THE FORMATION OF NEUTRON FIELDS FROM RADIONUCLIDE NEUTRON SOURCES FOR THE PURPOSES OF METROLOGICAL SUPPORT OF MEASUREMENTS OF DOSIMETRIC QUANTITIES

S. G. FEDOROV, P. F. MASLYAEV

The most significant influencing factor in the calibration of measuring neutron radiation is scattered radiation arising in the host calibration rig with a neutron source (from the walls, floor, ceiling and various items), which may lead to a change in the spectrum of neutron radiation with distance from the source of neutron radiation due to the different contribution of the scattered radiation, which leads to the violation of the inverse square law. This fact is true for an open geometry, and collimated geometry.

Generally collimated geometry value of HRo (flux, kerma, ambient dose equivalent (ADE)) at a distance Ri from the center of the source can be represented as a sum of contributions from three sources: HRis - from direct radiation of the neutron source placed at the origin (center of source); НRivs - from a virtual source, characterizing diffuse in the container-radiation collimator; Hsci - distributed ambient indoor radiation, i.e.:

HRi = HRis+HRivs+Hsci (1)

To transfer units: flux density (FD), kerma, ADE in the certification necessary to carry out measurements for the entire range of working distances with the setup that requires too much time and high stability and satisfactory energy sensitivity of the means of transmission units required values that is currently unattainable. It is therefore advisable that the dependence of the required values for different races distances from the center of the neutron source emission rate using the coefficients determined by calculation, taking into account the room dimensions, wall thickness and conditions of placement testing equipment in the room. The most appropriate method of calculation is the Monte Carlo method.

Given the presence of scattering and debilitating radiation of objects, equation (1) can be rewritten in the form:

HRi=HRo∙(1-aо)∙RoRi2∙k1 + HRo∙aо∙Ro-diRi-di2∙k2+Hsci (2)

where HRo – value on the distance Ro (the distance reference point from the center of the source);

ao - share value at the expense of neutron radiation scattered in the container-collimator at a distance of Ro;

k1 and k2 are coefficients taking into account the change of the field of neutron radiation by attenuation of radiation and accumulation of scattered radiation in the air for primary and scattered in the container-collimator neutron radiation;

Hsci - is the contribution to value from diffused indoor radiation;

di - conditional offset the center of the virtual source is scattered in the container-collimator neutron radiation relative to the center of the actual source (the origin).

It is obvious that the values of k1, k2, ao, and di depend on the neutron energy and the type of the considered quantities.

The calculated values of k1, k2 for the PuBe source is shown in Fig.1 and Fig. 2.

Fig. 1 the dependence of the coefficient k1 from the distance to the PuBe source.

Fig. 2 the dependence of the coefficient k2 from the distance to the PuBe source.

Calculations of the values of ao and di, based on calculations by the Monte Carlo values HRo and HRi, were conducted under the assumption of absence of air and scattering of neutron radiation in the room.

The calculated values of ao for the PuBe source is shown in table 1

Table 1

The neutron source / The flux density of energy: / Total flux density / Kerma / ADE
less
0,5 МэВ / more
0,5 МэВ / more
1 МэВ
PuBe / 0,609 / 0,208 / 0,195 / 0,284 / 0,189 / 0,220

To calculate di, the expression (3) should be written as:

HRi=HRo∙1-aо∙RoRi2+HRo∙aо∙Ro-diRi-di2 (3)

The results of calculations of values for di PuBe neutron source and various quantities: PP, density of neutron flux with energy above 1 MeV (FD > 1 MeV), ADE and kerma is presented in Fig. 3.

Fig. 3 to change the position of the center of multiple conditional in the container-collimator neutron radiation, depending on the distance from a PuBe source.

Driven GOST 8.355-79 amendments for distances when calculating values according to the inverse square law are not satisfactory, as seen in Fig. 4 amendments should be different for different quantities. It is convenient to use correction factors Fi are calculated by the Monte Carlo method and allows to apply the "inverse square law" to determine the value of HRi as

HRi=Fi∙HRo∙RoRi2 (4)

In Fig. 4 and 5 shows the dependence of the coefficient of Fi on the distance from the center of the PuBe source for the size of the room 3,5×6×6 m with the thickness of the walls in the room is 30 cm and the install location UCPN-1M in the center.

Fig. 4 the dependence of the factor F for the density of neutron flux (total – FD, with energy less than 0.5 MeV – FD < 0.5, with energy above 0.5 MeV – FD > 0.5) of the distance from the source.

Fig. 5 the dependence of the factor F for the density of neutron flux with energy above 1 MeV – FD > 1), kerma and ADE from the distance from the source.

Fig. 6 Dependence on the distance to the PuBe source values of the coefficient F for ADE and the relative sensitivity of the dosimeter-radiometer DKS-96.

Certification of installations it is advisable to carry out measurements at two different distances from the center of neutron radiation of 100 cm and 150 cm. With the measurement result obtained at a distance of 100 cm, is used to calculate values at different distances, using the transition rates shown in the respective charts. The measurement result corresponding to the distance of 150 cm, is used to verify compliance with the provisions of the central source, the zero point on the calibration line installation. This should satisfy the condition:

H100∙FH150-2,25∙100≤2∙∆2+δ2 (11)

where H100 and H150 – value is the value measured at distances of 100 cm and 150 cm, respectively;

F – conversion factor for the corresponding value of the point on the distance from the source 100 cm to a point on the distance from the source 150 cm;

∆ - error measurement in percent, used for certification;

δ – expanded uncertainty with a coverage rate 2 calculate F (less than 1 %).

The value of the ratio H100/H150 depends on the distance from the source to the walls. This dependence relationship of ambient dose equivalent is shown in Fig. 7. In Fig. 7 shows that this dependence on the size of the room is not very substantial.

Fig. 7 the ratio of the value of ADE at a distance of 100 cm to the value of ADE at a distance of 150 cm distance from the source to the walls of the room.

When testing measuring instruments, especially in determining the energy dependence of the sensitivity of measuring important factor is the possibility of modifying the spectra of neutron radiation. In table 2 and table 3 shows the values averaged over different values of neutron energies for some of the spectra from the set of spectra used in the FSUE "VNIIFTRI".

Table 2

Averaging in size: / Average energy for the types of spectra generated with the PuBe source:
1 / 2 / 3 / 4 / 5 / 6
The flux density of neutrons with energy more 0,414 eV / 3,41 / 2,23 / 0,96 / 3,33 / 1,05 / 1,33
Power ADE / 3,90 / 3,22 / 1,83 / 3,86 / 1,91 / 3,26
Power kerma / 4,50 / 4,00 / 2,30 / 4,48 / 2,33 / 3,95
Power of effective dose / 4,31 / 3,77 / 2,19 / 4,30 / 2,26 / 3,68

Table 3

Averaging in size: / Average energy for the types of spectra generated with the Cf-252 source:
1 / 2 / 3 / 4 / 5 / 6
The flux density of neutrons with energy more 0,414 eV / 1,89 / 0,90 / 0,50 / 1,75 / 0,60 / 0,53
Power ADE / 2,26 / 1,68 / 1,15 / 2,17 / 1,23 / 2,03
Power kerma / 2,71 / 2,08 / 1,36 / 2,63 / 1,46 / 2,51
Power of effective dose / 2,62 / 2,00 / 1,33 / 2,54 / 1,44 / 2,04

In table 2 and table 3 figures 1 ÷ 6 indicated the following conditions for obtaining the spectrum of different types:

1 - source is placed in the container installation UCPN-1M with the collimator;

2 - the source is placed in the container installation UCPN-1M with plastic over-the procrastinator;

3 the source is placed in the container installation UCPN-1M with the retarder in the form of a cone of iron length 19 cm;

4 - the source is placed in an open geometry;

5 - between the source and the point under consideration is placed an absorbent cone made of polyethylene with 5 % (by weight) of natural boron is 50 cm long;

6 - source placed in the center of the sphere with a diameter of 30 cm, filled with heavy water.

These spectra were obtained at a distance of 1 m from the centre of the source in the room size 6×3.5×9 m with a wall thickness of concrete 30 cm.