AN APPARATUS FOR DETECTING PARTICLES FROM RADIOACTIVE DECAY, AND A METHOD OF MEASURING RADIOACTIVITY CONCENTRATION

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: AN APPARATUS FOR DETECTING PARTICLES FROM RADIOACTIVE DECAY, AND A METHOD OF MEASURING RADIOACTIVITY CONCENTRATION Technical Field The present invention relates to an apparatus for detecting particles from radioactive decay, i.e. an apparatus which is adapted to detect and count a number of particles produced at radioactive decay or disintegration, such as alpha particles, and provide an output indicative of the count. More specifically, the present invention is directed at a radioactive particle detection apparatus suitable for radon gas monitoring. The invention also relates to a method of measuring radioactivity concentration by means of such an apparatus. Description of the Prior Art Throughout this specification, the expression "radioactive particles" refers to particles released at radioactive decay or disintegration, such as alpha particles. Moreover, the expression "radioactive particle detection apparatus" refers to an apparatus for detecting such radioactive particles. A radon gas monitor is a typical example of a radioactive particle detection apparatus according to the above. Throughout the rest of this document, a radon gas monitoring apparatus will serve as an example of a radioactive particle detection apparatus according to the invention. However, it is strongly emphasized that the invention is not limited to merely an apparatus for monitoring radon gas. Radon gas is very difficult to detect, since it lacks color and scent. The radon gas is radioactive, and therefore it- is important to avoid all unnecessary human contact with the gas. Unfortunately, radon is quite common in many houses, where the radon gas may originate both from the ground and from the materials, of which the house is built. Moreover, sometimes the radon gas may emanate from water. If such houses are not properly ventilated, the presence of the radon gas poses a potentially hazardous health problem. Scientists have estimated that 1 000 new cases of lung cancer are discovered each year in Sweden alone due to radon gas exposure. Therefore, it would have been desirable for owners and occupants of houses, that an inexpensive, compact and easy-to-use apparatus was available for measuring the amount of radon present in their homes or at work. Radon is an inert gas and is normally monoatomic, i.e. one gas molecule only consists of one atom (in contrast to for instance nitrogen or oxygen gas, which normally is diatomic and comprises two atoms per molecule) . Radon is generated when radium disintegrates. It appears in several different isotopes, among which 222 Rn is the most common in the ground and indoor air. Furthermore, the ground comprises large amounts of the 220 Rn isotope, which is generally referred to as thoron and appears where the element thorium is present. The 222 Rn isotope is unstable and disintegrates into a radon daughter 218 Po, wherein alpha particles (helium nuclei) are generated. The radioactive alpha particles are capable of travelling about 4.5 cm in air. The radon concentration is measured in Bq/m 3 , i.e. the decay per second and cubic meter. Also the radon daughters, which are formed when radon disintegrates, are radioactive and emanate alpha and beta particles. However, unlike radon, the radon daughters are not gases and will therefore, normally, remain at the location where they were generated. The radon daughters effectively attach to electrically charged particles of dust, water or soot, or to air molecules. The daughters also attach to walls and protruding parts of a space. The alpha and beta radiation generated when radon and radon daughters disintegrate is dangerous to the human body, since energy will be transferred from the alpha and beta particles to the cells of the human body. As is well known per se, such radiation may give rise to abnormal cells, such as cancer cells. In particular, the cells of the lungs are very sensitive to radiation caused by radon gas and radon daughters inside the lungs. Several different types of detectors of radon and radon daughters are previously known. Generally, such detectors are either of a momentary/continuously measuring type or an integrating type. The first type of detectors detects the individual alpha particles momentarily through an electric pulse, which is generated by a sensor when hit by the alpha particle and which is registered by appropriate electronic circuitry. The second type registers the radioactivity during a long period of time, for instance during several days or even several months. Integrating radon gas detectors include for instance track film with or without filter, active carbon spectro- metry, and thermal luminiscence dosimeter. The radioactive particle detection apparatus according to the invention belongs to the momentary/continuous type of detectors. Such detectors constantly provide a momentary or current value of the concentration of radon present in a given environment. To be able to operate properly, the volume of air upon which the measurements are made will have to be exchanged regularly through a filter by way of for instance diffusion or by means of a fan or pump. An ion chamber is perhaps the simplest type of momentary/continuous particle detectors. A high-voltage electric field is generated between two plate electrodes. When alpha particles pass between the plate electrodes, the air will be ionized and a discharge will occur between the electrodes, which may be registered by appropriate electronic circuitry. Ion chambers have a drawback in that they require high-voltage equipment and are often of a large size. Moreover, ion chambers are sensitive to virtually all kinds of ionizing radiation, which makes an ion chamber inappropriate, if only one type of radiation is of interest, such as alpha radiation. Other types of momentary/continuous particle detectors are teflon electrets and Lucas cells. The present invention belongs to a fourth type of particle detectors; the surface barrier detectors. Surface barrier detectors comprise a semiconductor sensor. When an alpha particle passes through a p-n silicon junction, an electric pulse is generated, which is registered and used for keeping record of the number of alpha particles detected. Commercially available radon gas monitors are for instance the ATMOS 12 from Gammadata Matteknik AB, Uppsala, Sweden; the RM3-B radon monitor from Studsvik Instrument AB, Nykόping, Sweden; and the AlphaGUARD PQ 2000/MC50 detector from ScandnoraX AB, Vallentuna, Sweden. These products have in common that they have a large apparatus size, heavy weight and are expensive. Summary of the Invention It is an object of the present invention to provide an apparatus for detecting particles from radioactive decay, which is more simple, inexpensive and compact than the commercially available products referred to above. The object has been achieved by an apparatus, which is provided with at least two separate and parallel semiconductor detector channels, each of which is adapted to detect a radioactive particle and in response generate an electric signal. The outputs from the semiconductor detector channels are independently coupled to a controller, which keeps track of a total number of the particle hits as detected by the independent semiconductor channels. Moreover, in order to reduce the influence from disturbances and consequently provide improved accuracy, the controller is adapted to ignore a second particle hit, as reported by either of the independent semiconductor detector channels, which follows within a predetermined time after a first particle hit. Since "real" radioactive particles essentially always occur one by one, simultaneous particle hits reported from more than one semiconductor channel at the same time, or with only a short time between them, are obviously caused by disturbances and shall therefore be ignored by the controller. The object of the present invention is also achieved by a method of measuring radioactivity concentration, where a counter is provided for representing a number of radioactive particles detected since a given start 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 time period is determined between the reception of the first and second detection signals; it is decided whether this time period is shorter than a predetermined limit; and if so, the first and second detection signals are ignored; or otherwise the counter is incremented. Other objects, advantages and features of the present invention appear from the following detailed disclosure of a preferred embodiment, from the drawings as well as from the appended dependent claims. Brief Description of the Drawings A preferred embodiment of the present invention will now be described in more detail, reference being made to the accompanying drawings, in which FIG 1 is a schematic block diagram of a radioactive particle detection apparatus according to the preferred embodiment , FIG 2 is a schematic circuit diagram of one independent particle detector circuit of the embodiment shown in FIG 1, and FIG 3 is a flowchart diagram of an operating principle of a controller used in the embodiment shown in FIG 1. Detailed Disclosure of a Preferred Embodiment FIG 1 illustrates a radioactive particle detection apparatus of the above-referenced surface barrier detector type. The apparatus comprises four independent particle detector circuits or channels 101-104, each of which comprises a semiconductor detector for detecting an incoming alpha particle and in response providing a voltage pulse on a respective output 111, 112, 113, 114, as will be described in more detail with reference to FIG 2. A comparator circuit 120 is connected to the outputs 111-114 from the particle detector circuits 101-104. In the preferred embodiment, the comparator circuit 120 is a LM339 comparator, which is commercially available from for instance Motorola Inc. and National Semiconductor Inc., USA. The comparator circuit 120 also has a reference input connected to a voltage reference source 122. The comparator circuit is adapted to compare the voltage pulses, which arrive from either of the particle detector circuits 101- 104 when being hit by an alpha particle, to a fixed reference voltage Vref from the voltage source 122. If such an incoming voltage pulse exceeds the fixed reference voltage Vref, the comparator circuit 120 will provide a logical 0 on a corresponding output among a set of outputs 141, which are coupled to a corresponding number of inputs on a microcontroller 140. Otherwise, the outputs 141 are normally constantly high. The microcontroller 140 of the preferred embodiment is a PIC16C64A, which is commercially available from Microchip Technology Inc., USA. However, the microcontroller 140 may be implemented by another commercially available microcontroller of the PIClδCxx family, such as a PIC16C73, or by PIC16F873 (which is also available from Microchip Technology Inc.), or any other microcontroller available on the market. The microcontroller 140 has a second input 142, which is coupled to a 12-position switch 130, by means of which the user of the apparatus may select an integration time at his own desire. In the preferred embodiment, the different positions of the switch 130 represent 1 hour, 4 hours, 8 hours, 12 hours, 1 day, 2 days, 7 days, 14 days, 30 days and 60 days, as well as one position for continuous measurements. When this last position is selected, the apparatus will count all electrical pulses from the detector circuits 101-104 from a given start moment. The value of the radon concentration will then be updated once every minute or whenever a new pulse arrives, wherein the apparatus will present an average of the concentration during the entire period of measurement. The values for entire days will be updated once every hour. The microcontroller 140 also receives two different clock signals (not illustrated in FIG 1) , the first of which sets the processor frequency of the microcontroller 140 md is set to 12 MHz in the preferred embodiment. The second clock signal is supplied from a real-time clock 170 across 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 coupled to a character display 150, through which the user may read the measured radon concentration as well as a marginal of error (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. The microcontroller is also operatively connected, through the IC bus 143, to an 8K serial memory 160, which is commercially from Microchip Technology Inc., under the name 24FC65. The serial memory 160 is used for storing temporary variables, momentary count results, etc., used by the microcontroller 140 during the operation thereof. The memory 160 may also store program instructions to be executed by the microcontroller 140. In the preferred embodiment, however, such program instructions are stored internally in an EPROM program memory of the microcon- troller 140. The microcontroller 140 may also provide an output signal to be used for controlling a fan (not shown in FIG 1) , which is located within the housing of the radioactive particle detection apparatus and has the purpose of exchanging the air inside the apparatus. The fan may either be constantly running or be driven regularly, for instance during one minute after each tenth minute. Alternatively, the air inside the apparatus may be exchanged by means of a pump, preferably provided with a filter. Finally, the microcontroller 140 is coupled, via an input 145, to an RS232 serial interface 190, which may be used for connecting external devices (such as a laptop computer) to the radioactive particle detection apparatus. In this way, measurement results may be communicated electronically from the radioactive particle detection apparatus to an external computer, where the results may be further processed and analyzed. Referring now to FIG 2, a circuit diagram of the first particle detector circuit 101 is illustrated. The second, third and fourth particle detector circuits 102-104 are identical to circuit 101. As previously mentioned, the radioactive particle detection apparatus of the preferred embodiment comprises four separate and independent detector channels, each of which is capable of detecting a radio- active particle (e.g. an alpha particle) and generating an electric voltage pulse in response. The particle detector circuit of FIG 2 is driven by a supply voltage V cc = 5 volt. A first amplifier step 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 (referenced I in in FIG 2) . The semiconductor sensor used in each particle detector circuit 101-104 according to the preferred embodiment is a semiconductor surface barrier detector, such as a photodiode or a solar cell. The semi-conductor detector comprises a p-n junction. When the p-n junction is hit by e.g. 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 electric pulse, as is well known within the field of semiconductor engineering. As shown in FIG 2, the output of the first operational amplifier 200 is fed back, through a resistor R3 connected in parallel with a capacitor C3 , to the negative input of the amplifier 200. The operational amplifier 200 is implemented by a TLC 274 operational amplifier and has a very high input impedance (10 GW) , which is required in order to amplify the extremely weak pulses from the photo- diode I ιn . The first operational amplifier 200 is coupled as a transresistance amplifier (i.e. a current-controlled voltage generator) , where resistor R3 and capacitor C3 form a first-order low-pass filter, which limits the frequency upwardly and eliminates disturbances at high frequencies. A second amplifier step is formed by a second operational amplifier 210, which is a TLC 274 amplifier just like the first operational amplifier 200. A high-pass filter, comprising a resistor R5 and a capacitor Cl, is arranged between the output of the first operational amplifier 200 and the positive input of the second operational amplifier 210. The high-pass filter is necessary, since the dark current from the photodiode normally amounts to approximately 100 pA and would immediately drive the amplifier 210 into saturation, had the filter not been provided in order to eliminate this "DC" current. The high- pass filter also eliminates other disturbances at low frequencies (such as mains hum at 50 Hz and 100 Hz) . Apart from the second amplifier 210, the second step of the circuit shown in FIG 2 comprises a feed-back loop from the output of the amplifier 210 to the negative input thereof, said loop comprising two resistors Rl and R2 , the values of which are chosen to obtain an amplifier gain of 1 000. Moreover, a capacitor C2 is connected in parallel with the resistor Rl and forms yet another low-pass filter, which eliminates high-frequency disturbances. At the output portion of the circuit in FIG 2, a high-pass filter is provided, comprising a capacitor C4 and a resistor R4. This filter eliminates the offset voltage from the second amplifier step. From an overall point of view, the different filters of the amplifier steps provide a second order band-pass filter, which only passes signals within the frequency range, where the signals from the photodiodes I ιn (caused by the incoming alpha particles) occur. As set out above, since the voltage pulses from the photodiodes are extremely weak, the particle detector circuits 101-104 are subjected to various electric and electromagnetic disturbances from the environment. According to the invention, a "real" alpha particle may be recognized and differentiated from a "spurious" alpha particle (i.e. an electric disturbance, which causes a false electric pulse from any of the particle detector circuits 101-104), since a pulse from a real alpha particle will generally always occur "alone". In other words, real pulses will occur one by one at large time intervals, which will be at least several seconds and may very well be as large as an hour or so at low radon concentrations. Disturbances, on the other hand, will generally always give rise to a burst of pulses. Therefore, the particle detection apparatus according to the invention is arranged to determine the time between two subsequent pulses and ignore such pulses, if the time interval between them is shorter than a predetermined limit. Consequently, irrespective of whether two pulses are generated essentially simultaneously on two different particle detector circuits 101-104, or whether two pulses are generated within a short time by the same particle detector circuit, such pulses will be ignored by the microcontroller 140. Referring now to FIG 3, a schematic flow chart diagram is presented and provides an illustration of the operating method of the microcontroller 140 in FIG 1. In a first step 300, a counter is initialized within the microcontroller 140. This counter is used for keeping track of the number of alpha particles detected by any of the particle detector circuits 101-104 since a predetermined start moment. Moreover, in step 300, the memory 160 is initialized, as well as internal registers in the microcontroller 140, the real-time clock 170 and the display 150. Then, a main endless loop 310 is entered. In this main loop, the microcontroller 140 monitors whether an interrupt is received from an external clock, wherein the real-time clock 170 is updated and the display 150 is refreshed in a subsequent step 320. Following termination of step 320, the control is transferred back to the main step 310. In step 310, the microcontroller 140 also monitors whether an interrupt is caused by an input signal from any of the particle detector circuits 101-104. As has been described above, an incoming radioactive particle will be detected by either of the particle detector circuits 101- 104, which generate an electric pulse on the respective output 111-114. The comparator circuit 120 compares the electric pulse to the fixed referenced voltage Vref from the source 122 and generates an output signal on a respective output 141 accordingly. If the microcontroller 140 is alerted, through a logically high signal on one of the inputs 141, that an alpha particle has been detected, the control continues to a step 330, wherein it is determined whether the signal represents a real alpha pulse or is just 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 apparently represent a disturbance, and the microcontroller 140 ignores this disturbance by immediately returning the control to step 310 without incrementing the particle counter. Next, if only one pulse was received from either of the particle detector circuits 101-104, the microcontroller 140 monitors for a subsequent pulse within a predetermined time interval (such as two seconds) . If indeed at least one subsequent pulse is generated within this time interval, the microcontroller 140 determines that the pulses are due to a disturbance and shall be ignored by immediately returning the control to step 310 without incrementing the counter. Otherwise, i.e. if only one single electric pulse has been received from either of the particle detector circuits 101-104 within the prescribed time limit, the microcontroller 140 will increment the number of pulses stored m the counter. The microcontroller 140 will also update appropriate variables etc. in the memory 160 and refresh the display 150 to reflect the incremented particle counter. The display 150 may present an average of the radon concentration as measured since a start moment (if the switch 130 is set to continuous measurements) , or alternatively the radon concentration as measured during the selected time period (as set by the switch 130) . The microcontroller 140 also determines a marginal of error (accuracy) of the determined radon concentration and presents this accuracy on the display 150, as 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 preceding disclosure. On the contrary, the invention shall only be restricted by the definitions in the appended independent patent claims. Other embodiments than the one disclosed herein are equally possible within the scope of the invention. In particular, it is to be understood that fewer than four detector circuits may be used, however not less than two. Also more than four detector circuits may be appropriate in certain circumstances. Moreover, when two particle hits are reported not simultaneously but within the predetermined time period, the controller may be programmed to ignore just one of them instead of both.

Claims: PATENTKRAV 1. An apparatus for detecting particles from radioactive decay, characterized by: a first detector circuit (101) and at least a second detector circuit (102) adapted to generate a respective electric signal on a respective output (ill, 112) for indicating a hit by a radioactive particle; and a controller (140) , which is operatively coupled to the output of the first and second detector circuit, respectively, wherein the controller is adapted to provide a result output indicative of a number of radioactive particles hits as indicated by the first and second detector circuits, and wherein the controller is furthermore adapted to ignore 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. 2. An apparatus according to claim 1, wherein the controller (140) is adapted to ignore the second particle hit when indicated by the same detector circuit (101) as the first particle hit. 3. An apparatus according to claim 1 or 2 , wherein the controller (140) is adapted to ignore the second particle hit when indicated by a different detector circuit (102) than the first particle hit (101) . 4. An apparatus according to any preceding claim, wherein the controller (140) is adapted to ignore also the first particle hit when ignoring the second particle hit. 5. An apparatus according to any preceding claim, wherein the first and second detector circuits (101, 102) comprise a semiconductor detector (I ιn ) . 6. An apparatus according to claim 5, wherein the semiconductor detector (101, 102) is a photodiode (I ιn ) . 7. An apparatus according to claim 5, wherein the semiconductor detector (101, 102) is a solar cell. 8. An apparatus according to claim 5, wherein the first and second detector circuits (101, 102) comprise surface barrier sensors (I ιn ) . 9. An apparatus according to any preceding claims, further comprising: a third detector circuit (103) and a fourth detector circuit (104), which are adapted to generate a respective electric signal on a respective output (113, 114) for indicating a hit by a radioactive particle; wherein the controller (140) is operatively coupled to the output of the third and fourth detector circuit, respectively, wherein the controller is adapted to provide a result output indicative of a number of radioactive particles hits as indicated by the first, second, third and fourth detector circuits, and wherein the controller is furthermore adapted to ignore 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. 10. An apparatus according to any preceding claim, wherein the controller (140) comprises a programmable processing device (140) having a memory for storing program instructions and said result output. 11. An apparatus according to claim 10, wherein the programmable processing device (140) is a PIC-16Cxx micro- controller. 12. An apparatus according to any preceding claim, used for radon gas measurements . 13. An apparatus according to any preceding claim, further comprising means for exchanging air in the apparatus . 14. An apparatus according to claim 13, wherein the means for exchanging air in the apparatus comprises a fan or pump . 15. A method of measuring radioactivity concentration, characterized by the steps of: providing a counter representing a number of radioactive particles detected since a given start moment; receiving a first detection signal indicative of a first radioactive particle ; receiving a second detection signal indicative of a second radioactive particle; determining a time period between the reception of said first and second detection signals; deciding whether the time period is shorter than a predetermined limit; and if so, ignoring at least one of said first and second detection signals; and otherwise, incrementing said counter. 16. A method according to claim 15, used for radon gas measurements .

Inventors: FLOBERG HENRIK, JOENSSON GILBERT, SCHUETZ ILGARS, TEIKARI THOMAS, FLOBERG, HENRIK, JOENSSON, GILBERT, SCHUETZ, ILGARS, TEIKARI, THOMAS
Applicants: FLOBERG HENRIK, JOENSSON GILBERT, RADONANALYS GJ AB, SCHUETZ ILGARS, TEIKARI THOMAS, FLOBERG, HENRIK, JOENSSON, GILBERT, SCHUETZ, ILGARS, TEIKARI, THOMAS
Application year: 2000
Main classification: G01T1
IPC codes: G01T1, G01T7