S. A. Pozzi

Requirements and quantitative comparison of fast waveform digitizers for data-acquisition systems designed for nuclear nonproliferation applications

Accurate pulse shape discrimination (PSD) is essential for organic scintillators such as EJ-301s (or EJ-309s with higher flash point) because they are sensitive to both neutrons and gamma rays. Because of the background gamma-ray presence the accurate neutron detection requires accurate discrimination of neutrons from gamma rays. This is especially important for applications where fast and robust systems are paramount, such as nuclear nonproliferation and safeguards. For nuclear nonproliferation and safeguards applications, accurate discrimination of neutrons from gamma rays significantly influences the outcome of material identification/characterization. Specifically, particle misclassification can lead to longer measurement times or even to false identification/misclassification of measured material. In this paper, various fast waveform digitizers are compared from the PSD-performance point of view. Specifically, a 12-bit, 250-MHz digitizer is compared to a 12-bit, 500-MHz digitizer and a 10-bit 1-GHz/2-GHz digitizer. The results presented in this paper indicate that the 12-bit-500-MHz resolution combination leads to the best PSD results. The results also show that a 10-bit digitizer can perform better than a 12-bit digitizer, if the 10-bit digitizer has a significantly better time resolution.

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.

Characterization of Deuterated-Xylene Scintillator as a Neutron Spectrometer

We have experimentally characterized the neutron light output response functions of a deuterated-xylene scintillator for neutron energies lower than 10 MeV. We then used the response matrix to unfold the energy distribution of neutrons produced via several reactions, i.e., spontaneous fission, d(d,n)3He, 27Al(d,n)28Si, and 9Be(alpha,n)12C. Organic scintillators based on deuterated compounds show a fast response and good gamma-neutron discrimination capability, similar to or better than proton-based scintillators. Deuterated scintillators can also effectively provide neutron energy spectra by unfolding measured data with the detector response matrix, without the need of time-of-flight. Deuteron recoils, produced by elastic collisions between deuterium and impinging neutrons, are preferentially forward-scattered. This non-isotropic reaction results in distinct peaks in the response functions to monoenergetic neutrons. In this work, we evaluated a custom-fabricated 7.62 cm

New generation plastic scintillators for fast neutron spectroscopy and Pulse Shape Discrimination

\times 7.62

Selective activation with all-laser-driven Thomson γ-rays

cm deuterated-xylene (EJ301D) liquid scintillator. This liquid has a low volatility and higher flash point, compared to benzene-based deuterated detectors, e.g., EJ315 and NE230. We measured the EJ301D detector neutron response matrix (up to 6 MeV neutron energy) using an intense 252Cf source and the time-of-flight technique. The number of response functions obtained using our method is only limited by counting statistics and by the experimentally achievable energy resolution. Multi-channel unfolding was then performed successfully for neutron sources with different energy spectra.

Fast multiplicity counter featuring stilbene detectors for special nuclear material assay

New generation of plastic scintillators have been developed at RMD for fast neutron detection technology. These plastics have peak emission wavelength ~ 440 nm, fast scintillation decay <; 10 ns, light output ~ 13,000 photons/MeV, and excellent Pulse Shape Discrimination (PSD) between gamma rays and neutrons. We have achieved a Figure-of-Merit (FOM) of 2.3 at 1.0 MeVee electron energy threshold for a 2 inch diameter right cylinder sample. At RMD, comparative measurements were made between the plastic scintillator and Eljen liquid scintillator EJ309 both 1 inch diameter × 1 inch length. RMD plastic showed competitive performance. Additionally, in an experiment performed at the University of Kentucky 7 MV Van De Graaff accelerator, RMD plastic scintillator was irradiated with mono-energetic fast neutron beam energies up to 20.8 MeV. The results from this experiment confirm fast neutron spectroscopy capabilities. These results and effects of different electronic systems on the PSD measurements are discussed in this paper.

WE-AB-BRB-11: Portable Fast Neutron and Photon Dose Meter

A bright, narrow band MeV γ-ray source-ray source based on Thomson scattering using a laser-driven electron accelerator has been developed. We discuss the application of this source for selective activation in regions of high particle (neutron or gamma) production, with minimal absorption in intervening materials.

Scintillator-based measurement of off-axis neutron and photon dose rates during proton therapy

A fast neutron multiplicity counting system based on organic scintillators, i.e. EJ-309 and stilbene, has been developed and experimentally tested at the University of Michigan. The system is able to detect correlated photon and neutron multiplets emitted by the fission reaction, within a gate time of tens of nanoseconds. This counting strategy is exploited to quantify fissile mass, without the need of complex electronic circuitry and unfolding procedures, otherwise required in moderated systems. Moderated systems are traditionally based on helium-3 detectors and feature a gate time of hundreds of microseconds. Measurement precision is thus negatively affected by accidental coincidences. A prototypal version of the proposed well-shaped counter was assembled and tested in the laboratory, using a spontaneous fission and an (α, n) neutron source, i.e. 252Cf and PuBe respectively. Measured results show excellent agreement with the simulated model of the system. The viability of the system to discriminate time-correlated fission neutrons from random, uncorrelated neutrons was proved. Preliminary results of an experimental campaign, carried out at INL (Idaho National Laboratory), to characterize plutonium metal samples are also shown. Results show a monotonic increasing trend for the range of measured 240-Pu effective masses, i.e. 0.024-0.5 kg, enabling the measurement of the mass of an unknown sample.

Characterization of secondary neutron production during proton therapy

Purpose: To develop an instrument for measuring neutron and photon dose rates from mixed fields with a single device. Methods: Stilbene organic scintillators can be used to detect fast neutrons and photons. Stilbene was used to measure emission from mixed particle sources californium-252 (Cf-252) and plutonium-beryllium (PuBe). Many source detector configurations were used, along with varying amounts of shielding. Collected spectra were analyzed using pulse shape discrimination software, to separate neutron and photon interactions. With a measured light output to energy relationship the pulse height spectrum was converted to energy deposited in the detector. Energy deposited was converted to dose with a variety of standard dose factors, for comparison to current methods. For validation, all measurements and processing was repeated using an EJ-309 liquid scintillator detector. Dose rates were also measured in the same configuration with commercially available dose meters for further validation. Results: Measurements of dose rates will show agreement across all methods. Higher accuracy of pulse shape discrimination at lower energies with stilbene leads to more accurate measurement of neutron and photon deposited dose. In strong fields of mixed particles discrimination can be performed well at a very low energy threshold. This shows accurate dose measurements over a large range of incident particle energy. Conclusion: Stilbene shows promise as a material for dose rate measurements due to its strong ability for separating neutrons and photon pulses and agreement with current methods. A dual particle dose meter would simplify methods which are currently limited to the measurement of only one particle type. Future work will investigate the use of a silicon photomultiplier to reduce the size and required voltage of the assembly, for practical use as a handheld survey meter, room monitor, or phantom installation. Funding From the United States Department of Energy and the National Nuclear Security Administration.

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

Dose rates, especially from photons and neutrons, inform safety procedures in any radioactive environment. A method for measuring dose rates from both particles with one device will simplify current methods for surveying dose fields. We have developed a technique using organic scintillators that is based on the definition of effective dose, extracted from energy deposited in the detector. Using pulse shape discrimination, energy deposited from either neutrons or photons can be separated. Radiation weighting factors are applied to obtain effective dose from neutrons and photons concurrently. This method was previously demonstrated for organic scintillators with a californium-252 source; this work presents measurements of off-axis secondary particle dose rates from a sub-clinical strength proton beam incident on various tissue-equivalent targets. The experiment was modeled using the Monte Carlo code MCNPX-PoliMi. Measured proton beam secondary neutron dose rates agree reasonably well with simulated results. The comprehensive analysis of this dose rate technique demonstrates the possibility for a dual particle dosimeter to be used in a variety of applications, including surveying, room monitoring, and shielding analysis. Enhancing and simplifying dose rate monitoring will allow for a safer and more productive radiation work environment.