 101. The second, third and fourth particle detector circuits 102-104 are identical to the circuit 101. As previously mentioned, the apparatus for detecting radioactive particles according to the preferred embodiment comprises four separate and independent detector channels. , each of which can detect a radio 10 H 20 25 30 35 2000-12-13 E: \ PUBLIC \ DOC \ P \ 414Û002 Sv överAioC BA / - a: 1 ø »e | k. active particle (for example an alpha particle) and as a result generate an electrical voltage pulse. The particle detector circuit in FIG. 2 is driven by a supply voltage Vw = 5 volts. A first amplifier stage comprises a first operational amplifier 200 with a grounded positive input and a negative input, to which the output of a semiconductor particle detector is connected (designated Im in FIG. 2). The semiconductor sensor used in each particle detector circuit 101-104 according to the preferred embodiment is a surface barrier type semiconductor detector, for example a photodiode or a solar cell.

The semiconductor detector includes a p-n junction. When the p-n junction is struck by, for example, an alpha particle, charge carriers will be excited and released, so that electrons and holes will move across the junction and give rise to an electrical pulse, which is well known in the art of semiconductor technology.

As shown in Fig. 2, the output signal from the first operational amplifier 200 is fed, via a resistor R3, which is connected in parallel with a capacitor C3, to the negative input of the amplifier 200. The operational amplifier 200 is implemented by an operational amplifier of the type TLC (10 GQ) , which is required to amplify the extremely weak pulses from 274 and has a very high input impedance photodiode Im. The first operational amplifier 200 is connected as a transresistance amplifier (ie a current-controlled voltage generator), where the resistor R3 and the capacitor C3 form a first-order low-pass filter, which limits the frequency upwards and eliminates interference at high frequencies.

A second amplifier stage is formed by a second operational amplifier 210, which is a TLC type 274 amplifier just like the first operational amplifier 200.

A high-pass filter, comprising a resistor R5 and a capacitor C1, is arranged between the output of the first operational amplifier 200 and the positive input of the .q now u o u v .. u »n u m v f; a: n i a: n I 1 0 'I é, .. »1- u v: o I 0 I I m,» Å »I 1 11 n. u - u I; I \ z v f n 1» n v - = s n 1. _. v, 10 15 20 25 30 35 15 336 10 2000-12-13 E: \ PUBLIC \ DOC \ P \ 4l40002 EN ÖVEISAÅOC BA / â other operational amplifier 210. The high-pass filter is necessary, since the dark current from the photodiode normally amounts to about 100 pA, which would immediately drive the amplifier 210 to saturation, if the filter had not been arranged to eliminate this "direct current". The high-pass filter also eliminates other disturbances at low frequencies (such as mains hum at 50 Hz and 100 Hz). In addition to the second amplifier 210, the second stage of the circuit shown in FIG. 2 comprises a feedback loop from the output of the amplifier 210 to its negative input, said loop comprising two resistors R1 and R2, the care of which is selected to achieve a gain of 1000. is a capacitor C2 connected in parallel with the resistor R1 and constitutes another low-pass filter, which eliminates high-frequency interference.

At the output part of the circuit in FIG. 2 there is a high-pass filter, comprising a capacitor C4 and a resistor R4. This filter eliminates the offset voltage from the second amplifier stage. From an overall point of view, the various filters in the amplifier stages form a second order bandpass filter, which only transmits signals within the frequency range where the signals from the photodiodes Im occur (caused by incoming alpha particles).

As described above, since the voltage pulses from the photodiodes are extremely weak, the particle detector circuits 101-104 are exposed to various electrical and electromagnetic disturbances from the environment. According to the invention, a "real" alpha particle can be recognized and distinguished from a "false" alpha particle giving rise to a false electrical pulse from any of the particles (ie an electrical disturbance which gives the detector circuits 101-104), since a pulse from a real alpha particle in principle will always appear "alone". In other words, real pulses will occur one by one with large time intervals, which will amount to _ .. <- 10 15 20 25 30 35 515 336 §_j = § | jj = _ = .___, = ll 2000-12- 13 E: \ PUBLIC \ DOC \ P \ 414D002 en Översxioc EJA / LJ L .. now for at least several seconds and which may well be as long as, for example, one hour at low radon levels.

Disturbances, on the other hand, will usually always give rise to a burst of pulses. The particle detection apparatus according to the invention is therefore arranged to determine the time between two subsequent pulses and to ignore such pulses, if the time interval between these is shorter than a predetermined limit value. Consequently, whether two pulses are generated substantially simultaneously by two different particle detector circuits 101-104, or whether two pulses are generated in a short time by the same particle detector circuit, such pulses will be ignored by the microcontroller 140.

Referring to FIG. 3, a schematic flow chart is presented which illustrates the operation of the microcontroller 140 in FIG. 1. In a first step 300, a counter is initialized in the microcontroller 140. This counter is used to keep track of the number of alpha particles. which has been detected by any of the particle detector circuits 101-104 from a predetermined start moment. In step 300, the memory 160 as well as internal registers are also initialized in the microcontroller 140, the real-time clock 170 and the display 150.

Thereafter, the method enters an endless main loop 310. In this main loop, the microcontroller 140 'monitors whether an interrupt is received from an external clock, updating the real-time clock 170 and the display 150 in a subsequent step 320. When step 320 ends, control returns to the main step. 310.

In step 310, the microcontroller 140 also monitors whether an interruption is caused by an input signal from any of the particle detector circuits 101-104. As described above, an incoming radioactive particle will be detected by one of the particle detector circuits 101-104, which generates an electrical pulse at the corresponding output 111-114. The comparator circuit 120 compares the electrical pulse with the fixed 10 15 20 25 30 (5.3 S, 336 12 2000-12-13 E; \ PUBLIC \ DOC \ P \ 4140002 en oversmioc... «N 1 v I. nun the reference voltage Vref from the source 122 and consequently generates an output signal at a respective output 141.

If the microcontroller 140 is informed via a logically high signal at one of the inputs 141 that an alpha particle has been detected, control proceeds to a step 330, where it is determined whether the signal represents a real alpha pulse or merely a disturbance. As mentioned above, the microcontroller 140 checks whether a pulse has been received from more than one of the particle detector circuits 101-104. If so, the pulses obviously represent a disturbance, with the microcontroller 140 ignoring this disturbance by immediately returning control to step 310 without counting the particle counter. If only one pulse was received from any of the particle detector circuits 101-104, then the microcontroller 140 awaits a subsequent pulse within a predetermined time interval (for example, 2 seconds). If at least one subsequent pulse is actually generated within this time interval, the microcontroller 140 determines that the pulses are due to a disturbance and should be ignored by immediately returning control to step 310 without counting the counter.

Otherwise, i.e., if only a single electrical pulse has been received from any of the particle detector circuits 101-104 within the prescribed time limit, the microcontroller 140 will count the number of pulses stored in the counter. The microcontroller 140 will also update appropriate variables and the like in memory 160 and update the display 150 to reflect the enumerated particle counter.

The display 150 may present an average value of the measured radon content after a start moment (if the switch 130 is set to continuous measurements), or alternatively the measured radon content during the selected time period (with setting according to the switch 130). f .. .f 10 15 20 25 2000> 12-13 E; \ PUBLIC \ DOC \ P \ 4l40002 Sv ÖVeISJiOC SÄS 1 5 3 3 6 13. . . . .- ~ *. . . »U The microcontroller 140 also determines an error margin (accuracy) for the determined radon content and presents this accuracy on the display 150, which has already been mentioned above.

The present invention has been described above with reference to a preferred embodiment. However, the invention is in no way limited to the foregoing description. On the contrary, the invention is to be limited only by the definitions in the appended independent claims. Embodiments other than those described here are equally possible within the scope of the invention.

In particular, it should be understood that drying circuits can be used, but not less than two. Even more that less than four detectors than four detector circuits may be suitable in certain circumstances.

Furthermore, when two particle hits have been reported not simultaneously but within the predetermined time period, the controller may be programmed to ignore only one of the hits instead of both.

Claims (16)

10 U 20 25 30 515 336 ~ 'f ... .. 2000-12-11 m = \ pus1.1c \ noc \ v \ 414ooo2 av Colonel: axx / La _' -; 14 '"PÄTENTKRÄ" Apparatus for detecting particles from a radio characterized by: (101) which are arranged to generate an active decay, a first detector circuit and at least one (102), respectively electrical signal on a respective output (111, 112) second detector circuit for to indicate a hit of a radioactive particle; and a control unit (140) operatively connected to the output of the first and second detector circuits, the control unit being arranged to output results representing a number of hits of radioactive particles indicated by the first and second detector circuits, and wherein the control unit further is arranged to ignore every second particle hit that follows a first particle hit, if the first and second particle hits are indicated within a time period which is shorter than a predetermined time period. The apparatus of claim 1, wherein the controller (140) is arranged to ignore the second particle hit, if this (101) is indicated by the same detector circuit as the first particle hit. Apparatus according to claim 1 or 2, wherein the control unit (140) is arranged to ignore the second particle hit, (102) if this is indicated by a detector circuit other than the first particle hit (101). Apparatus according to any one of the preceding claims, wherein the control unit (140) is arranged to ignore even the first particle hit, when the second particle hit is ignored. 10 15 20 25 30 35 515 356 ~ - -..-, = I 21 I; . . . . . ¿2000-12-13 s = \ 1> UBLxc \ DoC \ P \ 414ooo2 sv cvezsdoc BA / L _ ', "i”.': 2 I! 1 'j. G- An apparatus according to any preceding claim, wherein the first and second detector circuits (101, 102) comprise a semiconductor detector (Iin). The apparatus of claim 5, wherein the semiconductor detector (101, 102) is a photodiode (IiQ). The apparatus of claim 5, wherein the semiconductor detector (101, 102) is a solar cell. The apparatus of claim 5, wherein the first and second detector circuits (101, 102) comprise surface barrier sensors (Iin) - An apparatus according to any preceding claim, further comprising: a third detector circuit (103) and a fourth detector circuit (104), which are arranged to generate a respective electrical signal at a respective output (113, 114) for indicating a hit by a radioactive particle; wherein the control unit (140) is operatively connected to the output of the third and fourth detector circuits, the control unit being arranged to emit a result representing a number of radioactive particle hits, indicated by the first, second, third and fourth detector circuits, and wherein the control unit further is arranged to ignore every second particle hit following a first particle hit, if the first and second particle hits are indicated within a time period which is shorter than a predetermined time period. Apparatus according to any one of the preceding claims, wherein the control unit (140) (140) has instructions and said output. includes a programmable processing device with a memory for storing programmable B = \ PUBLIC \ DOC \ P \ 4l40002 EN Review BAII. . : - ~ .- 11. ll. The apparatus of claim 10, wherein the programmable processing device (140) is a PIC-16CXx type microcontroller. Apparatus according to any one of the preceding claims, used for radon gas measurements. Apparatus according to any one of the preceding claims, further comprising means for exchanging the air in the apparatus. Apparatus according to claim 13, wherein the means for exchanging the air in the apparatus comprises a fan or a pump. Method for measuring the level of radioactivity, characterized by the steps of: providing a counter representing a number of detected radioactive particles from a given starting moment; receiving a first detection signal indicating a first radioactive particle; receiving a second detection signal indicating a second radioactive particle; determining a time period between the reception of the first and second detection signals; determine whether the time period is shorter than a predetermined limit value; and if so, ignoring at least one of said first and second detection signals; and otherwise enumerating said counter. Method according to claim 15, used for radon gas measurements.