(669c) Neutron Energy Spectrum Correction Based On Capture Time

Accurate neutron energy measurement becomes a requirement for research in many realms. Time-of-flight, proton recoil, and spectrum de-convolution are techniques widely used in many applications. Capture-gated method using a loaded liquid scintillator has been studied before¹. This technique is based on the measurement of two signals generated by the same neutron interacting in the scintillator. An incident fast neutron is not easily absorbed because the absorption cross-section is small in fast range. It will be scattered by hydrogen nuclei and generate recoil protons. When the neutron has been thermalized, it has a very high probability of being captured in the neutron-absorbing medium, such as ¹⁰B, ⁶Li and ¹¹³Cd. The prompt recoil proton pulse is gated by the capture pulse of the same neutron. The primary (scattering) neutron pulse from the scintillator is accepted as the ‘true’ neutron pulse only if the subsequent (capture) neutron pulse is detected. The prompt pulse amplitude is strongly correlated to the incident neutron energy, and this fact can be used to estimate the incident neutron energy.

In current capture-gated technique, it is assumed that capture only happens when the incoming neutron loses all of its kinetic energy. In this work, we show that this assumption is not always correct. The degradation of energy resolution caused by this can be corrected based on capture time. The theory of our approach is demonstrated by simulation using Geant4 Monte Carlo toolkit. The simulation results will be benchmarked with experiment results. In addition, various digital signal processing techniques will be discussed for pulse shape analysis using this detector. Evaluation with experimental data will be presented.

This talk is built upon our previous simulation work. We will show some measurement results with an EJ-339A detector to benchmark our concept.

DESCRIPTION OF THE ACTUAL WORK

In this study, EJ-339A has been chosen as an example of boron loaded liquid scintillator suitable for capture-gated technique. In the EJ-339A, a scattered neutron is easily captured by¹⁰B, leading to absorption reaction with Q=2.3MeV and 94% branching. A combination of pulse height and pulse shape discrimination techniques may be employed to identify the prompt and delayed neutron pulses from amongst the gamma background.

Traditionally, it is assumed that the scattered neutron is captured with no kinetic energy. Thus the incident neutron energy can be solely determined by the prompt pulse amplitude. However, since the absorption cross-section is a function of neutron energy, the energy of neutron when captured should follow a distribution centered at a non-zero value. A considerable discrepancy between the energy deduced from the prompt recoil proton pulse and the real incident neutron energy could exist, which may lead to the degradation of energy resolution.

To correct this, we assume the energy discrepancy is correlated with the elapsed time between the neutron scattering pulse and the subsequent neutron capture. To study this relationship, we used Geant4 to build an EJ-339A model and then bombarded it with a fast neutron beam of certain energy. Two variables are generated via the Geant4 simulation: the energy of the neutron before being absorbed and the elapsed time between two pulses. The relation between them was then studied.

 Simulation

 In the simulation, the boron loaded liquid scintillator is bombarded by a monoenergetic neutron beam with energy of 1MeV. From the simulation, we can obtain the values of  and E. We can see that the neutron energy (E) before being absorbed is correlated with the elapsed time (Dt) between two pulses. And also, for a particular value of , E follows a certain distribution.

 RESULTS

From the simulation, we can see the neutron energy (E) before being absorbed is corresponding to the elapsed time (Dt) between two pulses. So we can make correction on a neutron energy spectrum through measuring the Dt, which is quite feasible.

To make a correction on a neutron energy spectrum, we have to establish a relation between E and Dt. As discussed above, for a particular value of , E follows a certain distribution. Instead of assuming the neutron is always captured with no kinetic energy, the measured energy based on the proton recoil pulse can be corrected following this distribution. By doing this, we expect to achieve better energy resolution than standard capture-gated method.