NE213 Liquid Scintillation Detector Research Articles

Wavelet-based digital pulse-shape method for discrimination between neutron and gamma-rays with organic scintillation detectors

Organic scintillator detectors are central instruments in many applications involving fast neutrons. Many organic scintillator detectors exhibit pulse-shape discrimination (PSD) properties and are widely used to discriminate between neutron events and background gamma rays. PSD methods have been traditionally implemented using analog electronic circuits; however, in recent years, digital techniques have proven to outperform analog methods in areas such as count rate capability. Many digital PSD algorithms with various levels of achievement have been proposed, and those based on the wavelet transform of digitized scintillation pulses have shown great promise for operation at high event rates, where the pulse pile-up effect limits the performance of PSD methods. However, the proposed wavelet-based PSD methods involve intricate calculations that limit their practical use. In this work, we describe a modified version of the wavelet-based PSD methods that offers significant simplification of the PSD process while still producing excellent PSD performance. The method employs the Haar wavelet transform, which is the simplest available wavelet function and is easily implemented on digital hardware, such as field-programmable gate arrays (FPGA). We describe the details of the method, and different aspects of its performance are experimentally demonstrated using an experimental setup comprising a NE213 liquid scintillation detector. A figure-of-merit (FOM) of 1.47 ± 0.07 is achieved with an energy threshold of 500 keVee (electron equivalent energy). An excellent FOM value (1.32) is achieved with a short pulse processing window of only 26 ns, indicating the resilience of the method against the pulse pile-up effect.

A technique for the reduction of pulse pile-up effect in pulse-shape discrimination of organic scintillation detectors

Abstract A technique for the reduction of pulse pile-up effect in digital pulse-shape discrimination (PSD) of neutrons and gamma-rays with organic scintillation detectors is presented. The technique is based on an electronic reduction of the effective decay-time constant of scintillation pulses while retaining the PSD information of the pulses. The experimental results obtained with a NE213 liquid scintillation detector in a mixed radiation field of neutrons and gamma-rays are presented, demonstrating a figure of merit (FOM) of 1.20 ± 0.05 with an energy threshold of 350 keVee (electron equivalent energy) when the effective length of the pulses is reduced to 50 ns.

Initial operation results of NE213 scintillation detector for time-resolved measurements on triton burnup in KSTAR.

In time-resolved measurement for triton burnup in Korea Superconducting Tokamak Advanced Research (KSTAR) deuterium plasmas, an NE213 liquid scintillation detector was installed and operated during the 2017 KSTAR campaign. The detector is composed of an NE213 scintillator (50 mm in diameter and 10 mm in thickness) and a photomultiplier tube (PMT). The PMT anode signal was processed under a data acquisition system which contains a field programmable gate array circuit and pulse processing software that is capable of discriminating gamma-ray and neutron pulse signals. In order to determine an appropriate threshold level for the 14 MeV neutron signal resulting from triton burnup, the NE213 scintillation detector was calibrated by using d-d and d-t neutron generators at the National Fusion Research Institute and Intense 14 MeV Neutron Source Facility, OKTAVIAN, Osaka University, Japan. The detector was installed on KSTAR with a 10 mm thick soft-iron stray magnetic field shield and a radiation shield which consists of 100 mm thick lead blocks and 200 mm thick borated polyethylene blocks. A discrimination range for d-t neutron was determined based on test results from neutron generators and KSTAR. Data points selected from the discrimination range were consistent with the classical triton confinement characteristics. In conclusion, under condition of an input counting rate of 1.9 × 105 counts per second (CPS), the detector is able to measure triton burnup signals up to 500 CPS for various plasma parameters.

Threshold Rejection Mode Active Interrogation of SNMs Using Continuous Beam DD Neutrons With Centrifugal and Acoustic Tensioned Metastable Fluid Detectors

A threshold-mode based, first of its kind, portable, lightweight SNM active interrogation mode assessment system is under development by our group with the goal of detecting 1 kg HEU at 1 m under 2-min scan time. This system is comprised of a lightweight ( $\sim 5\times 10^{{\mathbf {7}}}$ n/s) and lightweight ( 104:1 (i.e., over 15,000 DD neutrons rejected for each 252 Cf neutron detected). Similar experiments were also conducted with the ATMFD architecture (in which the sensing state oscillates at over 40 kHz from active to inactive). It was found that the rejection ratio rises from about 1:1 at a drive powers above ~7 W, towards ~10:1 at ~3 W, and to >102:1 below 1 W. In sharp contrast, using a 100cc NE-213 liquid scintillation detector under identical conditions, the rejection ratio varied from a high of ~12 (with 0% gamma rejection) down to only ~1.4 (with 99.9% gamma rejection).

Tagging fast neutrons from a 252Cf fission-fragment source

Coincidence and time-of-flight measurement techniques are employed to tag fission neutrons emitted from a 252Cf source sealed on one side with a very thin layer of Au. The source is positioned within a gaseous 4He scintillator detector. Together with α particles, both light and heavy fission fragments pass through the thin layer of Au and are detected. The fragments enable the corresponding fission neutrons, which are detected in a NE-213 liquid-scintillator detector, to be tagged. The resulting continuous polychromatic beam of tagged neutrons has an energy dependence that agrees qualitatively with expectations. We anticipate that this technique will provide a cost-effective means for the characterization of neutron-detector efficiency in the energy range 1–6MeV.

A comparison of untagged gamma-ray and tagged-neutron yields from 241AmBe and 238PuBe sources

Untagged gamma-ray and tagged-neutron yields from 241AmBe and 238PuBe mixed-field sources have been measured. Gamma-ray spectroscopy measurements from 1 to 5MeV were performed in an open environment using a CeBr3 detector and the same experimental conditions for both sources. The shapes of the distributions are very similar and agree well with previous data. Tagged-neutron measurements from 2 to 6MeV were performed in a shielded environment using a NE-213 liquid-scintillator detector for the neutrons and a YAP(Ce) detector to tag the 4.44MeVgamma-rays associated with the de-excitation of the first-excited state of 12C. Again, the same experimental conditions were used for both sources. The shapes of these distributions are also very similar and agree well with previous data, each other, and the ISO recommendation. Our 238PuBe source provides approximately 2.6 times more 4.44MeVgamma-rays and 2.4 times more neutrons over the tagged-neutron energy range, the latter in reasonable agreement with the original full-spectrum source-calibration measurements performed at the time of their acquisition.

The light-yield response of a NE-213 liquid-scintillator detector measured using 2–6 MeV tagged neutrons

The response of a NE-213 liquid-scintillator detector has been measured using tagged neutrons from 2 to 6MeV originating from an Am/Be neutron source. The neutron energies were determined using the time-of-flight technique. Pulse-shape discrimination was employed to discern between gamma-rays and neutrons. The behavior of both the fast (35ns) and the combined fast and slow (475ns) components of the neutron scintillation-light pulses were studied. Three different prescriptions were used to relate the neutron maximum energy-transfer edges to the corresponding recoil-proton scintillation-light yields, and the results were compared to simulations. The overall normalizations of parametrizations which predict the fast or total light yield of the scintillation pulses were also tested. Our results agree with both existing data and existing parametrizations. We observe a clear sensitivity to the portion and length of the neutron scintillation-light pulse considered.

Portable spectroscopic fast neutron probe and 3He detector dead-time measurements

This paper presents dead-time calculations for the Portable Spectroscopic Fast Neutron Probe (N-Probe) using a combination of the attenuation law, MCNP (Monte Carlo N-particle Code) simulations and the assumption of ideal paralyzing and non-paralyzing dead-time models. The N-Probe contains an NE-213 liquid scintillator detector and a spherical 3He detector. For the fast neutron probe, non-paralyzing dead-time values were higher than paralyzing dead-time values, as expected. Paralyzing dead-time was calculated to be 37.6 μs and non-paralyzing dead-time was calculated to be 43.7 μs for the N-Probe liquid scintillator detector. Dead-time value for Canberra 3He neutron detector (0.5NH1/1K) was also estimated using a combination of subcritical assembly measurements and MCNP simulations. The paralyzing dead-time was estimated to be 14.5 μs, and the non-paralyzing dead-time was estimated to be 16.4 μs for 3He gas filled detector. These results are consistent with the dead-time values reported for helium detectors.

Fast-neutron induced background in LaBr3:Ce detectors

The response of a scintillation detector with a cylindrical 1.5-in. LaBr3:Ce crystal to incident neutrons has been measured in the energy range En = 2–12MeV. Neutrons were produced by proton irradiation of a Li target at Ep = 5–14.6MeV with pulsed proton beams. Using the time-of-flight information between target and detector, energy spectra of the LaBr3:Ce detector resulting from fast neutron interactions have been obtained at 4 different neutron energies. Neutron-induced γ rays emitted by the LaBr3:Ce crystal were also measured in a nearby Ge detector at the lowest proton beam energy. In addition, we obtained data for neutron irradiation of a large-volume high-purity Ge detector and of a NE-213 liquid scintillator detector, both serving as monitor detectors in the experiment. Monte-Carlo type simulations for neutron interactions in the liquid scintillator, the Ge and LaBr3:Ce crystals have been performed and compared with measured data. Good agreement being obtained with the data, we present the results of simulations to predict the response of LaBr3:Ce detectors for a range of crystal sizes to neutron irradiation in the energy range En = 0.5–10MeV.

Simple algorithms for digital pulse-shape discrimination with liquid scintillation detectors

The development of compact, battery-powered digital liquid scintillation neutron detection systems for field applications requires digital pulse processing (DPP) algorithms with minimum computational overhead. To meet this demand, two DPP algorithms for the discrimination of neutron and γ-rays with liquid scintillation detectors were developed and examined by using a NE213 liquid scintillation detector in a mixed radiation field. The first algorithm is based on the relation between the amplitude of a current pulse at the output of a photomultiplier tube and the amount of charge contained in the pulse. A figure-of-merit (FOM) value of 0.98 with 450keVee (electron equivalent energy) energy threshold was achieved with this method when pulses were sampled at 250MSample/s and with 8-bit resolution. Compared to the similar method of charge-comparison this method requires only a single integration window, thereby reducing the amount of computations by approximately 40%. The second approach is a digital version of the trailing-edge constant-fraction discrimination method. A FOM value of 0.84 with an energy threshold of 450keVee was achieved with this method. In comparison with the similar method of rise-time discrimination this method requires a single time pick-off, thereby reducing the amount of computations by approximately 50%. The algorithms described in this work are useful for developing portable detection systems for applications such as homeland security, radiation dosimetry and environmental monitoring.

Some Peculiarities of Digital Gamma-ray Spectroscopy with Germanium Detectors Performed in Presence of Neutrons

We measured the time-differentiated spectrum of the 252Cf source as seen by an high-purity germanium detector triggered by the NE213 liquid scintillator detector. The detectors are off-line coincided from the event-by-event list formed with 10ns resolution by the quad FADC unit of the CAEN N1728B type. The signatures of the processes induced by fast and slow neutrons, both within the detector itself and in its environment, appear completely separated. The processes induced by fast neutrons are found in the prompt part of the time spectrum and those induced by thermal neutrons in the long tail of delayed coincidences. We analyse the time behaviour of spectral structures at 692 keV, which are the signatures of inelastic neutron scattering on Ge-72, and comment on its suitability for determination of the fast neutron flux in general, and in the digital gamma-ray spectroscopy in particular.

Measurements of electron-induced neutrons as a tool for determination of electron temperature of fast electrons in the task of optimization laser-produced plasma ions acceleration

High intensity laser-plasma interaction has attracted considerable interest for a number of years. The laser-plasma interaction is accompanied by generation of various charged particle beams, such as high-energy proton and ions with high charge to mass ratio (Q/M; same as multi-charged ions). Results of simultaneous novel measurements of electron-induced photonuclear neutrons (photoneutron), which are a diagnostic of the laser-plasma interaction, are proposed to use for optimization of the laser-plasma ion generation. The proposed method is demonstrated by the laser irradiation with the intensity of 1 × 10(21) W/cm(2) on the metal foil target. The photoneutrons are measured by using NE213 liquid scintillation detectors. Heavy-ion signal is registered with the CR-39 track detector simultaneously. The measured signals of the electron-induced photoneutrons are well reproduced by using the Particle and Heavy Ion Transport code System. The results obtained provide useful approach for analyzing the various laser based ion beams.

Application of a digital pileup resolving method to high count rate neutron measurements

The possibility of working in high count rate conditions is an important issue for neutron measurements applied to fusion experiments in order to extend the measurement dynamic range and time resolution. One of the main issues of high count rate operation is the overlapping of closely spaced events (pileup) that produces a distortion in their amplitude and therefore in the pulse height spectra. The usual way to deal with such pileup events using standard analog electronics is to reject them in the energy analysis process. Thanks to digital acquisition techniques, the waveforms of pileup events can be recorded and elaborated in order to reconstruct the contributing original single events. This work presents the results of the application of a pileup resolving method for the reconstruction and analysis of overlapping events to pulses acquired digitally from a NE213 liquid scintillator detector. The measurements have been carried out at the PTB accelerator facility with 14 MeV neutrons at total count rates over 400 kHz.

Modeling, analysis and prediction of neutron emission spectra from acoustic cavitation bubble fusion experiments

Self-nucleated and external neutron nucleated acoustic (bubble fusion) cavitation experiments have been modeled and analyzed for neutron spectral characteristics at the detector locations for all separate successful published bubble fusion studies. Our predictive approach was first calibrated and validated against the measured neutron spectrum emitted from a spontaneous fission source ( 252Cf), from a Pu–Be source and from an accelerator-based monoenergetic 14.1 MeV neutrons, respectively. Three-dimensional Monte-Carlo neutron transport calculations of 2.45 MeV neutrons from imploding bubbles were conducted, using the well-known MCNP5 transport code, for the published original experimental studies of Taleyarkhan et al. [Taleyarkhan, et al., 2002. Science 295, 1868; Taleyarkhan, et al., 2004. Phys. Rev. E 69, 036109; Taleyarkhan, et al., 2006a. PRL 96, 034301; Taleyarkhan, et al., 2006b. PRL 97, 149404] as also the successful confirmation studies of Xu et al. [Xu, Y., et al., 2005. Nuclear Eng. Des. 235, 1317–1324], Forringer et al. [Forringer, E., et al., 2006a. Transaction on American Nuclear Society Conference, vol. 95, Albuquerque, NM, USA, November 15, 2006, p. 736; Forringer, E., et al., 2006b. Proceedings of the International Conference on Fusion Energy, Albuquerque, NM, USA, November 14, 2006] and Bugg [Bugg, W., 2006. Report on Activities on June 2006 Visit, Report to Purdue University, June 9, 2006]. NE-213 liquid scintillation (LS) detector response was calculated using the SCINFUL code. These were cross-checked using a separate independent approach involving weighting and convoluting MCNP5 predictions with published experimentally measured NE-213 detector neutron response curves for monoenergetic neutrons at various energies. The impact of neutron pulse-pileup during bubble fusion was verified and estimated with pulsed neutron generator based experiments and first-principle calculations. Results of modeling-cum-experimentation were found to be consistent with published experimentally observed neutron spectra for 2.45 MeV neutron emissions during acoustic cavitation (bubble) fusion experimental conditions with and without ice-pack (thermal) shielding. Calculated neutron spectra with the inclusion of ice-pack shielding are consistent with the published spectra from experiments of Taleyarkhan et al. [Taleyarkhan, et al., 2006a. PRL 96, 034301] and Xu et al. [Xu, Y., et al., 2005. Nuclear Eng. Des. 235, 1317–1324] where ice-pack shielding was present, whereas without ice-pack shielding the calculated neutron spectrum is consistent with the experimentally observed neutron spectra of Taleyarkhan et al. [Taleyarkhan, et al., 2002. Science 295, 1868; Taleyarkhan, et al., 2004. Phys. Rev. E 69, 036109] and Forringer et al. [Forringer, E., et al., 2006a. Transaction on American Nuclear Society Conference, vol. 95, Albuquerque, NM, USA, November 15, 2006, p. 736; Forringer, E., et al., 2006b. Proceedings of the International Conference on Fusion Energy, Albuquerque, NM, USA, November 14, 2006] and also that from GEANT computer code [Agostinelli, S., et al., 2003. Nuclear Instrum. Methods Phys. Res. A 506, 250–303] predictions [Naranjo, B., 2006. PRL 97 (October), 149403] in which ice shielding was also absent. The results of this archive confirm for the record that the confusion and controversies caused from past reports [Reich, E., 2006. Nature (March) 060306. news@nature.com; Naranjo, B., 2006. PRL, 97 (October) 149403] have resulted from their neglect of important details of bubble fusion experiments. Results from this paper demonstrate that ice-pack shielding between the detector and the fusion neutron source, gamma photon leakage and neutron pulse-pileup due to picosecond duration neutron pulse emission effects play important roles in affecting the spectra of neutrons from acoustic inertial confinement thermonuclear fusion experiments.

Improved response function calculations for scintillation detectors using an extended version of the MCNP code

The analysis of (e,e′n) experiments at the Darmstadt superconducting electron linear accelerator S-DALINAC required the calculation of neutron response functions for the NE213 liquid scintillation detectors used. In an open geometry, these response functions can be obtained using the Monte Carlo codes NRESP7 and NEFF7. However, for more complex geometries, an extended version of the Monte Carlo code MCNP exists. This extended version of the MCNP code was improved upon by adding individual light-output functions for charged particles. In addition, more than one volume can be defined as a scintillator, thus allowing the simultaneous calculation of the response for multiple detector setups. With the implementation of 12C(n,n′3α) reactions, all relevant reactions for neutron energies E n<20 MeV are now taken into consideration. The results of these calculations were compared to experimental data using monoenergetic neutrons in an open geometry and a 252Cf neutron source in the complex Darmstadt setup, where in both cases excellent agreement was found.

Response of NE213 liquid scintillation detectors to high-energy photons (7 MeV < Eγ < 20 MeV)

Two NE213 liquid scintillation detectors of 206 and 412 cm 3 in volume were calibrated in high-energy photon fields ( E γ > 7 MeV) at the PTB accelerator facility. The measured pulse height spectra are in excellent agreement with the response functions simulated with the EGS4 Monte Carlo code. The generally assumed linear dependence of the electron light output on the energy deposited does not hold for electron energies above 1.6 MeV. The nonlinear electron light output function was therefore included in the calculation of the response functions. The production of high-energy electrons and positrons in the target assembly and in the air was also taken into consideration.

Developments of a long liquid scintillation detector for fast neutron time-of-flight experiments

We have developed a long NE213 liquid scintillation detector (LLSD, 80 cm × 6.5 cm × 10 cm) which is useful as a position sensitive detector and a high-efficiency neutron detector for measurements of neutron emission cross sections. LLSD is a long rectangular shaped NE213 scintillator coupled with two photomultipliers at both ends of the scintillator cell. Position information along the long cell axis is obtained by the time difference method. The time compensated signal needed for time-of-flight (TOF) experiments is provided by a mean timer. Data are acquired for TOF, position and pulse-height signals gated by n-γ discriminator signals using a three-parameter data acquisition system. The updated data acquisition system and a position-dependent biasing analysis could achieve a remarkably lower bias level down to ∼1 MeV neutrons with good n-γ discrimination compared to the similar type NE213 detectors and remove position dependence of detection efficiency. LLSD with the sophisticated data taking and processing techniques proved to be very effective for neutron emission cross section measurements for 10–18 MeV incident neutrons as a wide-angle scattering detector and a high-efficiency single detector.

Energy resolution tests of 125 mm diameter cylindrical NE213 detector using monoenergetic gamma rays

Abstract Pulse height tests have been carried out for cylindrical NE213 liquid scintillation detectors of diameters 50 and 125 mm, using monoenergetic gamma rays. The 125 and 50 mm NE213 scintillator cells, each having a thickness of 50 mm, were directly coupled to 130 mm and 52 mm diameter photomultipliers respectively. Gamma-gamma coincidence technique was used to determine the detector's energy resolution and pulse height corresponding to the maximum energy of Compton electrons over the gamma rays energy range of 0.5–1.2 MeV. For the 125 mm detector, the energy corresponding to half height of Compton maxima is 14.4-6.6% higher than the maximum energy of Compton electrons while its energy resolution varies from 24.0 to 18.0%. For the 50 mm detector the energy corresponding to half height of Compton maxima is 9.8-4.0% higher than the maximum energy of Compton electrons while its energy resolution varies from 18.8 to 12.2%. For the 125 mm diameter detector results are presented for the first time.

Transmission characteristics of Cf 252 neutrons passing through rare earth and boron loaded concrete slab shields

Experiments that measure the transport of fission neutrons through gadolinium, samarium and boron loaded slab shields were carried out using a Cf 252 source. The effectiveness of rare earths in comparision to boron as suitable additives in concrete is investigated. A NE-213 liquid scintillation detector with pulse shape discrimination to simultaneously resolve neutron and gamma ray events was employed. The measurements were compared with the spectra calculated using the ‘MORSE’ Monte Carlo code. The calculated and measured spectra are in favourable agreement in the energy range 0.8 MeV up to 12 MeV depending on the concrete slab thickness. Gadolinium mixed concretes exhibit a much higher attenuation as compared to samarium as well as boron loaded types.

Neutron elastic and inelastic differential scattering cross-sections by proton recoil spectrum unfolding

A neutron scattering system is described with which elastic and inelastic differential cross-section can be measured over the angular range 20° to 150°. Three NE213 liquid scintillation detectors are used to provide proton recoil spectra due to the scattered neutrons. These spectra are then unfolded to give the energy spectra of the scattered neutrons which can, in turn, give the differential scattering cross-sections. Results for 2.5 MeV neutron scattering by Fe, Sn and Bi are compared with previous cross-section measurements by conventional time-of-flight techniques. The potential usefulness of the proton recoil spectrum unfolding technique for differential cross-section measurement is assessed. Examples of measurements on nuclei with closely spaced excited states are also discussed.