RU2351953C1 - Method of registering neutrons in presence of gamma radiation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: gamma radiation is constantly checked using neutron detector. Thus measuring of the direct current I_γ arising in the neutron detector under influence of gamma radiation is carried out. The following values are used: for determination and installation of such threshold of discrimination which provides peak efficiency of the detector at the power of exposition dose of gamma radiation which are taking place in current cycle of measuring. For determination of a degree of decrease in the relative efficiency of the detector under influence of gamma radiation and its reduction to greatest possible; for determination of exposition dose power of gamma radiation.

EFFECT: expansion of functionality.

2 cl, 6 dwg

Description

The invention relates to the field of registration of nuclear radiation, and more specifically to methods of measuring the neutron flux density (PPN) in the presence of other types of radiation, mainly hard gamma radiation (GI), and can be used in all industries related to fissile materials determination of other parameters derived from PPN, and ensuring nuclear safety. For example, at facilities for the processing of irradiated nuclear fuel (SNF), which are widely used neutron control devices, to determine the concentration and mass of plutonium; accumulation and distribution of plutonium in equipment, SNF burnout; level of solutions and solid components in special apparatuses where other methods of level measurement cannot be applied; in neutron absorptiometry; in neutron control methods using external radiation sources.

The classical method of registration of PPN, used with minor variations throughout the world, is implemented by devices, the construction of which is illustrated by the functional block diagram in figure 1. A similar block diagram is given, for example, in [Douglas Reilly, Norbert Esslin, Hastings Smith, Jr. and Sarah Krainer. Passive non-destructive analysis of nuclear materials: Per. from English - M.: Publishing House Binom CJSC, 2000, p. 409]. On it, unlike the circuit in figure 1, there is no source of high voltage detector. But without it, the functioning of the detector is impossible. And the fact that such a source is implied is indicated in the same book of the block diagram for switching on gas-filled detectors (p. 42, 378), in which a high-voltage source is shown.

The resistor R, installed between the high voltage source and the detector, is called load and matching. It provides a match between the low output impedance of the high voltage source and the high internal resistance of the detector.

The amplitude of the pulses from the neutron detector is not enough to carry out measurement actions on them, therefore, their amplification is necessary. This function is performed by the preamplifier and amplifier.

The functional purpose of the integrated discriminator is to pass only pulses generated by neutrons and cut off the noise of the equipment, electrical noise and pulses generated by other types of radiation. The pulses at the discriminator output are normalized by amplitude and duration for the convenience of their subsequent registration.

The purpose of the recounting device is to record the number of pulses arriving after the discriminator in a time specified by the timer, and thus determine the neutron count rate (CCH), which then, taking into account the sensitivity of the detector and the discrimination threshold, determine the PPI at the measurement site.

To implement the classical method of detecting neutrons using the described scheme, mandatory procedures are required, such as setting a high voltage value in accordance with the type of detector and its mode of operation (for example, proportional mode or corona discharge mode), providing maximum detector efficiency, setting a discrimination threshold that provides registration of neutron pulses only, setting in the timer of a set of pulses in accordance with a given statistical error .

Gamma radiation is a serious obstacle to the detection of neutrons in measurements of SNF, as well as in monitoring technological parameters for neutron radiation of solutions with SNF, since the ratio of the number of gamma rays to the number of neutrons can reach 10 ¹² , and the sensitivity of the detector to gamma radiation often becomes determining factor.

The effect of GI on SSN is a consequence of two main processes that occur in the detector under the influence of GI.

The first is the formation of spurious pulses of large amplitude, comparable in amplitude to pulses from neutrons, due to multiple gamma-gamma overlays. Therefore, in order to get rid of their registration, a discrimination threshold is set slightly higher than the pulse amplitude from the GI.

The second process is a drop in the gas gain under the action of GI.

If the discrimination threshold is always set no lower than the amplitude of the pulses from the GI, both of these processes lead to a decrease in the SSH, i.e. to a decrease in the relative efficiency η (η is defined as the ratio of CCH at a given discrimination threshold U _D to CCH at a discrimination threshold U _D0 in the absence of gamma radiation). In the first case, this occurs due to an increase in the discrimination threshold, when neutron pulses whose amplitude is lower than the set threshold are not recorded, and in the second, due to a decrease in the amplitude of neutron pulses [V.V. Frolov. Nuclear-physical methods for controlling fissile substances. - M.: Energoatomizdat, 1989, p. 67], [Douglas Reilly, Norbert Esslin, Hastings Smith, Jr. and Sarah Krainer. Passive non-destructive analysis of nuclear materials ... - p. 382-383], [A.I. Dokuchaev, PTE, 1959, No. 6, p. 43].

When implementing the considered method for detecting neutrons in the presence of HI, various techniques are used to protect against it or reduce its negative impact.

The first trick is to protect against the action of GI by shielding the detector with lead.

But it has flaws. For thick (more than 5 cm) lead screens, the decrease in the neutron detection efficiency due to an increase in the distance between the source and counter significantly exceeds the exponential attenuation factor. In addition, the use of thick lead screens is not technologically advanced, and in some tasks of nuclear physical control it contradicts the conditions for their successful solution [V.V. Frolov. Nuclear-physical methods for controlling fissile substances. - M .: Energoatomizdat, 1989, - p. 71]. In the vast majority of cases, a screen of sufficient thickness cannot be placed inside the controlled environment. The detector has to be moved beyond its limits, which, on the one hand, is not always possible, and on the other, leads to loss of sensitivity even when using a neutron reflector. In addition, a violation of selectivity to the object of control is possible. For example, when the detector is moved outside the controlled stage of the extraction apparatus, it begins to register neutron radiation from other nearby stages.

The second method is that instead of neutron detectors with high efficiency but low noise immunity to GI, detectors are installed that are less efficient but more noise immunity to GI.

The disadvantage of this approach is that, although it ensures compliance with nuclear safety by registering excesses of specified standards, it often does not allow the detector to monitor the progress of the process due to the low efficiency of the detector. This happens when, for example, instead of the SNM-18 helium counters, SNM-11 boron counters of comparable length are used, the noise immunity of which to the GI is 100 times higher, but the efficiency is 10 times lower than that of the SNM-18. This technique does not save from possible salvo emissions of gamma emitters. At the very least, constant parallel measurement of the exposure dose rate (ED) of the HI at the location of the neutron detector is required. This not only increases the cost of control, most often it is simply impossible to place neutron and GI detectors together.

Closest to the claimed and adopted as a prototype of the invention is a method for detecting neutrons in the presence of gamma radiation with all the elements of the classical method and the procedures for its implementation, having the feature that, in the conditions of inconstancy of the MED GI, neutron registration is carried out with a constant threshold of discrimination, which should be somewhat higher than the maximum possible amplitude of the pulses from the GOP (U _E> U _Dmax) corresponding to the value DER _max, above which the amplitude pulses from the GOP ceases to increase, allowing It is in all cases to avoid spurious pulses occurring due to gamma-gamma overlays, and thereby provide registration of a neutron pulses [A.I.Dokuchaev, PTE, 1959, №6, p.43].

This method has disadvantages. Firstly, the relative efficiency η decreases, even in cases when DER GI << DER _max . If, in this case, the SSH is still sufficient to determine the controlled parameter, then a decrease in η leads only to an increase in the statistical error, since a smaller number of pulses will be accumulated in a given time. If CCH is initially small, for example, when controlling low concentrations of plutonium, then a decrease in η can lead to a lack of sensitivity with respect to the controlled parameter and its measurement will become impossible. Secondly, work at the maximum threshold of discrimination can lead to violations of the technological process and nuclear safety, the possibility of which follows from the consideration of the following situation: PPI measurements are carried out at high values of DER GI (DER _n ) but, as a rule, do not exceed the limit values of DER GI (DER _{n np} ) established by the specifications for the neutron detector; the relative efficiency η is sufficient for monitoring and its error due to fluctuations in the gas amplification coefficient does not exceed admissible; There is no constant monitoring of the EDM of the GI at the location of the neutron detector. Measurements can be carried out until DER _n does not exceed DER _{n np} . But there are cases of excess. They arise, for example, at the end of a spent nuclear fuel reprocessing campaign during the gradual accumulation of gamma-emitting suspensions at the interface (aqueous and organic) in extraction apparatuses and during the formation of gamma-emitting sediments at their bottom. Volley emissions (over a time ten times less than the usual accumulation time) of gamma-ray emitters are also not excluded. By themselves, their appearance and accumulation do not have a noticeable negative effect on the quality of the technological process and its safety, since aqueous and organic solutions at the points of placement of neutron detectors remain "clean" (without gamma emitters). But they disrupt the operation of neutron devices, which are under technological control, - there is a drop in the gas gain of the counter, causing a decrease in SSN at a constant value of the controlled parameter, for example, the concentration of plutonium. An unacceptably high systematic error appears in the direction of underestimation, which is the higher, the greater the excess of DER _n over DER _{n np} . The operator, trying to "restore" the current value of the parameter, causes the process to deviate from the norm without suspecting it, which, in turn, can lead not only to marriage, but also to a dangerous violation of the conditions ensuring the nuclear safety of the process.

The technical objectives of the proposed method are:

- elimination of the negative consequences caused by the action of GI when registering neutron radiation due to the fact that with each measurement a discrimination threshold is set that corresponds to the current value of the GIE DER, and the degree of decrease in the gas gain under the action of GI is taken into account;

- expanding the scope of application of neutron detectors both by using them for GIE DER values significantly exceeding the maximum allowable ones, as well as for measuring GEM DER by neutron detectors themselves.

The tasks are solved in the following way.

Measure direct current I _γ arising in the neutron detector under the influence of GI. The values of this current during preliminary calibration are compared with the independently measured values of the GEM DER, the degree of deformation of the integral spectra of the detector pulses caused by the GI is determined, and the corresponding dependencies are found, which are discussed in detail below. These dependences make it possible in the future, during ongoing measurements, to select the optimal neutron pulse registration mode based on the results of current measurements I _γ and to bring the neutron count rate N _Iγ obtained under the conditions of GI (I _γ > 0) to the count rate N _I0 , taking place in the absence of GI (I _γ = 0).

Thus, the continuously measured current I _γ becomes the only valid criterion for the quantitative assessment of the negative impact of GI on the neutron detection efficiency, i.e. the degree of its reduction, which occurs due to an increase in the discrimination threshold (for cutting off pulses from gamma-gamma overlays) and a decrease in the gas gain coefficient, which causes deformation of the integral spectrum of neutron pulses. A quantitative assessment allows, in turn, to compensate for the decrease in efficiency by restoring the original integrated spectrum, i.e. to reduce, as mentioned above, the recorded counting rate N _Iγ to the value N _I0 and, as experiments show, to use neutron detectors in the ED of GI, tens of times higher than the maximum permissible values established for each type of detector. In addition, only current measurements I _γ allow full automation of measurements, and also at any point in the neutron control to obtain information about another important technological parameter - the power of the exposure dose of gamma radiation using the dependence I _γ = f (DER GI).

The current I _γ can be determined by measuring, for example, the voltage drop across the resistor R, shown in figure 1.

The authors experimentally established that the meter has the highest sensitivity for current to the GI when the supply voltage U _B is higher than for the proportional mode U _P and less than the ignition voltage of the corona discharge U _K. At this voltage, the initial current in the absence of GI remains of little significance and can either be neglected or taken as a reference point, and the current I _{γ is} much higher (four times in the SNM-16 and SIM-18 counters) than in the proportional mode.

Therefore, in the proposed method, in order to ensure maximum efficiency, both when registering a CCH and current I _{γ, it is} proposed to use a clock measurement procedure. In the first cycle, the counter supply voltage U _B <U _{P is set} , at which the current from the influence of the GI is negligible, and the current measurement circuit was set to zero. In the second step, the counter supply voltage U _P <U _B <U _{K is established} , at which the maximum efficiency of current recording I _{γ is} ensured, and its measurement is performed. According to the dependence of the discrimination threshold on the current I _γ , U _Д = f (I _γ ), found during preliminary calibration, its value is determined and fixed. In the third step, the counter supply voltage is set (U _K - for corona or U _P - for proportional modes), at which neutron radiation should be recorded, CCH is measured and its value is adjusted by the dependence of the relative efficiency on the current I _γ , η found upon preliminary calibration = f (I _γ ). If necessary, additional clock cycles with other operating modes can be introduced, for example, for diagnosing recording equipment by a non-working branch of the integral pulse spectrum.

The mentioned dependences were obtained by the authors for helium counters SNM-16 and SNM-18. Since, in principle, they do not differ from each other (taking into account the differences between the counters themselves), the data are only for SNM-16.

Figure 2 shows a functional block diagram of a classical method for recording PPN with a current measuring circuit I _γ made on the basis of a differential operational amplifier.

Figure 3 shows the dependence I _γ = f (DER GI) obtained at U _P <U _B <U _K. This dependence, after applying it to the form of DER GI = f (I _γ ), allows you to determine the DER GI.

Figure 4 presents the integral spectra of the pulses (discriminatory characteristics) obtained in the mixed fields of neutron and gamma radiation in a stable corona discharge mode. The measurement data in the absence of a neutron source at various GEM EDRs and the corresponding current values I _γ (non-working branches — corona noises and gamma-gamma overlays) are shown by dashed lines. Measurement data with a neutron source at a constant neutron flux density and various values of I _γ (DER GI) are shown by solid lines. The characteristics clearly reflect the mechanism of action of the GI considered above: an increase in noise from gamma-gamma overlays, accompanied by a shift in the non-working branch and causing the need to increase the discrimination threshold, reaching their maximum, and then a decline (reverse shift of the non-working branch) at increased values of I _γ (DER MED) ; deformation of the working branches of the integrated spectra due to a decrease in the gas amplification coefficient.

The discrimination thresholds for each value of I _γ (DER GI) were found at the noise counting rate (non-working branch extrapolated to the discrimination axis) N _Ш = 1 s ^-1 . The degree of shift of the non-working branch (crown noise level and gamma-gamma overlays) is constructed in the form of the dependence К _i = U _Дi / U _Д0 = f (I _γ ) shown in Fig. 5, where U _Дi and U _Д0 are discrimination thresholds for N _W = 1 s ^-1, respectively, for different values of I _γi (DER GI) and in the absence of I _γ (MEDGI).

For all discrimination thresholds U _Di from the integrated spectra of Fig. 4, we found the values of CCH - N _Di and plotted the change in the relative efficiency of the counter under the influence of GI in the form of a dependence and η = N _Di / N ₀ = f (I _γ ). This graph is shown in Fig.6. It was obtained using the dependence К _i = U _Дi / U _Д0 = f (I _γ ) and the clock procedure considered above: setting the current measurement circuit to "zero" at U _B <U _П , in this case U _B = 1300 V; measuring the current I _γi at the meter supply voltage U _P <U _B <U _K , in this case U _B = 1740 V, determining and fixing the discrimination threshold U _Di = K _i · U _D0 ; fixing the meter supply voltage for measuring the CCH, in this case U _K = 2000 V, and determining the CCH. It is important to note that the time of setting “zero” and measuring the current I _γi does not exceed 10 s and practically does not affect the statistical error in determining the SSN, since it takes no more than 10% of the total measurement time, which, depending on the values of the SSN, fluctuates in limits from 100 to 1000 s.

The dependence η = N _Di / N ₀ = f (I _γ ) in FIG. 6 indicates the possibility of using the counter for the DER DER, which significantly exceeds the maximum permissible value. The meter’s operability expands to at least 1000 R / h (20 times higher than the limit value set for the SNM-16 counter - 50 R / h), despite the decrease in efficiency, since this decrease can be compensated. For example, with a GEM EDR equal to 1000 R / h, the current I _γ = 1.2 · 10 ^-6 A (dependence in Fig. 3) and

N _Di = 68 s ^-1 (CCH in FIG. 4). According to the graph in FIG. 6, K _N = N _Di / N ₀ = 0.38, i.e. CCH with a GEM EDR equal to = 1000 R / h is 2.6 lower than the maximum possible (initial) one, which occurs in the absence of gamma radiation. From here we get the initial neutron count rate N ₀ = 2.6 · N _Di = 177 s ^-1 . Thus, the maximum possible detector efficiency is realized at a given neutron flux density. Naturally, in order not to increase the statistical error, it is necessary proportionally, i.e. 2.6 times, increase the measurement time (set the total number of pulses).

Claims (2)

1. A method for detecting neutrons in the presence of other types of radiation, mainly hard gamma radiation, comprising setting a high voltage value in accordance with the type of detector and its operating mode; setting a constant discrimination threshold, which should be slightly higher than the maximum possible amplitude of the pulses from gamma radiation, characterized in that they additionally measure the direct current I _γ arising in the neutron detector under the influence of gamma radiation, the values of which are found from the dependences found during preliminary calibration - current I _γ from the power of the exposure dose of gamma radiation, the working level of discrimination from the current I _γ , the degree of deformation of the integral spectra of the neutron detector pulses from t eye I _γ - is used to determine and set such a discrimination threshold that ensures maximum detector efficiency at the exposure dose rate of gamma radiation that takes place in the current measurement cycle to determine the degree of decrease in the relative efficiency of the detector under the influence of gamma radiation and bring it to the maximum possible to determine the exposure dose rate of gamma radiation. 2. The method according to claim 1, characterized in that the clock measurement procedure is used: in the first clock cycle, set the maximum supply voltage of the meter at which the current I _γ is close to zero, and set the “zero” current measuring circuit I _γ ; in the second clock, the meter supply voltage is set to ensure the maximum efficiency of recording current I _γ , its value is measured and the discrimination threshold is fixed; in the third step, the counter voltage is set, at which neutron radiation should be recorded, and the neutron count rate is measured.

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