Detection of Delayed Neutrons from Fissionable Samples: Monte Carlo Modelling and Physical Assumptions for a Design of the DET12 Device
Abstract
An activation of fissionable materials with neutrons has been considered as a possible neutron diagnostic of D–D and D–T fusion plasma. Fission reaction caused by fusion neutrons leads up to emission of secondary neutrons: prompt and delayed. Physical assumptions have been outlined to design a new device (DET12) for measurements of delayed neutrons emitted from samples of fissionable materials activated with neutrons at big fusionplasma devices. The aim is to support a classic neutron activation method used as one of plasma diagnostics at tokamaks or stellarators. An interpretation of the time decay of delayed neutrons enables an assessment of the primary neutron flux which induced fission reaction. Monte Carlo calculations have been carried out in order to elaborate the method considered. Nuclides like: pure ^{235}U, ^{238}U and ^{232}Th, have been selected as possible materials to be irradiated. Physical fundamentals of generation of the delayed neutrons are mentioned and a resulting concept of the DET12 device, built in the Institute of Nuclear Physics, Poland, is presented. A general size and dimensions of particular constituent material layers, and a number and placement of neutron detectors are optimized by means of Monte Carlo modelling. Recommendations for a technical design of the measuring chamber were formulated. Detection efficiency of DET12 has been also estimated.
Keywords
Nuclear fusion diagnostics Neutron diagnostics Neutron activation method Fission reaction Delayed neutronsIntroduction
The neutron activation method is one of the standard methods for the plasma diagnostics at tokamaks. The activation detectors (targets of chosen isotopes) are placed in exposition locations, where the neutron yield from fusion plasma (D–D or D–T) is measured. After the activation the samples are delivered with a pneumatic transport system to a gammaray spectrometer. A deconvolution of the spectra allows calculating the primary neutron yield in given points.
Conceptual Design of the Device and Modelling of the Radiation Transport
Detectors are connected to separate electronic lines (H.V. supply, preamplifier, amplifier, counting electronics) and finally to a control PC unit with the data acquisition and analysing systems.
All elements of the experimental setup have been optimized by detailed Monte Carlo calculations, i.e. type of the fissionable material, geometry and size of the target sample, number and location of ^{3}He detectors, shape and sizes of all covers. The setup has been modelled using MCNP numerical code for radiation transport [4] and details of calculations are presented in the reports [3, 5, 6].
DET12 device has been optimized for measurement of delayed neutrons produced during fission reactions of ^{235}U induced by thermal and fast neutrons and of ^{238}U and ^{232}Th induced by fast neutrons. These two last nuclides are characterized by energy threshold for neutroninduced fission reaction (about 1 MeV for ^{238}U and 0.5 MeV for ^{232}Th) which is a valuable feature for the detection of fast neutrons.
Size and shape of the target sample were optimized taking into account the typical shape of the pneumatic system applied at JET [1] and a magnitude of neutron current through the sample surface. The cylindrical sample of height 1.8 cm and diameter 1.8 cm has been chosen.
Twelve ^{3}He neutron detectors: 1^{″} diameter, 30 cm length (25 cm active), 5 atm. pressure have been chosen as the optimum number. The total efficiency of detection, calculated as a sum from all 12 detectors has been evaluated from the MCNP simulations on 24%. The simulations were performed either for all 16 detectors or for 12 detectors, i.e. when the detectors in corners are removed (Fig. 1b). The contribution of these detectors is smaller than of the others. The efficiency decreases less than it results only from the ratio of 12/16. As expected MCNP calculations confirmed, the contribution of the corner detectors is smaller than of the others. A decrease of the efficiency, ratio ε(N12)/ε(N16) ≈ 0.87, is smaller than that which would result from the ratio of numbers of the detectors, 12/16 = 0.75.

Square horizontal size: 58 cm × 58 cm, height 74 cm

Central hole for the pneumatic transport: 6 cm × 6 cm

Consecutive layers from the hole towards outside (see Fig. 1): (6) bismuth: 2 cm, (4) polyethylene (moderator of delayed neutrons): 12 cm, (3) B_{4}C (absorber): 3.8 cm, (2) Cadmium (absorber): 0.2 cm, (1) Polyethylene (external protection): 8 cm.
Electronic System for Neutron Detection and Data Acquisition
Detection Efficiency of DET12 Device [5]
Detection efficiency of the DET12 device was tested using the ^{252}Cf source of activity = 8.777 kBq placed in the middle of the measuring channel of DET12, i.e. in the position presumed for the activated sample to be measured. Spontaneous fission of ^{252}Cf brings the neutron emission with the coefficient of 11.627% [7]. This results in the neutron emission rate of 1020 cps. The obtained average count rate of the DET12 device is 191.4 ± 0.2 cps. Thus, the neutron detection efficiency can be assessed at 18.8%. Comparing the experimental results with those obtained from the Monte Carlo simulations the difference of the estimations is 20%. Probable reason may be inaccurate knowledge of the active volume of the detector and its gas composition, which were used for numerical calculations. The Monte Carlo simulations do not include neither dead time nor efficiency of the detectors (problem of the detector active volume). Real composition of the gas in the detectors may be different from the one assumed in the MC simulations—manufacturer does not release even principal information and additionally some admixtures, like CO_{2} or Ar as a quench gas, (below 1%) are generally known to be present.
Summary
Measurement of delayed neutrons which are emitted from fissionable samples irradiated in a neutron field originated from thermonuclear plasma is one of types of the neutron activation method applicable for a neutron diagnostic of fusion plasma.
The experimental device (DET12) has been designed, optimized and constructed for the measurements of delayed neutrons from the neutron induced fission reactions. A small amount of delayed neutrons generated during the fission reactions dedicates this method to the measurement of very high neutron yields generated in large fusion devices. Analytical methods, MCNP calculations and experiments indicate that DET12 device can works with good statistics with minimal neutron fluxes about 10^{8}–10^{9} n/cm^{2}s.
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