David Murer

Fast neutron detection with pressurized 4He scintillation detectors

Measurement result and performance parameters are presented for fast neutron detectors exploiting the scintillation of natural helium at high pressure. This detection medium has a very low electron density, minimizing the sensitivity to gamma radiation and thus enabling neutron detection also in high gamma radiation environment. Contrary to proportional counters, scintillation detection enables fast (nanosecond) timing and pulse shape discrimination, a technique that enables a lower neutron detection threshold. In this work, the basic principles of the detector are described, followed by a study of gamma rejection capabilities. Methods to calibrate the detector are discussed. Finally, a brief description of a 4He scintillation based detector system including data acquisition electronics is given.

Suitability of high-pressure xenon as scintillator for gamma ray spectroscopy

In this paper we report the experimental study of high-pressure xenon used as a scintillator, in the context of developing a gamma ray detector. We measure a light yield near 2 photoelectrons per keV for xenon at 40 bar. Together with the light yield, we also measured an energy resolution of 9% (FWHM) at 662 keV, dominated by the statistical uctuations in the number of photoelectrons.

A scalable DAQ system based on the DRS4 waveform digitizing chip

Many current and future experiments require the highest temporal resolution together with large numbers of channels in the data acquisition system (DAQ) for a minimum cost. The DRS4 waveform digitizing chip allows data sampling at up to 5 Giga-samples per second (GSPS) with high amplitude resolution. The domino wave sampling method offers a significant cost and power reduction compared to a traditional flash analog-to-digital converter (ADC). This work presents a new DAQ system based on the DRS4 chip that allows continuous digitization of analog signals at 120 Mega-samples per second (MSPS) with the possibility to sample a region of interest up to a rate of 5 GSPS, thereby allowing a long event record with small dead-time in the read-out. The signal-to-noise ratio (SNR) of the system is measured to be 9.3 bit for the 120 MSPS signal and 9.6 bit for the DRS4 readout signal. Arbitrarily complex trigger logic can be built entirely in the digital domain in the read-out field-programmable gate array (FPGA). A Gigabit Ethernet link provides high-speed connectivity from the DAQ board to the backend system. Built-in board-to-board communication and the modular design of the system offer great scalability and flexibility with respect to the number of supported data channels.

Real-time data analysis using the WaveDREAM data acquisition system

The WaveDREAM data acquisition (DAQ) system, based on the DRS4 switched capacitor array waveform digitizing chip, provides Giga-samples per second (GSPS) digitization in a region of interest together with continuous sampling of the input signals at 120 Mega-samples per second (MSPS). In the Field-programmable gate array (FPGA), the 120 MSPS signal can be used to build arbitrarily complex trigger logic and to perform real-time data analysis with hard real-time constraints in the microseconds range. As an application example, the system is configured and optimized for the readout of an array of high-pressure 4He fast neutron detectors. At each end of the detector, photomultiplier tubes (PMTs) collect and amplify the light. Variable gain amplifiers (VGAs) scale the PMT signals such that they optimally fit into the dynamic range of the DRS4 integrated circuit (IC). The trigger in the FPGA employs a coincidence logic between the two PMT signals of a detector to effectively filter out PMT dark counts. Real-time data analysis includes energy deposit measurements and a pulse shape discrimination (PSD) algorithm to reject events originated by gamma radiation, thus greatly reducing the data rate to be processed offline. The high-resolution GSPS signal from the DRS4 IC is used to obtain precise event timing information, thereby enabling neutron time of flight (ToF) measurements with nanosecond time precision. In addition, this signal can be used during offline data processing to strengthen the analysis results. Thus, the WaveDREAM DAQ provides both efficiency (real-time data analysis) and precision (GSPS signal for offline analysis) at low cost.

Analysis for in-situ fission rate measurements using 4 He gas scintillation detectors

Active neutron interrogation is a powerful NDA technique that relies on detecting and analyzing fission neutrons produced in a fuel sample by an interrogating high neutron flux. 4He scintillation gas fast neutron detectors are investigated in this paper for use in a novel fission rate measurement technique The He-4 detectors have excellent gamma rejection, a fast response time, and give significant information on incident neutron energy allowing for energy cuts to be applied to the detected signal. These features are shown in this work to allow for the detection of prompt fission neutrons in-situ during active neutron interrogation of a 238U sample. The energy spectrum from three different neutrons sources (252Cf, AmBe, AmLi) is measured using the 4He detection system and analyzed. An initial response matrix for the detector is determined using these measurements and the kinematic interaction properties of the elastic scattering with the 4He.

4 He detectors for Mixed Oxide (MOX) fuel measurements

A neutron spectroscopic technique for plutonium content measurement is described. The technique exploits the kinematic cutoff of neutron emission in (α,n) reactions on oxygen. The Watt spectrum of fission neutron emission extends to higher energies without such a cutoff. 4He scintillation detectors were calibrated with an energy cut to reject neutrons of low energies, thereby making the detectors sensitive only to fission neutrons but not to neutrons from the (a,n) reaction on oxygen.

BENEFITS OF TIME CORRELATION MEASUREMENTS FOR PASSIVE SCREENING

The “FLASH Portals Project” is a collaboration between Arktis Radiation Detectors Ltd (CH), the Atomic Weapons Establishment (UK), and the Joint Research Centre (European Commission), supported by the Technical Support Working Group (TSWG). The programs goal was to develop and demonstrate a technology to detect shielded special nuclear materials (SNM) more efficiently and less ambiguously by exploiting time correlation. This study presents experimental results of a two-sided portal monitor equipped with in total 16 4He fast neutron detectors as well as four polyvinyltoluene (PVT) plastic scintillators. All detectors have been synchronized to nanosecond precision, thereby allowing the resolution of time correlations from timescales of tens of microseconds (such as (n, γ) reactions) down to prompt fission correlations directly. Our results demonstrate that such correlations can be detected in a typical radiation portal monitor (RPM) geometry and within operationally acceptable time scales, and that exploiting these signatures significantly improves the performance of the RPM compared to neutron counting. Furthermore, the results show that some time structure remains even in the presence of heavy shielding, thus significantly improving the sensitivity of the detection system to shielded SNM.

Measurement of the Fast Neutron Response for

Time-of-flight (TOF) and coincident scattering measurements were conducted to measure the light response of a pressurized 4He fast neutron scintillation detector as a function of deposited energy up to 5 MeV. The energy deposited in the detector by a neutron was measured by its angle of scatter and compared to the resulting light output. Whereas previous research has exclusively focussed on the energy information contained in the slow component, this work demonstrates that the fast component is also sensitive to neutron energy, and the entire scintillation signal can therefore be used. The gamma rejection capability of the detector was also measured for a variety of gamma sources. The detector demonstrated a inherent gamma rejection rate of 97.31%, which was increased to 99.89% after the application of pulse shape discrimination (PSD) algorithms. The characterization of gamma rejection and light response parameters will enable implementation of these detectors for neutron spectroscopy in mixed radiation fields.

^4 {\rm He}

The spatial response of pressurized helium-4 fast neutron scintillation detectors is characterized using collimated neutron source measurements and MCNPX-PoliMi simulations. A method for localizing the position of each detected event is also demonstrated using the two-sided photomultiplier readout. Results show that the position of particle interaction along the axis of the active volume has a measurable effect on the scintillation light response of the detector. An algorithm is presented that uses the probability distribution of relative interaction positions to perform source localization, further demonstrating the applicability of these detectors as tools for the detector of hidden shielded nuclear material.

Scintillation Detectors Using a Coincidence Scattering Method

In this study we report on the scintillation response of Xe gas under irradiation of gamma-rays in the energy range between 50 keV and 1.5 MeV. Xe gas was pressurized to 50 bar and tested as a detector for gamma spectroscopy. The gas was confined in a titanium vessel of 200 mm length and 101 mm diameter with 2.5 mm thick walls. The vessel was sealed with two 3 inch diameter UV transparent windows. The inner surface of the vessel was covered with a reflecting wavelength shifter. Two photomultipliers coupled to both windows at the end of the vessel allowed for registration of 3700 photoelectrons/MeV, which resulted in 7.0% energy resolution registered for 662 keV γ-rays from a 137Cs source. The non-proportionality of the photoelectron yield and intrinsic resolution was studied with gamma photoabsorption peaks. Due to the thickness of the detector vessel, the response of the Xe gas as a scintillator in the low energy range was performed by means of a Compton Coincidence Technique and compared with the gamma absorption results. The shape of the non-proportionality characteristics of Xe gaseous scintillator was compared to the results obtained for NaI:Tl, LaBr3:Ce and LYSO:Ce. A correlation between non-proportionality and intrinsic resolution of Xe gaseous scintillator was pointed out.