FR2669438A1 - Method and apparatus for measuring radiation composed of particles emitted by a body contaminated by radioactive products - Google Patents

Abstract

The apparatus comprises: (a) a single particle detector (5) (b) a plurality of screens (E0, E1, E2), each capable of absorbing particles with energy less than a predetermined energy level (c) means for successively placing these screens in front of the detector for predetermined successive time intervals, and (d) means (17) for counting the particles detected by the detector (5) during these time intervals and for taking differences of particle counts thus carried out with several such screens and for various corrections, evaluation of the population of particles detected in several energy bands, each bounded by at least one of the said predetermined energy levels. Application to the measurement of the contamination of food products by radioactive products.

Description

 The present invention relates to a method and an apparatus for measuring radiation composed of particles emitted by a body contaminated by radioactive products. More particularly, the present invention aims to provide such a method and to produce such an apparatus allowing separate counts of particles a, p and'b contained in such radiation.

 We know of such devices provided with several specialized or mixed probes: probes a, p, put or X for example. The presence of these multiple probes obviously increase the price and the distribution of these devices.

However, in the event of a large-scale nuclear accident, the protection of the populations concerned can only be ensured if very numerous contamination measures can be carried out, in particular on possibly contaminated food products whose ingestion could be fatal.

 The object of the present invention is therefore to propose a method and to provide a device for measuring a radiation composed of particles emitted by a body contaminated by radioactive products, this method making it possible to produce a simple device and inexpensive to manufacture, so as to be suitable for wide circulation.

 The present invention also aims to provide such a device which is light, compact, with autonomous power supply and direct reading.

 These aims of the invention are achieved with a method for measuring radiation composed of particles emitted by bodies contaminated with radioactive products, characterized in that the procedure is carried out using a same particle detector. , to a succession of particle counts transmitted through a plurality of screens each absorbing particles of energy less than a predetermined energy and an evaluation of the population of particles detected is obtained from differences in particle counts carried out with several such screens in several energy bands each bounded by at least one of said predetermined energy levels.

 The use of a single particle detector, in combination with a suitable set of screens, makes it possible to contain the price of the device according to the invention, suitable for allowing wide dissemination of this device.

According to the invention, for measuring radiation made up of particles of at least one of types a, p and
there are, for three successive time intervals, numbers of particles M0 ,. M1, M2 emanating from the same surface of the body, separated from the detector successively by a space devoid of a screen, provided with a screen active with respect to the particles a, then with an active screen with respect to the particles a and ss respectively.

According to a preferred implementation of the method according to the invention, the particles are counted for three time intervals of the same duration. One then calculates counting rates Na, Np,, N γ in particles a, p, f respectively, such that
Na = (Mo - M1) x Ra NF = (M2 - M1) x
Np = M2 / RX, with Ra, R, Rf, detector efficiency for particles a, p and respectively, and from these rates Na, N, Ns, an evaluation of the populations of particles detected emanating from said surface of the body .

 Thus, according to the invention, three types of particles are separated with only two screens.

 The invention also provides an apparatus for implementing this method. The apparatus comprises a screen-carrying disc mounted in rotation so that the screens carried by the disc can be presented successively in front of an input window of the detector, and means for rotating the disc between various angular positions each corresponding to the installation of one of the screens in front of the window.

 According to one embodiment of this disc, it is pierced with three openings angularly spaced from each other and arranged so as to each be able to occupy a position situated opposite the window of the detector after a suitable rotation of the disc, one of the openings being empty, the two other openings each being closed by a screen designed to absorb the particles a of radiation and the particles a and ss of this same radiation, respectively.

 According to another embodiment of this disc, it has four openings, one of the openings being empty, another being provided with a screen capable of absorbing substantially all the particles has radiation while the last two the openings are lined with screens absorbing, respectively, p particles from two separate energy domains, this to ensure a distinction between "soft" p radiation and "hard" p radiation.

 The apparatus includes counting means associated with a microprocessor programmed to trigger and measure counting time intervals and to calculate, from the counts made, counting rates Na, N, N, in particles a,,! , respectively.

Other characteristics and advantages of the method and of the radiation measurement device according to the invention will appear on reading the description which follows and on examining the appended drawing in which
FIG. 1 is an elevation view of the radiation measuring device according to the present invention, and
FIG. 2 is a functional diagram of electronic detection, counting and calculation means incorporated into the device of FIG. 1.

 Referring to Figure 1 of the accompanying drawing where it appears that the apparatus according to 1 invention comprises a housing 1 from which protrudes a handle 2 which gives a portable character to this apparatus, this handle containing batteries 3 of power supply of the 'apparatus. A stand 4 can possibly ensure the device a stable position when it is placed on a work surface. A particle detector 5 is disposed in a fixed position inside the housing 1, in the vicinity of a window 6 pierced in the wall of the device and possibly fitted with a protective grid 7. The window 6 is centered on a window 8 of the detector 5, the particles to be detected must thus pass through these two windows.

 The particle detector can consist of a Geiger-Müller detector, a zinc sulfide scintillation detector for example, a semiconductor detector or another particle detector of one of the known types.

 According to the invention, a screen-carrying disc 9 is interposed between the windows 6 and 8. The disc is rotatably mounted in the housing around an axis 10 parallel to the common axis 11 of the windows 6 and 8. As a result of the offset of the axes 10 and 11, it is possible to bring by rotation, different parts of the disc between the windows 6 and 8.

According to a first embodiment of the invention, designed to separate counts of particles a, p, emanating from a body contaminated by radioactive products and presented in front of the window 6 of the device, the disc 9 is pierced with three substantially circular openings regularly distributed at 1200 from one another, these openings being substantially the same diameter as that of windows 6 and 8. One (Eo) of the disc openings is empty and the other two are closed by screens E1 and E2 designed to absorb particles a of radiation and particles a and P of this same radiation, respectively. The screens used can conventionally be made of a suitable material such as aluminum, the selective absorption of particles by the two screens being ensured by adjusting the thickness of these screens. If e1 is the thickness of the screen E1 intended to absorb substantially all the particles a of radiation, the thickness e2 of the screen E2 intended to further absorb the particles P will be linked to the thickness e1 for example by the relationship
e2 Z 100 x e1
Means are provided for installing the empty opening Eo or the screens E1 and E2 in front of the detector 5, by rotation of the disc 9.

 According to a first embodiment of these means, these are entirely manual and include a button 15 rotatably mounted on an outer face of the housing 1 opposite to that against which the disc 9 rotates, this button being connected to a shaft 12 of support of the disc 9 by a rod 13 provided at its ends with connections 14, 14 ′ of the flexible joint type, for example. Markers (not shown) allow the button 11 to be placed in one or other of the three positions which bring the empty opening Eo or the screens E1, E2 opposite the detector 5.

 FIG. 1 also shows schematically electronic cards 16, 17, 18 enclosed in the housing 1 of the device.

 As shown in the functional diagram of FIG. 2, the electronic card 16 comprises all the components and circuits necessary for the amplification and the shaping of the signals coming from the detector 5, the signals thus processed being delivered to the card 17 which provides particle counts and calculations prior to display on a display 19 mounted on a card 18, particle count rates a, ss,, as will be seen below. Such amplification and shaping, counting and calculation, and display electronic cards are of conventional structures well known to those skilled in the art. It should however be noted that card 17 must be equipped with a microprocessor associated with counters and memories to carry out the calculations and countings which will be described below, in conjunction with the description of the measurement method according to the invention. Programming the microprocessor based on the information that will be given in the description of the process does not exceed the normal skills of a programmer.

We will now describe the measurement method according to the invention, implemented using the apparatus described above. This involves measuring the contamination of radioactive products in a body such as, for example, a fresh food product. The window 6 of the device according to the invention is applied against a surface of this body. To avoid contamination by contact with the measuring device, the product, a salad for example, is previously enclosed in a transparent bag for the radiation to be detected, for example Mylar (registered trademark). We then successively bring the opening Eo of the disc 9, then the openings filled with screens E1 and
E2, in front of the detector 5, a Geiger counter for example. The microprocessor of the card 17 then proceeds to three successive counts of a predetermined duration for each screen, each counting being executed with a screen or opening E ,. E1, E2 different. The microprocessor then corrects these counts with a "background noise" previously measured with the device. This background noise is due to the ambient particulate radiation, independent of product contamination. Accounts M ,, M1, M2 are then memorized, these corresponding to the numbers of particles detected by the counter 5 while the opening Eo and the screens E1 and E2 were placed in front of the counter, respectively.

The Mo account follows from a count of all the parts emitted by the contaminated body, a, p and, since the opening Eo is empty. The account M1 follows from a count of particles P and t only, because the screen E1 then interposed in front of the counter absorbs the only particles a emitted by the contaminated body. The account M2 follows from a count of particles X only, because the screen E2 then interposed (of thickness 100 times greater than that of E1) stops all the particles a and ss and lets pass substantially all the particles
A well-known "dead" time correction can be made by the microprocessor on the raw counts thus produced by application of the formula
Twink = Mxbrut (1 -t. M1 gross) with
i = O, or 1 or 2 and
T: average duration of a pulse emitted by the counter.

If, for simplicity, we still denote by M0, M1 and
M2 the counts thus corrected and by t, Rp and R γ the (known) yields of the counter with respect to the particles a, ss, γ , respectively, it is understood that one can deduce Na count rates. Nss, N in particles a, ss, respectively, such that
Na = (M0 - M1) / Ra
N, = (M2 - M1) / Rss NX = M2 / R γ
These counting rates are calculated by the microprocessor which also controls their display on the display 19. The measurements M0. Ml, M2 having been established during predetermined time intervals, it is understood that these rates are well representative of the respective intensities of the radiations concerned. The measurements thus made allow, using contamination standards formulated in Bq / cm2 or Bq / m3 for example, to establish whether a food product is fit for human or animal consumption, from the point of view of its possible contamination by radioactive products.

 Thanks to the calculation means incorporated in the apparatus according to the invention, it is possible to further improve the precision of the measurements carried out by correcting it for "rejections". Indeed a screen supposed to stop only the particles a can also stop a sensitive fraction of the particles p, for example. This distorts the counting of particles a which can then be increased by the counting of certain particles p. Preliminary tests carried out with standard radiation sources make it possible to measure this well-known phenomenon and to quantify it by rejection rates stored in the calculation means. The counts made are then corrected using a specific program executed by the microprocessor.

 The counting time fixed by the microprocessor can be a few seconds, 5 s for example. The program for controlling the counting time executed by the microprocessor can be designed to automatically modify this time and increase it for example to 50 s if the number of particles counted during the first 5 seconds is not high enough to be significant, taking into account the accuracy of the measurements.

 As a variant of the measurement method described above, the durations of the three counts to be made could be different. These durations could be modified to compensate for the various yields Ra, R, R of the counter with respect to the three types of particles envisaged.

 As a variant of the measuring device described above, the rotation of the disc can be automated and controlled by the microprocessor.

 To this end, the disc is mounted on the output shaft of an electric motor, the power supply of which is controlled by the microprocessor. Proximity detectors control the cut-off of the motor supply when the disc has reached a desired angular position corresponding to the installation of a particular screen in front of the detector 5. However, such an embodiment requires shielding of the circuits supply and electronic filtering of the signals formed to prevent interference of the measurements.

 It thus appears that, in general, the invention makes it possible to carry out particle counts in predetermined energy bands, the limits of such a band being fixed by a particular choice of screens, each absorbing particles of energy lower than one of the two limits of the predetermined energy band. Different energy bands, adjacent or not, can thus be delimited from screens made, for example, of the same material but of different thicknesses. Particles of various types, a, ss, essentially belonging to different energy bands can thus be counted separately. We can deduce from the counts thus operated the importance of radioactive contamination of any body, this contamination being measured by counts of decay products, particles a, ss, or, of the contaminant (s).

 The number of discriminated energy bands could be increased as necessary, within the same category of particles for example. Thus, it will be possible to split the screen E2 absorbing particles a and p into two screens absorbing respectively, in addition to the particles a, the "soft" p radiation and the "hard" ss radiation belonging to two adjacent energy bands. More generally still, it is possible to discriminate n energy bands by using n screens, by assimilating the empty opening Eo of the disc to a screen of zero thickness.

Claims (14)

 1. A method of measuring radiation composed of particles emitted by bodies contaminated with radioactive products, characterized in that one proceeds, using the same particle detector, to a succession of counts of transmitted particles through a plurality of screens each absorbing particles of energy lower than a predetermined energy level and from the particle counts obtained with several such screens, an evaluation of the population of particles detected in several energy bands is obtained each bounded by at least one of said predetermined energy levels.  2. Method according to claim 1, applied to the measurement of radiation consisting of particles of at least one of types a, p,, characterized in that there are, for three successive time intervals, numbers of particles M, M1, M2 emanating from the same surface of the body separated from the detector successively by a space (Eo) devoid of a screen, provided with a screen (E1) active with respect to the particles a, then d 'a screen (Ez) active with respect to particles a and p, respectively.  3. Method according to claim 2, characterized in that the particles are counted for three time intervals of predetermined durations and in that counting rates Na, Nss, N Nd in particles a, ss, respectively are calculated. , such as  Na = (M0 - M1) / Ra  = = (M (M2 - M1) / Rss  Nss = = M2 / R γ with R α, Rss, R γ, detector yields for particles a, p, respectively, and from these rates Na, Nss, N an evaluation of the populations of particles detected emanating from said surface of the body.  4. Apparatus for implementing the method according to claim 1, characterized in that it comprises (a) a single detector (5) of particles (b) a plurality of screens (E ,. EX, E2) each capable of absorbing particles of energy below a predetermined energy level, (b) means for successively placing these screens in front of the detector for successive predetermined time intervals and (d) means (17) for counting the particles detected by the detector (5) during these time intervals and to derive from differences in particle counts thus operated with several such screens an evaluation of the population of particles detected in several energy bands each bounded by at least one of said particles predetermined energy levels.  5. Apparatus according to claim 4, characterized in that it comprises a screen-carrying disc (8) rotatably mounted so that the screens (E0, E1, E2) carried by the disc can be presented successively in front of a detector input window (5), and means for rotating the disc (8) between various angular positions each corresponding to the installation of one of the screens in front of the detector window (5).  6. Apparatus according to claim 5, designed for measuring radiation consisting of particles of at least one of types a, p and, characterized in that the disc is pierced with at least three openings angularly spaced apart one from the other and arranged so as to each be able to occupy a position situated opposite the detection window, after a suitable rotation of the disc, one of the openings (E,) being empty, the other two openings being closed by screens (E1) and (E2) designed to absorb particles a of radiation and particles a and p of this same radiation, respectively.  7. Apparatus according to claim 6, characterized in that the disc (8) is pierced with four openings, one of the openings being empty, another being provided with a screen capable of absorbing substantially all the particles a of radiation while the last two openings are lined with screens absorbing, respectively, p particles from two distinct energy domains.  8. Apparatus according to any one of claims 4 to 7, characterized in that it comprises means (16) for amplifying and shaping the signals formed by the detector (8) and counting means ( 17) sensitive to these signals to count the numbers of particles detected during a predetermined time interval, behind each of the screens on the disc.  9. Apparatus according to claim 8, characterized in that the counting means (17) comprise a microprocessor programmed to trigger and measure the counting time intervals and to calculate, from the counts made, particle count rates a,, y, respectively.  10. Apparatus according to claim 9, characterized in that the microprocessor is responsible for a program for calculating at least one of the corrections for background noise, dead time, detection efficiency and rejection.  11. Apparatus according to any one of claims 9 and 10, characterized in that it comprises a motor mechanically coupled to the screen-carrying disk (8) and means controlled by the microprocessor for actuating the motor and triggering a sequential rotation of the screen-holder disk, prior to the counts to be made.  12. Apparatus according to any one of claims 4 to 10, characterized in that it comprises means (13, 15) for manually actuating the rotation of the screen-carrying disc (8).  13. Apparatus according to any one of claims 4 to 12, characterized in that the detector is formed by one of the detectors of the assembly consisting of a Geiger-Müller detector, a scintillation particle detector and a detector semiconductor particles.  14. Apparatus according to any one of claims 9 to 13, characterized in that it comprises display means (18) counting rates in particles a, p ,, calculated by the microprocessor.