FR2509870A1 - METHOD AND INSTALLATION FOR QUANTITATIVELY DETECTING RADIOPHOTO LUMINESCENCE, PROPORTIONAL TO DOSES, OF DOSIMETERS WITH SOLID BODY - Google Patents

Abstract

The invention relates to a method and a device for quantitatively detecting the dose-proportional radiophotoluminescence of a solid-state dosimeter into a dose range which corresponds to the natural environmental radiation, the solid state dosimeter being irradiated by means of a radiation generating a luminescence and the luminescent radiation emerging from the solid state dosimeter being detected. Parasitic luminescence which is not generated by ionisation radiation is superimposed on the radiophotoluminescence (RPL). To eliminate this parasitic luminescence, it is provided that the solid state dosimeter is irradiated with a pulsed repetitive excitation radiation and after each excitation pulse a time-resolved detection of single photons of the luminescent radiation is carried out, that a luminescence decay curve is set up from adding together the single photons, and that from this two exponential decays which are associated with the radiophotoluminescence and an interference luminescence are separated by means of a computer and the dose-proportional radiophotoluminescence signal is integrated over time.

Description

 The invention relates to a method and an installation for quantitatively detecting the dose-proportional radiophoto-luminer of a dosimeter with a solid body down to a dose range corresponding to the surrounding natural radiation, the dosimeter with a solid body being irradiated by means of radiation causing luminescence, and the luminescence radiation from the solid-body dosimeter, being detected.

 At the radiophoto-luminescence (RPL) a parasitic luminescence is superposed, which is not produced by ionization radiation 0 In the case of CW excitation, this luminescence simulates a dose signal, called predoses, which limits the use for example metaphosphate glasses in the lower range of doses. The parasitic signals are manifested by centers of fluorescence on the surface of the glass, while the centers of the radiophoto luminescence AgO, or Ag ++, are in the volume of the glass. The excitation and emission spectra of the fluorescence of predoses and luminescence radiophoto, are superimposed and, therefore, cannot be separated from each other by means of special troscopic installations. This parasitic fluorescence can be markedly reduced by appropriate glass compositions. (R. Yokota et al, US patent 3,463,664).

Even in the case of optimal I1 "dosimeter specimens (Y. Nishiwaki et al, 5th Int. Conge. Rad. Prot. Ass.,
Jerusalem (1980)), the predoses are always in the mGy domain and are therefore an order of magnitude 1 times higher than the lower dose domain, interesting for the dosimetry of people, less than about 0.1 mGy, which also corresponds to the surrounding natural monthly radiation.

The measurement values of radiophoto-luminescence are therefore affected by a very significant insecurity.

Excitation by individual laser pulses (P. Killenkamp, Dy Regulla, 3rd Int. Congr. Lumin
Dosimetry, Ris; (1971)), has certainly shown that a predose signal associated with a superficial luminescence decreases ten times faster in time than radiophotooluminescence, and can be distinguished from this in the case of a detection of signals triggered periodically . However, the processing of the analog measurement signal RPL (using an oscillograph) is relatively imprecise. In addition, this measurement signal oscillates about + 30 ffi due to the oscillations of the energy ies laser pulses. Reliable quantitative values are only obtained for doses greater than 0.1 mGy corresponding to approximately 100 mR .

 It was tempting to seek to eliminate the influence of the oscillation of the energy of the laser pulses by using weighting or normalization as proposed by R. Yokota et al, 3rd Int. Congr. Bright. Dosi metry, Risd 1971; G. Gorger et al, Sth Int. Congr. Bright. Dosimetryl, Sao Paulo, 1977. However, the operating methods of these authors do not allow any detection of the signal with periodic triggering, and therefore no compensation of the basic luminescence corresponding to the components described for example by J. Barthe et al. , Health Phys. 29, 219 (1975) and therefore also no quantitative exploitation of radiophoto-luminescence as a function of doses.

 The object of the present invention is therefore to create a method and an installation for making it possible to demonstrate, with periodic triggering, the luminescence, this method making it possible to detect the other contributions with respect to the predoses, and to detect thus quantitatively radiophotooluminescence with very high precision, even in the range of 100 pGy and below.

 To this end, the invention relates to a method for quantitatively detecting radiophoto-luminescence, proportional to the doses of a solid-body dosimeter up to a range of doses corresponding to the surrounding natural radiation, the solid-body dosimeter being irradiated by means of radiation causing luminescence, and the radiation of luminescence from the solid-state dosimeter being detected, a method characterized in that the solid-body dosimeter is irradiated with pulsed and repetitive excitation radiation, while a detection, triggered periodically , individual photons of the luminescence radiation is carried out after each excitation pulse, a luminescence decay curve being established from the totalization of the individual photons, curve from which two exponential decays, associated respectively with radiophoto- luminescence and parasitic luminescence, are separated by means of a calculator r, while the luminescence radiophoto signal proportional to the doses, is calculated by integration as a function of time.

 Other characteristics of the invention make it possible to envisage advantageous complements to the process defined above, as well as facilities for its implementation.

The particular advantages of the invention are therefore based in particular on the following peculiarities of the method: - repetitive measurement method with a large number of measurements
individual (typically 104 to 10). This results in good
measurement statistic and weighting with respect
the fluctuating energy of the laser pulses, - a high signal / noise ratio by counting individual photons
duels.

- periodic digital detection of photons
individual. The summation of these gives the curve of
decrease in fluorescence. The time interval is
chosen between 75 ns and 40rus after excitation by pulses
laser. Thus, very short-lived components (influence of
predoses by surface luminescence and laser dispersion)
and very long-lived components are eliminated.

- Adjustment of the measurement curve using the microcomputer
by superposition of several exponential curves. The ex
exploitation gives exponential decreases which
can be respectively associated with luminescen radiophoto
this and, on the other hand, to the parasitic luminescence of the system
optical. Lumines radiophoto decay times
cence and parasitic luminescences are for example respec
tive of (3 - + 1) 88 and (30 + 10) ryes, and are, contrai
other authors' indications, in the context of the
measurement accuracy increased here, independent of doses
(J. Barthe et al, Health Physics 29, 213-216, 1975).

- the dose proportional RPL signal is completely separated
parasitic luminescence and exploited by integration in
function of time, - there is discrimination in time between the signal RPL and
all parasitic luminescence, that is to say the whole
predoses.

- due to the large number of individual measurements and the ratio
high signal / noise, elimination of the determining factor
the lower limit of measurement and increase in accuracy;
measurement.

The invention will be explained in more detail with reference to an exemplary embodiment shown in the accompanying drawings in which
FIG. 1 schematically represents an installation for implementing the method according to the invention,
- Figures 2 to 4 are diagrams representing the results of measurements obtained.

An installation allowing the implementation of the method according to the invention is shown diagrammatically in FIG. 1. All the measurements. (Figures 2 to 4) are carried out with a metaphosphate 1 dosimeter (type DOS02
Schott und Gen., ERG), which can be placed on an automatic specimen changer 2, operated by the microcomputer control (control conductor 3) which will be described a little later. The dimensions of the dosimeter 1 glasses are 8 x 8 x 4.7 mm3. The glasses of dosimeter 1 are irradiated and so that the centers of fluorescence are located inside them.

To make it possible to obtain individual and multiple photons 4, the dosimeter glass 1 is placed in a light-tight chamber and excited by excitation radiation 6 coming from the pulsed laser 5. (laser with
N2; wavelength 337 nm; pulse duration 10 ns; repetition frequency 40 Hz). It is also possible to use as light source 5, a light bulb with N2 or a similar device. The excitation radiation 6 is oriented on the glass dosimeter 1 by means of a mirror 7, a weakening filter 8 and an interference filter 9. A light receptacle It receives the light 10 passing through the glass dosimeter 1. The emitted radiation 4 (perpendicular to the excitation radiation 6), before it reaches the photomultiplier 14, passes for example through two wideband interference filters 12, 13 (the second filter 13 absorbing in to a large extent the natural fluorescence of the first filter 12), an N2 mirror for the selective reflection of the excitation light 6, and an interference filter for the emitted light 4 or else the intense monochromatisms.

 The circuit device for determining the luminescence radiophoto proportional to the doses, provides a detection of the exciting light 6 ′ by means of the radiation divider 15 and the photodiode 16 as well as a detection of the fluorescence 17 of the individual photons (from the radiation emitted 4) via the photomultiplier 14. The individual excitation and emission signals 18, 19 are applied separately to the start and stop channels 20, 21 of the time converter 22 compared to the pulse levels. This converter 22 generates voltage signals 23 respectively proportional to the delay time 1 between the detected individual photon 17 and the excitation signal 6 '.

 The multi-channel analyzer 24 connected after the converter 22 is used for the assortment and the totalization of the voltage pulses 23 and for the constitution of the fluorescence decay curves, as for example indicated on the Figures 2 and 3, (respectively for doses of 0.3 mGy or 10 mGy), The excitation pulses as well as the pulses of individual photons 18, 19, are applied via an amplifier respectively or 26 and a discriminator 27 or 28 to the converter 22. In the start-up channel 20, however, a pulse-retarding conductor 29 (of for example 75 ns) is still placed, so that only the fluorescent signals delayed 17, be measured, eliminating diffuse excitation light and the influence of short-lived predoses (surface luminescence).

 An integration amplifier 30 and a single-channel converter 31 are connected in parallel to the stop channel 21. It constitutes what is called the tracking channel 32 which means that only the individual photons are recorded by the converter 22. Multiple photon processes which cannot be measured quantitatively significantly are thus eliminated. Their contribution to luminescence is however determined using Poisson's statistical law. The time constant of the installation according to Figure 1, is better than 2 ns.

 For the determination of the luminescence radiophoto and for the control of the installation, a microcomputer 33 is provided for a multi-exponential adaptation, this microcomputer being connected following the multi-channel analyzer 24 and comprises between others, a data output 34 as well as a control 35 of the light pulse source 5.

 Figures 2 and 3 show fluorescence decay curves of two metaphosphate glasses 1 which are irradiated with gamma doses of 0.3 mGy and 10 mGy and which are established by the installation according to Figure 1 as results. intermediaries. In these two decay curves, two parts are distinguished, to which are associated by means of a bi-exponential adaptation (mid-calculator 33) two different decay times # 1 = (3 # and 1) #s and # 2 = (30 # 10 s. The decay time # 0 is to be associated with the surface luminescence, but is eliminated by the pulse delay 29 (see Figure 1).

 The two curves imply that the long-term component (Y which is associated with the parasitic luminescence (for example with optical components), does not depend on the irradiation sources. The short-term component (# 1) increases however monotonic with the absorbed doses (from 0.3 mGy to 10 mGy). This component is associated with the luminescence radiophoto superimposing on the background noise. The surface 36, which can be determined by # 1, # 2, #o thanks to the microcomputer 33 restores the measurement result proportional to the doses.

 In Figure 4 is reported the integrated number of individual photons of luminescence radiophoto, detected in a time interval of 0.5 to 6 s 8 after the laser excitation, depending on the absorbed doses (mGy). The results are corrected according to the background noise. The count rate is visibly proportional to the doses below 0.3 mGy.

An increase in the frequency of the pulses of the excitation source up to 5 × 104 s-1 for the measurement times which can be used in practice, results in an additional reduction in the doses which can be demonstrated quantitatively (as is indicated by extrapolation in Figure 4), and to an even better accuracy of the measurement.

Claims (6)

 R E V EN DI C A T I O NS  1.- Method for quantitatively detecting radiophoto luminescence (RPL), proportional to the doses of a solid-body dosimeter up to a range of doses corresponding to the surrounding natural radiation, the solid-body dosimeter being irradiated by means of provo radiation - as for the luminescence, and the luminescence radiation coming from the solid-body dosimeter being detected, a method characterized in that the solid-body dosimeter (1) is irradiated with pulsed and repetitive excitation radiation (6), while a detection (17), triggered periodically, individual photons of the luminescence radiation (4) is carried out after each excitation pulse (6), a curve of decay of the luminescence being established from the totalization of the individual photons (17), curve from which two exponential decays, respectively associated with luminescence and parasitic luminescence, are separated by means of a calculator r (33), while the dose proportional luminescence radiophoto signal is calculated by integration as a function of time.  2.- Method according to claim 1, characterized in that only the individual photons (17) are offset in time relative to the excitation pulse (6).  3.- Method according to any one of claims 1 and 2, characterized in that the multiple photon processes are in fact eliminated during the measurement, their contribution to the luminescence radiophoto signal being however detected statistically.  4.- Installation for implementing the method according to any one of claims 1 to 3, installation characterized in that it comprises a source of light pulses (5), which irradiates the dosimeter with solid body (1) , a detector (14) which detects individual photons (17), another detector (16) for detecting the excitation radiation (6 '), and a circuit device with a converter of time with respect to the pulse levels (22) and a multi-channel analyzer (24), which detects individual photons (17) and the time of their appearance after excitation (6, 6 ').  5.- Installation according to claim 4, characterized in that it comprises a microcomputer (33) which develops the luminescence radiophoto signal proportional to the doses, and which controls the circuit device as well as the pulse source light (5) and the sample changer (2).  6.- Installation according to claim 4, characterized in that it comprises optical elements (7 to 9, 12, 13, 15) on the path (6, 4) of the excitation radiation and the luminescence radiation, to control- the excitation of the solid-body dosimeter (1) and the selection of the individual photons RPL (17) as well as for dividing (15) the excitation radiation (6, 6 ').