Brian M. Wieger

Correlated neutron emissions from 252Cf

Abstract This paper presents new experimental results of correlated, prompt neutron emission from the spontaneous fission of 252Cf. Specifically, we present correlated-neutron emission probabilities and average energies for two detected neutrons as a function of the angle between the two neutrons. Experimental results are compared to several Monte Carlo models that include the number, energy, and angular distributions of prompt neutrons from fission.

Digital data acquisition and processing for a neutron-gamma-ray imaging system

A digital, data-acquisition system for use with a large number of detectors was set up at the University of Michigan. Fast waveform digitizers from CAEN Technologies with 8 channels were synchronized to create a fully scalable system, with a current set-up of 32 channels. While some of the systems limitations are still being investigated, the excellent time resolution of the system enabled accurate time-of-flight measurements at the Los Alamos Neutron Science Center (LANSCE).

Validation of MCNPX-PoliMi fission models

We present new results on the measurement of correlated, outgoing neutrons from spontaneous fission events in a Cf-252 source. 16 EJ-309 liquid scintillation detectors are used to measure neutron-neutron correlations for various detector angles. Anisotropy in neutron emission is observed. The results are compared to MCNPX-PoliMi simulations and good agreement is observed.

An Implicit Correlation Method for Cross-Correlation Sampling, with MCNPX-PoliMi Validation

Abstract Monte Carlo particle transport codes used to model detector responses are traditionally run in analog mode. However, analog simulations of cross-correlation measurements are extremely time-consuming because the probability of coincident detection is small, approximately equal to the product of the probabilities of a single detection in each detector. The new implicit correlation method described here increases the number of correlated event scores, thereby reducing variance and required computation times. The cost of the implicit correlation method is comparable to the cost of simulating single-event detection for the lowest absolute detector efficiency in the problem. The new method is especially useful in the nuclear nonproliferation and safeguards fields for simulating correlation measurements of shielded special nuclear material. The new method was implemented in MCNPX-PoliMi for neutron-neutron cross-correlations with a 252Cf spontaneous fission source measured by 14 detectors at various angles. The method demonstrated good agreement with analog simulation and reference measurement results. Small differences between nonanalog and analog cross-correlation distributions are attributed to discretization errors that are often not present in practical applications. Improvement in the figure of merit was greater than a factor of 100 in all tested cases.

Characterization of secondary neutron production during proton therapy

Proton therapy facilities use high-energy proton beams to destroy cancerous cells. In this approach, secondary radiation is produced due to proton interactions with the patient and surrounding materials. This secondary radiation field, which includes both neutrons and photons, must be accurately characterized in order to determine its effect on patients and medical personnel. The MCNPX-PoliMi code has been used to characterize the secondary neutrons produced during proton irradiation of radiation therapy phantoms. Measurements have been performed at the Loma Linda University Medical Center proton therapy research beamline in order to validate the Monte Carlo models. Proton beams of 155- and 200-MeV were used to irradiate a variety of phantoms and the secondary particles were detected using organic liquid scintillators. These detectors are sensitive to fast neutrons and gamma rays. Pulse shape discrimination was used to classify each detected pulse as either a neutron or a gamma ray. Preliminary analysis has shown good agreement between the simulations and the measurements.

SU-E-T-329: Tissue-Equivalent Phantom Materials for Neutron Dosimetry in Proton Therapy

PURPOSE To characterize tissue equivalence of phantom materials in terms of secondary neutron production and dose deposition from neutrons produced in radiation therapy phantom materials in the context of proton therapy using Monte Carlo simulations and measurements. METHODS In order to study the influence of material choice on neutron production in therapeutic proton beams, Monte Carlo simulations using the Geant4 and MCNPX-PoliMi transport codes were performed to generate the neutron fields produced by protons of 155 and 200 MeV. A simple irradiation geometry was used to investigate the effect of different materials. The proton beams were stopped in slab phantoms to study the production of secondary neutrons. The investigated materials were water, Lucite, and tissue-equivalent phantom materials (CIRS Inc., Norfolk, VA). Neutron energy spectra and absorbed dose by neutrons and their secondary particles were scored. In addition, simulations were performed for reference tissues (ICRP/ICRU) to assess tissue equivalence with respect to neutron generation and transport. In order to benchmark the simulation results, measurements were performed with a system developed at the University of Michigan; organic liquid scintillators were used to detect the neutron emissions from the irradiation of tissue-equivalent materials. Additionally, the MPPost code was used to calculate the scintillator response from the MCNPX-PoliMi output. RESULTS The simulated energy spectra and depth dose curves of the neutrons produced in different phantom materials showed similar shape. The differences of spectra and fluences between all studied materials and reference tissues were well within the achievable precision of neutron dosimetry. The shape of the simulated detector response of the liquid scintillators agreed well with measurements on the proton beamline. CONCLUSION Based on Geant4 and MCNPX-PoliMi simulations, the investigated materials appear to be suitable to study the production of neutrons in proton therapy. MC simulations were verified with neutron measurements in therapeutic proton beams. This work was funded in part by the ANDANTE grant of the European Commission in the 7th Framework Program.