One of the most significant problems in ensuring the uniformity of measurements of dosimetric quantities of neutron radiation fields is the problem of determining the energy dependence of the sensitivity of the relevant measuring instruments.
Since various neutron dosimeters have different energy dependence of sensitivity, it is necessary to know the spectrum of neutrons, or more precisely, the average spectrum energy. In the conditions of measuring instruments testing and checking the most widely used sources were: a source with half-life of 87 years based on the reaction of (α, n) and a source of spontaneous fission, such as , due to large half-life and release of neutrons.
To ensure the uniformity of measurements it is also necessary to know the average energy not only of the neutron sources placed in the open geometry, but also of the sources with different Bonner spheres. Bonner spheres allow changing the average energy of the neutron spectrum (see Table 1).
Table 1
Values of the average energies of neutron spectra with different Bonner spheres
|
Average energy <E>, МeV |
|||||
| Bonner sphere | «opened» | «collimated» | D2O | Polyethylene | Iron |
| Cf-252 | 2,15 | 1,89 | 1,42 | 0,84 | 1,03 |
| PuBe | 4,50 | 4,25 | 2,86 | 1,77 | 1,70 |
A measuring unit with Bonner spheres was used as a measuring instrument. The measuring unit is a scintillation counter with a LiI cylindrical crystal of 8×8 mm. The Bonner spheres are chosen so that they have different sensitivity to energy. Figure 1 shows energy dependence of sensitivity of the Bonner spheres of different diameters.
Figure 1. The dependence of sensitivity of the meter with Bonner spheres of different diameters on the neutron energy
As shown in Figure 1, at energies above 8 MeV the sensitivity curves of Bonner spheres are almost parallel, which leads to an increase in systematic error when solving the system of equations (1)
(1)
whereis count rate with a ball moderator number “i”;
f(E) — differential flux density;
(E) — function of sensitivity;
l — the number of ball moderators.
Despite the fact that ball moderators are made of the same material (polyethylene), each sphere appears to be slightly different in the measurement of the count rate (figures 2 and 3).
Figure 2. Count rates of the measuring unit for moderators at a distance of 60 cm from the source
Figure 3. Count rates of the measuring unit for moderators at a distance of 60 cm from thePuBe source
Therefore, to restore a neutron spectrum it is necessary to know two things: the dependence of the Bonner spheres sensitivity on energy and the count rate for each ball. Neutron radiation spectra are calculated using the MCNP5 program (Monte Carlo N-Particle Transport Code). At the same time as the input spectra there were used spectra of and of radionuclide sources recommended by ISO (ISO 8529-1.2006). There is strict control over abroad, so they use sources. The average energy of spectra of sources can be considered the same. These spectra are presented for a PuBe neutron source in Figures 4-7.
Figure 4. Neutron spectra from the PuBe source placed in a container-collimator
Figure 5. Neutron spectra from the PuBe source placed in a sphere filled with heavy water
Figure 6. Neutron spectra from the PuBe source placed in a container-collimator with a generator made of iron
Figure 7. Neutron spectra from the PuBe source placed in a container-collimator with a generator made of polyethylene
References:
- ISO 8529-1, Reference neutron radiations – Part 1.2.3;
- Maslyaev P.F. Formation of neutron fields from radionuclide neutron sources: «ANRI» journal №4(71) р. 32-38 2012.[schema type=»book» name=»FORMATION OF NEUTRON FIELDS FROM RADIOACTIVE NEUTRON SOURCES FOR DOSIMETRY» description=»The algorithm of neutron spectrum reconstruction by using neutron spectrometers with a scintillation detection unit placed in a ball moderator is described. The average values of energy of spectra from various radioactive sources, which were obtained by using different Bonner spheres, are presented.» author=»FEDOROV SERGEY» publisher=»Басаранович Екатерина» pubdate=»2016-12-12″ edition=»euroasia-science_28_28.07.2016″ ebook=»yes» ]