Andreas Enqvist

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).

Characterization of a Mixed Multiplicity Counter Based on Liquid Organic Scintillators

A measurement system capable of multiplicity measurements for both neutrons and gamma rays has been developed. The benefit of such an approach is in the increased number of available observables. A pure neutron assay results in three observables for third-order multiples, while a combined neutron/gamma-ray assay results in nine observables for the same order of multiples. The idea is to use the additional observables to achieve greater accuracy when determining unknown parameters of the sample such as the fissile mass. The measurement system is based on liquid scintillation detectors (EJ-309s) which feed detected pulses to a digital data-acquisition system. The excellent pulse shape discrimination capabilities of the EJ-309s allow for accurate differentiation between gamma-ray pulses and neutron pulses. The PSD is vital to correctly identify the different multiples up to the third order: n, γ , nn, nγ , γγ , nnn, nnγ , nγγ and γγγ . Previous investigation of the measurement system showed that good counting statistics can be achieved within minutes for spontaneous-fission sources such as 252Cf. In this paper, we present new measurement results and corresponding Monte Carlo simulations aimed at characterizing the measurement system. Comparison of the measured and simulated multiples is discussed in detail and a relatively good agreement is found.

Time-of-flight measurement for energy-dependent intrinsic neutron detection efficiency

Shortage in the current 3He supply has prompted a search for potential alternatives to the neutron detectors currently used in many nuclear nonproliferation and safeguards applications. An alternative detector must be efficient in detecting fission neutrons, and in rejecting or discriminating against gamma-ray radiation. For characterization of numerous detector types, it is helpful to have a technique for evaluating these two characteristics which is relatively fast and easy to perform. Here, a bench-top time-of-flight technique is presented which is based on a coincidence measurement with two ‘independent’ liquid scintillators (no direct source tagging is employed). The neutron source used is 252Cf. The technique can be used to measure energy-dependent intrinsic neutron detection efficiency for incident neutron energies of 0.5–5 MeV, as well as gamma-neutron discrimination efficiency. Measurement results are presented for three 2×2-inch cylindrical liquid scintillation detectors: EJ309, EJ315, and an additional EJ315 with naphthalene added.

Optimization of a mixed multiplicity counter using Monte Carlo simulations and measurements

A measurement system capable of multiplicity measurements for both neutrons and gamma rays has been developed at the University of Michigan. The benefit of such an approach is in the increased number of available measurables. A pure neutron assay results in three measurables for third-order multiples, while a combined neutron/gamma-ray assay results in 9 measurables for the same order of multiples. The idea is to use the additional measurables to achieve greater accuracy when determining unknown parameters of the sample such as the fissile mass. The measurement system is based on liquid scintillation detectors (EJ-309) which feed detected pulses to a digital data-acquisition system. The excellent pulse shape discrimination capabilities of the EJ-309s allow for accurate differentiation between gamma-ray pulses and neutron pulses. The PSD is vital to correctly identify the different multiples up to the third order: n, γ, nn, nγ, γγ, nnn, nnγ, nγγ and γγγ. Previous investigation of the measurement system showed that good counting statistics can be achieved within minutes for spontaneous-fission sources such as 252Cf. In this paper, we present new measurement results and corresponding Monte Carlo simulations aimed at charaterizing the measurement system. Comparison of the measured and simulated multiples count rates is discussed in detailed and a relatively good agreement on trends and count rates is found.

Low-Cost Depth and Radiological Sensor Fusion to Detect Moving Sources

Tracking radioactive sources in 3D can impact homeland security, airport/port surveillance and the military. Unfortunately, the radiological sensors with the highest SNR and the lowest price are unidirectional - i.e. They integrate radiation from a sphere of directions centered at the sensor. We combine such devices with commercially available depth sensors to break this directional ambiguity. We first introduce radiological sensing as an application area for the 3D vision community. Next, we propose a joint calibration algorithm for 3D sensors and unidirectional, or single cell, radiological sensors. Finally, we show applications for tracking people carrying radiological sources.

Gamma-ray and fast neutron imager with thermal neutron detection capability for detecting shielded and bare nuclear materials

We are working on a revolutionary scintillating plastic material in the form of a fused fiber structure. The material has different responses to fast neutrons, gamma rays and thermal neutrons. Energy deposited in 3-D voxels will be recorded using SiPM technology. This material development is directed towards the urgent need for high resolution imaging of both fast fission neutrons and gamma rays, and detection of thermal neutrons. This multi-functionality is driven by the demanding needs to detect various types of special nuclear material (SNM). This unique scintillating material is the basis for an imaging camera capable of performing, for the first time, all three of the above fundamental measurements for passive detection of SNM and RDDs. It does this with increased signal/background by order of magnitude or more. A Nanoptics, Inc. patent application is being submitted for the unique material and structure of the camera.

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.

The estimation of neutron energy spectra of nuclear materials by passive measurements for nuclear nonproliferation applications

The development of technologies to accurately characterize nuclear materials is becoming an area of significant importance in the United States and internationally. The development of new measurement systems with improved characterization capabilities is a top priority. Specifically, the study of neutron detection via passive measurement yields many opportunities. In this research, passive measurements were performed on mixed-oxide samples (MOX) and several isotopic neutron sources at the Joint Research Centre in Ispra, Italy in June of 2010. These measurements are used to test the capabilities of a neutron detection system and analysis algorithms developed at the University of Michigan. The measurements are also utilized to evaluate the use of liquid scintillation detectors for the determination of neutron energy distributions emitted from MOX samples and from various isotopic neutron sources. This measurement campaign provided the opportunity to identify and evaluate new candidates for the replacement of current detector technologies.

Measurement of fast neutron/gamma-ray cross-correlation functions with Cf-252 and Pu-Be neutron sources

A novel technique is presented for measurements of neutron and gamma-ray time-dependent cross-correlation (cc) functions from alpha-n and spontaneous-fission sources. A system based on a fast multi-channel waveform digitizer, organic liquid scintillation detectors, and analysis algorithms was developed at the University of Michigan by the Detection for Nuclear Nonproliferation Group. Measured total and partial cc functions are presented for Cf-252 and Pu-Be sources. The results indicate that an alpha-n source can be distinguished from a spontaneous-fission source by (visual) inspection of the shape of measured cc functions.