RU2297647C1 - Method for evaluating characteristics of high-power pulse source hard gamma-rays - Google Patents

Abstract

FIELD: nuclear and radiation physics.

SUBSTANCE: proposed method that can be used to evaluate characteristics of hard gamma-rays, including fluence and spectral composition of bremsstrahlung, from high-power pulse sources depends on multichannel recording of electric charges induced by radiation energy absorbed in similar-shape and equal-size sensing areas of detectors at outputs corresponding to respective detector channels; in the course of recording collimated gamma-ray stream is converted into fast electron and positron streams using working set of converters installed in front of sensing areas of detectors and made of materials having different atomic numbers out of entire periodic-system table, their thickness being chosen so as to ensure that selective sensitivities of recording channels formed in the process in various gamma-quanta energy areas covered entire range of gamma-ray energies; radiation pulses of source under investigation are pre-recorded using in all channels equal-size converters made of material similar to that of converters out of working set.

EFFECT: enhanced precision of evaluating recording-channel relative sensitivity, facilitated measurement procedure, enlarged range of experimental data obtained.

5 cl, 4 dwg

Description

The present invention relates to the field of nuclear and radiation physics and can be used to determine the characteristics of hard gamma radiation, in particular, the fluence and spectral composition of bremsstrahlung quanta of powerful single-shot pulsed sources, as well as to determine the change in these values over time during a radiation pulse.

An analogue of the proposed method is an activation method for measuring the characteristics of hard gamma radiation of powerful pulsed sources (fluence and spectral composition of quanta (spectrum shape) of bremsstrahlung) [1]. It consists in the simultaneous use of several detectors from materials activated by bremsstrahlung, the induced activity of which after pulsed irradiation is determined on a specialized radiometric installation, certified for sensitivity to external photon radiation.

где

- энергетический порог ядерного фотопоглощения для i-ого индикатора. Восстановление на основе этих оценок данных о спектральном составе квантов тормозного излучения из системы интегральных уравнений весьма сомнительно и, скорее всего, невозможно из-за высокой степени неопределенности получающегося результата.The main disadvantage of the activation method is the poor knowledge of the cross sections of photonuclear reactions due to the extremely low probability of their occurrence, which is the reason for the large errors of these cross sections. As a result, this method allows one to obtain only rough estimates of the integral fluences of gamma rays with energies

Where

- energy threshold of nuclear photoabsorption for the i-th indicator. The restoration of the data on the spectral composition of quanta of bremsstrahlung from the system of integral equations based on these estimates is very doubtful and, most likely, impossible due to the high degree of uncertainty of the resulting result.

Another drawback of this method is that the activation cross sections for photonuclear reactions are mainly resonant in the range of quantum energies 10–20 MeV, which limits the possibility of using this method in the energy range below 10 MeV.

Therefore, in the practical use of the analogue method, measurements by the activation method were accompanied by simultaneous measurements of absorbed doses by a completely independent method [1]. The final results were obtained only after a joint analysis of the experimental data of both methods and subsequent iterative computational procedures.

The prototype of the proposed method for determining the characteristics of hard gamma radiation of powerful pulsed sources — fluence and spectral composition of bremsstrahlung quanta is the method of absorbing filters [2], based on multichannel registration of electric charge at the outputs of the detectors, induced by the energy (and proportional to it) of the source bremsstrahlung absorbed in the same shape and size sensitive regions of these detectors located behind the gamma-radiation absorbing filters egg thickness.

The filters used in the prototype method attenuate the gamma radiation passing through them in various ways in different energy ranges of gamma rays. To form the selective sensitivity of the registration channels to various energy regions of the bremsstrahlung spectrum, filters are used from materials having the highest rate of change of the attenuation coefficient of the gamma-ray flux when their energy changes in the entire energy range of the registration.

The main disadvantage of this method is the problem of taking into account the contribution of the background radiation to the recorded signals, since the value of this contribution varies in different recording channels by several dozen times in accordance with the difference in the attenuation level of the primary bremsstrahlung in the filters. For this reason, insurmountable obstacles arise in reliable determination of the relative sensitivity of the registration channels, the large error of which leads, as in the analogue method, to the complete uncertainty of the desired spectral composition of gamma quanta in solving the system of integral equations. And to obtain the gamma-ray fluence values, as in the analogue method, a priori data are needed on the spectral composition of bremsstrahlung.

Another disadvantage of the prototype method is the complexity of the implementation of the measuring device, which requires to use a massive protective structure to reduce the level of background gamma radiation, which does not allow you to quickly make changes to the measurement setup. If it is necessary to make such changes, all disassembly and subsequent assembly of the structure must be repeated using heavy-duty technical devices and with the involvement of specially trained technical personnel. In addition, each such operation should be accompanied by a repetition of all the setup and verification procedures for the measurement paths of registration.

Thus, the impossibility of achieving a technical result on the basis of the analogue method and the prototype method is ultimately associated with large errors in determining the relative sensitivity of the registration channels. This leads to almost complete uncertainty of the desired characteristics of hard gamma radiation, in particular the spectral composition of gamma radiation, and to obtain estimates of the gamma-ray fluence, data on their spectral composition must be used as a priori.

The task is to develop a method for determining the characteristics of hard gamma radiation, in particular, the fluence and spectral composition of bremsstrahlung of high-power pulsed sources, with the ability to accurately determine the relative sensitivity of registration channels and thereby significantly improve the accuracy of determining the desired characteristics of bremsstrahlung. In addition, it is necessary to simplify the implementation of the proposed method so that it becomes possible to quickly change the geometry of measurements without the involvement of special equipment and additional maintenance personnel.

The task is due to the need for extensive research in the field of nuclear and radiation physics, as well as research related to the study of the operating modes of high-current pulsed accelerator installations.

The technical result in the claimed method for measuring the characteristics of hard gamma-ray radiation of high-power pulsed sources (fluence and spectral composition of bremsstrahlung, as well as their change in time during the radiation pulse) consists in the possibility of precision determination of the relative sensitivity of the registration channels, which in turn will provide the desired characteristics of bremsstrahlung with significantly higher accuracy.

In addition, the inventive method simplifies the formulation of the measurement of the desired characteristics through the use of a more compact and mobile measuring device, which will provide the ability to quickly change the geometry of measurements without the involvement of special equipment and additional maintenance personnel.

In addition, the technical result consists in expanding the amount of experimental information obtained, since the availability of reliable data on such characteristics of hard bremsstrahlung gamma radiation of powerful pulsed sources as fluence and its spectral composition, as well as changing them over time, will provide reliable experimental data on the characteristics of accelerated charged particles, leading to the formation of bremsstrahlung in the target device of accelerator installations. This will ensure the determination of such characteristics of accelerated charged particles as the flow, spectral composition and their effective angle of incidence on the brake target, as well as changes in these values over time during the pulse.

The technical result in the claimed method is achieved by the fact that, in contrast to the known method, based on multichannel registration at the outputs of the detectors of an electric charge induced by the energy (and proportional to it) of the initial bremsstrahlung gamma radiation absorbed in the same sensitive shape and size of these detectors, and determining, taking into account the registered values of the characteristics of the studied bremsstrahlung, in the proposed method during the registration process carry out the conversion of a limited flux of bremsstrahlung gamma radiation into fast electron and positron fluxes using a working set of converters installed in front of the sensitive areas of the detectors and made of materials with different atomic numbers from the entire range of the periodic table, as well as having thicknesses selected so that they form the selective sensitivities of the registration channels in various areas of gamma-ray energies uniformly covered the entire range of energies of the bremsstrahlung tions, while producing the pre-registration of the test pulse source radiation using all channels registration converters of identical size, made of the same material identical to the material of one of the working set of converters.

Thus, in the inventive method, multichannel registration of the electric charge is carried out at the outputs of the detectors corresponding to the channels, induced by the energy (and proportional to it) of the secondary electron and positron radiation, as a result of the conversion on the working set of converters of the initial bremsstrahlung absorbed in sensitive regions of the same shape and size detectors located behind the working set converters. The converters are made of materials with different atomic numbers from the entire range of the periodic table. The thicknesses of the working set converters are selected in such a way that the selective sensitivities of the recording channels formed in this case in different areas of gamma-ray energies uniformly cover the entire range of bremsstrahlung energies. In order to accurately determine the relative sensitivity of the registration channels, a preliminary registration of the gamma-ray pulse of the source under study is performed using all the recording channels of the same size converters from the same material from which one of the working set converters is made.

In addition, scintillation detectors or semiconductor detectors can be used in the registration channels.

That is, in contrast to the method of absorbing filters, which records bremsstrahlung gamma radiation transmitted through the filters by means of detectors installed behind them, the proposed method uses the principle of converting the collimated flux of this gamma radiation in the material of the converter into streams of fast electrons and positrons with their subsequent registration , for example, scintillation detectors with identical scintillators in shape and size. At the same time, the selective sensitivities of the registration channels (scintillation detectors + high-frequency cable lines + oscillographic recorders) in the necessary energy regions of the gamma-ray spectrum are formed by a working set of converters from materials with the corresponding atomic numbers (from the entire range of the periodic table) and thicknesses. The specific type of material and converter dimensions for each recording channel are selected in optimization calculations when conducting direct numerical simulation of experiments in real measurement geometry in such a way that the selective sensitivity of the recording channels uniformly covers the entire energy range of the gamma radiation under study. The very procedure for precision determination of the relative sensitivity of the registration channels consists in conducting a preliminary calibration experiment using exactly the same converters from all the registration channels from the same material from which one of the working set converters is made. The ratio of the amplitudes of the pulses recorded in the various two channels will be their relative sensitivity. To reduce the contribution to the detector signals from primary gamma radiation, it is necessary to use thin scintillators having high sensitivity to charged particles and low sensitivity to gamma radiation.

The basis for a significant increase in measurement accuracy is the following physical premises.

(в единицах Кл/МэВ). Конкретное значение чувствительности детектора сначала в единицах Кл·см²/квант определяется в градуировочном эксперименте на рабочем эталоне мощности экспозиционной дозы с радиоизотопным источником гамма-квантов, например, ⁶⁰Со со средней энергией Е_γ=1.25 МэВ и известным хранителю эталона значением эквивалентной плотности потока этих гамма-квантов, создающей такую же мощность экспозиционной дозы на измерительной позиции. Затем расчетным образом, например, с использованием предварительно протестированных монте-карловских программ [3] определяется чувствительность i-ого детектора к поглощенной в сцинтилляторе энергии в единицах МэВ·см²/квант. С использованием этих двух величин переход к единицам Кл/МэВ легко осуществим. Тогда чувствительность канала с i-ым конвертором из рабочего набора к гамма-квантам с произвольной энергией Е_γ составитLet us denote the sensitivity of the ith recording channel (out of the total number n) to the energy of fast electrons (positrons) absorbed in the sensitive region of the detector, for example, as

(in units of C / MeV). The specific value of the detector sensitivity, first in units of C · cm ² / quantum, is determined in a calibration experiment on the working standard of the exposure dose rate with a radioisotope source of gamma quanta, for example, ⁶⁰ Co with an average energy E _γ = 1.25 MeV and the equivalent flux density known to the reference keeper of these gamma rays, creating the same exposure dose rate at the measurement position. Then, in a calculated way, for example, using previously tested Monte Carlo programs [3], the sensitivity of the ith detector to the energy absorbed in the scintillator is determined in units of MeV · cm ² / quantum. Using these two quantities, the transition to C / MeV units is easily feasible. Then the sensitivity of the channel with the i-th converter from the working set to gamma rays with arbitrary energy E _γ will be

where S is the area of the plane face of the converter (cm ² ), and ε (E _γ ) _i (MeV / quantum) is the energy of fast electrons and positrons absorbed in the volume of the scintillator when a single gamma-ray stream is incident on the i-th converter surface with energy E _γ .

When gamma rays are incident on all detectors, the differential energy dependence of the fluence of which is described by an expression of the form

, измеряемый в единицах квант/см², a f(E_γ) - функция распределения гамма-квантов по энергии, имеющая размерность МэВ^-1, нормированная на единицу в соответствии с условиемwhere P is the total fluence of quanta in the energy range from zero to

measured in units of quantum / cm ² , af (E _γ ) is the energy distribution function of gamma quanta, having the dimension MeV ^-1 , normalized to unity in accordance with the condition

and determining the shape of the spectrum of gamma rays, the amount of charge (in C) recorded through the channel with the i-th converter will be

where is the value

represents, by definition, the average energy of fast electrons and positrons absorbed in the scintillator volume when a single gamma-ray stream with a function f (E _γ ) of their energy distribution is incident on the i-th converter surface of the i-th converter.

Then, the ratios of the measured electric charges q _i / q _m , where m is the fixed number of the converter and the corresponding registration channel, relative to which the sensitivity of all other registration channels will be determined, are determined by the following expressions with one unknown function f (E _γ )

whence follows

The choice of the material of the converters of all the registration channels (i.e., the material of the converter of the fixed registration channel with the number m) for the preliminary calibration experiment is determined only by the manufacturability of the material from which it is easiest to make completely identical converters in shape and size.

Следовательно, соответствие между снимаемыми с детекторов и регистрируемыми на осциллографах зарядами также выполняется с точностью до ±2%.The magnitude of the electric charge at the detector output fully corresponds to the amplitude of the recorded signal only in the absence of reflections at the ends of the cable lines. With a passport tolerance for wave impedance, for example, of a coaxial cable PK75-9-13 ρ = (75 ± 3) Ohm and using load resistances of a strictly fixed value R _Nag = 75 Ohm in all recording channels, the reflection coefficients of the signals at the inputs of the oscilloscopes can vary

Consequently, the correspondence between the charges recorded from the detectors and the charges recorded on the oscilloscopes is also fulfilled with an accuracy of ± 2%.

в (6) является так называемый коэффициент режима, используемый для перехода от абсолютной чувствительности детектора, определенной в градуировочных измерениях, например, на эталонном источнике ⁶⁰Со при напряжении U_град=100 В, к чувствительности при рабочем напряжении на детекторах U_раб=1.8 кВ. Процедура определения коэффициентов режима, например, для фотоэлементов СДФ7 надежно отработана, а погрешность измерений этих коэффициентов не превышает ±2.5%.Another source of error in determining relationships

in (6) is the so-called mode coefficient used to switch from the absolute sensitivity of the detector, determined in calibration measurements, for example, at a reference source of ⁶⁰ Co at a voltage of U _deg = 100 V, to sensitivity at a working voltage on detectors U of _work = 1.8 kV . The procedure for determining the mode coefficients, for example, for SDF7 photocells, has been reliably worked out, and the measurement error of these coefficients does not exceed ± 2.5%.

Thus, in any case, the error in determining the right-hand side of equations (6) cannot be made less than ± 4.5%. It is this circumstance that is the main limiter of the accuracy of determining the distribution function f (E _γ ) from equations (6), and the claimed method in this form does not have a decisive advantage over the analogue method and the prototype method.

At the same time, in contrast to the analogue method and the prototype method, the claimed method initially laid down the possibility of drastically improving the situation by conducting a calibration experiment using completely identical converters in the same shape and size from the same material from which it is made one of the working set converters, for example, converter number m. In this case, ε (Е _γ ) _i = ε (Е _γ ) _m for all i, and from relations (6) it follows

(6), получимSubstituting the obtained values for

(6), we obtain

It can be seen from relations (8) that, as a result of a preliminary calibration experiment, it is possible to “construct” channels for recording exactly the same sensitivity (even the systematic errors in determining the sensitivity of the oscilloscope recorders used in the measuring channels are completely compensated). We rewrite relations (8) in their final form

в заявляемом способе могут быть получены с высокой точностью (≤0.5%), и в этом смысле заявляемый способ в отличие от способа-аналога и способа-прототипа по своим возможностям полностью соответствует дифференциальным методам измерений.The errors in determining the distribution function f (E _γ ) from the system of equations (9) are directly determined by the errors in the dependences ε (E _γ ) _i obtained by the calculation method, for example, according to Monte Carlo programs, as well as implicitly, by the errors in the measurement of the right-hand side of equations (9) . Practice shows that the error in determining the distribution function f (E _γ ) for various methods of solving the system of integral equations (9) worsens by about an order of magnitude compared with the errors in the right-hand sides of these equations. Experimental error in determining the ratio

in the claimed method can be obtained with high accuracy (≤0.5%), and in this sense, the claimed method, in contrast to the analogue method and the prototype method, fully complies with the differential measurement methods in its capabilities.

от моноэнергетических электронов с фиксированными значениями энергии

, если измерения проводятся на электронном ускорителе. Указанные функции распределения

также определяются расчетным способом по монте-карловским программам.The definition of the function f (E _γ ) from (9) is substantially facilitated by the fact that its search can be carried out by selecting from a narrow class of functions, namely, distribution functions

from monoenergetic electrons with fixed energy values

if the measurements are carried out on an electronic accelerator. The specified distribution functions

also determined by calculation using Monte Carlo programs.

не совпадает с левыми частями системы уравнений (9) ни при каком конкретном значении энергии электронов

, то искомое решение этой системы уравнений определяется в виде суперпозиции функций

If the experimentally obtained set of values

does not coincide with the left sides of the system of equations (9) for any specific value of the electron energy

, then the desired solution to this system of equations is determined as a superposition of functions

и подбирается таким образом, чтобы полученная при этом из уравнения (10) функция распределения f(E_γ) удовлетворяла системе уравнений (9) лучше всех других функций. Полученный при этом набор коэффициентов a_j будет представлять собой функцию распределения электронов по энергии. Полный флюенс тормозных гамма-квантов П легко определяется затем из соотношения (4).where the set of coefficients a _j satisfies the normalization condition

and is selected so that the distribution function f (E _γ ) obtained from equation (10) satisfies the system of equations (9) better than all other functions. The resulting set of coefficients a _j will be a function of the electron energy distribution. The total fluence of inhibitory gamma rays P is then easily determined from relation (4).

In the prototype method, the error in determining the right-hand side of equations (9) increases to ~ 10% due to, as mentioned above, additional difficulties in accounting for the background, which has a different and significantly different level for different recording channels. Therefore, the error of the desired distribution function f (E _γ ) in the known method becomes close to 100%, which indicates its complete uncertainty. And to assess the magnitude of the total fluence P from equation (4) in the known method requires the presence of a priori information about this distribution function f (E _γ ).

для ряда фиксированных углов θ_е, падения электронов на поверхность тормозной мишени ускорителя. При этом описанная процедура подбора весовых коэффициентов a_j будет использоваться без изменения для более широкого набора функций

, полученных для различных углов падения электронов θ_е. Несмотря на эти дополнительные сложности, указанная величина погрешности функции f(E_γ) в полной мере соответствует представлениям о современном уровне ядерно-физического эксперимента и является заведомо достаточной для решения подавляющего числа задач, возникающих при проведении исследований на ускорительных установках.In contrast to the known method, in which the detectors are placed behind filters of various thicknesses, in the inventive method, the detectors are compactly placed in a direct flow of collimated bremsstrahlung with obviously identical background conditions, which does not create difficulties in taking into account the contribution of background radiation to the detector signals by performing an additional measurement with a closed collimator. Therefore, the error in determining the desired distribution function f (E _γ ) can be made approximately an order of magnitude smaller, i.e. ± 10%, in strict accordance with the fact that the error of the right-hand side of equations (9) in the claimed method during the procedure of "self-calibration" becomes an order of magnitude smaller. In practice, most likely, this error will be somewhat larger, including due to the fact that in real accelerator setups the spectral composition of bremsstrahlung f (E _γ ) is determined not only by the electron energy, but also by the angle of their incidence on the target. The angular distributions of incident electrons are not well understood, which will require the calculation of distribution functions

for a number of fixed angles θ _e , the incidence of electrons on the surface of the brake target of the accelerator. Moreover, the described procedure for the selection of weights a _j will be used without change for a wider range of functions

obtained for different angles of incidence of electrons θ _e . Despite these additional difficulties, the indicated error value of the function f (E _γ ) fully corresponds to the notions of the modern level of the nuclear physical experiment and is obviously sufficient to solve the overwhelming number of problems arising from research at accelerator facilities.

That is, based on an analysis of the experimental errors of the known and claimed methods, it is shown that the proposed approach is justified from the point of view of increasing the accuracy of determining the desired characteristics.

The implementation of the proposed method boils down to a careful selection of the working set of converters and their use in accordance with the stated procedure.

определяется подбором из набора функций

(получаемых расчетным способом по монте-карловским программам) таким образом, чтобы все уравнения системы (9) одновременно обратились в тождество в пределах экспериментальных погрешностей величин

. Флюенс гамма-квантов П на измерительной позиции определяется затем из любого из уравнений системы (4), в которых величина П является единственной неизвестной величиной.Thus, the spectral composition of hard bremsstrahlung (function f (E _γ )) in the inventive method from the measured charge ratios

determined by selection from a set of functions

(obtained by calculation using Monte Carlo programs) in such a way that all equations of system (9) simultaneously become identical within the experimental errors of

. The fluence of gamma rays P at the measuring position is then determined from any of the equations of system (4), in which the value of P is the only unknown quantity.

The simplification of the measurement of the characteristics of the hard bremsstrahlung, in contrast to the known method in the present method is based on the following premises.

As already mentioned above, in the known method for measuring the characteristics of hard bremsstrahlung, the detectors are installed behind filters of different thicknesses, which creates a very different level of background radiation in different registration channels. In order to minimize the level of this background radiation at the detector locations, the experimenters are forced to increase the mass of the protective structure around the detectors, trying to weaken the strongly penetrating scattered gamma radiation to the maximum extent. In this case, the total mass of the measuring device and the protective structure reaches several tons. When taking measurements, the assembly of such a structure requires the use of special heavy equipment and additional specially trained technical personnel. If it is necessary to make any changes to the measurement setup, for example, when changing the gamma-radiation registration angle, disassembling and reassembling these massive structures requires a lot of labor. In addition, in this case, it is necessary to repeat all the expensive work on the assembly and adjustment of registration paths, which can only be performed by highly qualified specialists.

In the inventive method, the detectors in the measuring device are placed in such a way that they are in a direct flow of inhibitory gamma radiation with obviously the same background conditions. The contribution of background radiation to the detector signals is easily determined by performing an additional measurement with the dimming collimator of the measuring device. This feature of the proposed method allows to reduce the mass of protective material by approximately an order of magnitude and to develop a mobile measuring device, after which it can be manually moved around the measuring hall and adjusted manually by personnel directly involved in the measurements. At the same time, unlike the known method, it is not necessary to carry out repeated work on setting up the measuring complex, since all registration paths remain intact.

The expansion of the informative capabilities of the proposed method is directly associated with a dramatic increase in the accuracy of measuring the characteristics of bremsstrahlung and the presence of an unambiguous relationship between the characteristics of beams of accelerated charged particles and the measured characteristics of the bremsstrahlung formed by them in the target device. An unambiguous relationship between the spectral distributions of bremsstrahlung and the spectral as well as angular distributions of accelerated electrons is expressed by equation (10). The relationship between the flux of electrons incident on the target and the fluence P of inhibitory gamma quanta at the installation site of the detectors can be easily obtained by calculation using the same Monte Carlo programs in the form of the coefficient of conversion of electron radiation into inhibitory

where φ _e is the electron flux on the target surface.

Thus, in contrast to the known method, the inventive method for determining the characteristics of radiation will provide a qualitatively new opportunity to determine not only the total fluence P of bremsstrahlung at the measurement position and its spectral composition (function f (E _γ )), but also the electron flux (in accordance with the expression (11)), their spectral composition (i.e., the set of coefficients a _j in expression (10)) and the effective angle of their incidence θ _е on the accelerator's brake target.

, где l меняется от 1 до k, для i-ого канала регистрации и l-ого номера временного интервала. С использованием этих парциальных зарядов для каждого временного интервала l отдельно повторяются все процедуры определения характеристик тормозного гамма-излучения, а затем и характеристик заряженных частиц аналогично описанным выше, что обеспечивает получение искомых временных зависимостей.In addition, in the inventive method, a rigid time reference is made (with an error of no more than 0.1 ns) of the signals received through different channels of registration, which will allow to determine the changes in the characteristics of bremsstrahlung gamma radiation and electronic radiation in time during the pulse. For this, the entire recording time interval T equal to the duration of the detected current pulse from the detector is divided into a fixed number k time intervals, for example, equal to the duration Δt = T / k. By integrating the current signal within each such time interval, the fraction of charge is determined

, where l varies from 1 to k, for the i-th registration channel and l-th number of the time interval. Using these partial charges for each time interval l, all the procedures for determining the characteristics of bremsstrahlung gamma radiation and then the characteristics of charged particles are repeated separately, similarly to those described above, which ensures obtaining the desired time dependences.

In addition, in the inventive method in all registration channels can be used and scintillation detectors, and semiconductor detectors. These two types of detectors are the most studied and affordable, which will ensure the absence of any fundamental difficulties in the practical implementation of the proposed method.

Figure 1 schematically shows a multi-channel measuring device (DUT) for determining the differential energy dependence of the output of the bremsstrahlung of powerful pulsed sources, which implements the inventive method using scintillation detectors. In figure 1, the positions denote: 1 - target device of the electron accelerator, 2 - polyethylene filter, 3 - collimating compartment of the IUT, 4 - protective compartment of the IU, 5 - detector compartment of the IU, 6 - photosensitive element of the scintillation detector, 7 - thin-walled air duct with reflective inner surface, 8 - converter, 9 - polystyrene-based plastic scintillator, 10 - detector connector, 11 - transport platform.

уравнений (9) от энергии падающих электронов на тормозную мишень ускорителя, рассчитанные с использованием монте-карловских программ для случая, когда в качестве номера m выбран номер канала регистрации с железным конвертором.Figure 2 shows the dependence of the left parts

equations (9) on the energy of incident electrons on the accelerator's brake target, calculated using Monte Carlo programs for the case when the registration channel number with an iron converter is selected as the number m.

(см. уравнение 9) ±4.5%.Figure 3 shows the same relationships, each of which is given in the form of two smooth curves, the distance between which corresponds to the magnitude of the experimental errors of the measured charges

(see equation 9) ± 4.5%.

, (см. уравнение 9) ±9%.Figure 4 shows the same relationships, each of which is given in the form of two smooth curves, the distance between which corresponds to the magnitude of the experimental errors of the measured charges

, (see equation 9) ± 9%.

The bremsstrahlung is generated in the target device of the accelerator (Fig. 1) (pos. 1) under the influence of a pulsed beam of accelerated electrons lasting several tens of nanoseconds and with a maximum boundary energy of up to 40 MeV and falls on a measuring device consisting of several compartments.

A 4 cm thick polyethylene filter (item 2) plays the role of an additional moderator and scatterer of fast electrons passing through the accelerator target and flying towards the measuring device, preventing their main fraction from reaching the detectors and thereby reducing the contribution from them to the detector signals to negligible values ~ 0.1%.

The collimating compartment (pos. 3) forms a strict parallelism of the beam of bremsstrahlung gamma radiation necessary to ensure the conditions of normal incidence of gamma rays on the flat surface of the converters (pos. 8).

The protective compartment (pos. 4) is designed to protect the detectors from lateral incident on them of background gamma radiation scattered in the air and the walls of the measuring room.

The detector compartment (pos. 5) is designed to accommodate a set of scintillation detectors in it. In this case, the photocell housing (pos. 6) of each detector is mounted behind the protective compartment (pos. 4) and is thus derived from the direct flow of gamma rays. Thus, in the direct flow of inhibitory gamma radiation, only scintillators (pos. 9) appear together with converters mounted on them (pos. 8). To improve the collection of photons of the scintillator luminescent radiation on the photocathode of a sensitive photocell, a lightweight construction (item 7) of thin Plexiglas (~ 1 mm thick) is used, coated inside with reflective material and outside with opaque black paper.

The transport platform (pos. 11) is designed to install, move and adjust the measuring device. The mass of the measuring device does not exceed 500 kg, which makes it quite accessible for manual movement using a transport platform.

The optimal number of detectors, providing sufficient accuracy, is five to six. All of them are placed in a circle in the detector compartment similarly to the detector (item 6), shown in figure 1. In accordance with this, the developed measuring device uses five independent recording channels, consisting of scintillation detectors based on SDF7 high-current photocells and plastic polystyrene scintillators, PK75-9-13 cable coaxial lines, and TDS 3052 oscilloscope recorders.

All detectors use identical thin cylindrical scintillators with a diameter of 20 mm and a thickness of 4 mm. As materials for the converters of the working set, materials with atomic numbers uniformly covering the entire range of the periodic table are used. Converters having a cylindrical shape and fixed with flat faces close to the scintillators have diameters equal to the diameter of the scintillators and various thicknesses, the specific values of which are selected in optimization calculations in such a way as to form the required selective sensitivity of the registration channels in the energy ranges uniformly covering the entire energy spectrum bremsstrahlung. In this case, converters are used from polyethylene 4 mm thick, aluminum 4 mm thick, iron 2 mm thick, cadmium 3 mm thick and lead 4 mm thick. At the same time, to accurately determine the relative sensitivity of the registration channels, the radiation pulse of the source under study is pre-registered using the same converters made of iron in all registration channels as the most technologically advanced material from which it is easiest to make completely identical converters in size.

на погрешности определения функции распределения f(E_γ) из системы уравнений (9) и сравнительные возможности способа-прототипа и заявляемого способа демонстрируют результаты прямого численного моделирования.The degree of influence of the error in determining the ratio

the errors of determining the distribution function f (E _γ ) from the system of equations (9) and the comparative capabilities of the prototype method and the proposed method demonstrate the results of direct numerical simulation.

To this end, using the Monte Carlo programs for all five detectors of the developed measuring device, the left-hand sides of equations (9) were calculated, the dependences of which on the energy of the electron accelerator incident on the target for all five recording channels are shown in Fig. 2.

The calculations were carried out in the real geometry of the placement of the detectors in the detector compartment of the developed measuring device, taking into account the contribution of scattered gamma quanta and secondary electrons and positrons to the detector signals generated on various structural parts of the measuring device. Figure 2 clearly shows the energy ranges in which each of the recording channels has the highest sensitivity compared to all others. So, for example, in the range of energies of accelerated electrons up to ~ 5 MeV, the highest sensitivity is shown by the registration channel with a polyethylene converter, in the energy range from ~ 5 to ~ 10 - the registration channel with an aluminum converter, etc.

, характерных для рассматриваемых способов измерений.We consider three cases of possible error values

characteristic for the considered measurement methods.

не превышает 0.5%, как в заявляемом способе измерений с использованием процедуры «самокалибровки» каналов регистрации, то искомая функция распределения гамма-квантов по энергии f(E_γ) будет подобрана с такой точностью, что определенные на ее основе зависимости левых частей уравнений (9) будут отличаться от приведенных на фиг.2 зависимостей на величину, не большую, чем фактически размеры изображенных на кривых специальных символов, а все уравнения системы (9) превращаются в тождество с такой же погрешностью. Именно в этом случае появляется качественно новая возможность определения не только характеристик тормозного излучения, но также и характеристик электронов, падающих на тормозную мишень ускорителя. При этом дополнительное увеличение точности получаемых результатов может быть достигнуто за счет увеличения количества детекторов и, соответственно, числа каналов регистрации. Как уже указывалось, в разработанном измерительном устройстве используется пять детекторов как некоторый компромисс между достаточной для решения многих практических задач точностью измерений и простотой реализации заявляемого способа, а также приемлемым уровнем сложности обработки их результатов.If the charge measurement error

does not exceed 0.5%, as in the claimed measurement method using the “self-calibration” of the registration channels, the desired energy distribution function of gamma rays f (E _γ ) will be selected with such accuracy that the dependences of the left parts of equations determined on its basis (9 ) will differ from the dependences shown in Fig. 2 by an amount not greater than the actual sizes of the special characters shown on the curves, and all equations of system (9) turn into an identity with the same error. It is in this case that a qualitatively new opportunity arises for determining not only the characteristics of bremsstrahlung, but also the characteristics of electrons incident on the braking target of the accelerator. Moreover, an additional increase in the accuracy of the results obtained can be achieved by increasing the number of detectors and, accordingly, the number of recording channels. As already indicated, the developed measuring device uses five detectors as a compromise between the accuracy of measurements sufficient to solve many practical problems and the ease of implementation of the proposed method, as well as an acceptable level of complexity of processing their results.

не может быть сделана лучше 4.5%. Эта ситуация показана на фиг.3 в виде коридоров ошибок шириной±4.5% для каждой зависимости. Очевидно, в этом случае речь может идти лишь о более или менее надежном определении среднего значения энергии ускоренных электронов

. Однако полученной информации о спектральном составе тормозного излучения

вполне достаточно для надежного определения флюенса гамма-квантов П на измерительной позиции (см. соотношение (4)).If measurements by the claimed method are carried out without the "self-calibration" procedure, the error of charge measurements

cannot be made better than 4.5%. This situation is shown in figure 3 in the form of error corridors with a width of ± 4.5% for each dependence. Obviously, in this case we can only talk about a more or less reliable determination of the average energy of accelerated electrons

. However, the information obtained on the spectral composition of bremsstrahlung

it is enough for reliable determination of the gamma-ray fluence P at the measuring position (see relation (4)).

достигают величины ±9% (с доверительной вероятностью 0.95). Видно, что в этом случае вообще не может идти речь об измерении функции распределения тормозных квантов (функции f(E_γ)), а для определения флюенса гамма-квантов П необходимо наличие, как и в способе-прототипе, априорной информации о форме спектра гамма-излучения. Либо, как в способе-аналоге, необходимо привлекать результаты измерений другими методами и использовать сложные математические процедуры обработки и восстановления результатов.Figure 4 shows the last third situation when the error

reach a value of ± 9% (with a confidence level of 0.95). It can be seen that in this case there can be no talk of measuring the distribution function of inhibitory quanta (functions f (E _γ )), and to determine the fluence of gamma quanta P, it is necessary to have, as in the prototype method, a priori information about the shape of the gamma spectrum -radiation. Or, as in the analogue method, it is necessary to involve the measurement results by other methods and use complex mathematical procedures for processing and restoring the results.

Thus, the achievement of the technical result in the claimed method is due to a radical increase in the accuracy of determining the relative sensitivity of the registration channels (which is impossible both in the analogue method and in the prototype method) by performing the procedure of their "self-calibration" in a preliminary experiment using a working set converters, which allows to obtain the desired radiation characteristics, namely the fluence and spectral composition of bremsstrahlung with a significantly higher accuracy Tew. At the same time, a simplification of the measurement method is provided and its informative capabilities are expanded.

Information sources

1. Yu.M. Odintsov, A. A. Kryzhanovsky, G. N. Maslov and others. Determination of the bremsstrahlung spectrum of the LIU-30 accelerator by the activation method. VANT. Ser .: Physics of nuclear reactors. Issue 3-4, pp. 35-42, 1999.

2. A.I. Veretennikov, V. M. Gorbachev, B. A. Predein. Research methods for pulsed radiation. Moscow, Energoatomizdat, pp. 108-110, 1985.

3. Shmarov A.E. The VISUAL TRIADA program is a numerical simulation tool for the joint transfer of gamma rays, electrons, and positrons in matter by the Monte Carlo method. // VI Interdisciplinary conference on radiation resistance, October 14-20, 2002, Sarov.

Claims (2)

1. A method for determining the characteristics of hard gamma radiation of high-power pulsed sources, based on multichannel registration at the outputs corresponding to the channels of electric charge detectors induced by radiation energy absorbed in the same sensitive shape and sizes of the sensitive areas of the detectors, and determining, taking into account the registered characteristics of the investigated gamma -radiation, characterized in that the registration process converts the collimated gamma radiation stream into streams fast electrons and positrons using a working set of converters installed in front of the sensitive areas of the detectors and made of materials with different atomic numbers from the entire range of the periodic table, and also having thicknesses selected so that selective sensing channels of registration channels are formed in different energy ranges gamma rays uniformly covered the entire range of gamma radiation energies, while the pulse is pre-registered from investigated source using the teachings of all recording channels of identical size converters made of the same material identical to the material of one of the working set of converters. 2. The method according to claim 1, characterized in that scintillation or semiconductor detectors are used in the registration channels.

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