Pulse Shape Discrimination Method Research Articles

New Limits on Interactions between Weakly Interacting Massive Particles and Nucleons Obtained with CsI(Tl) Crystal Detectors

New limits are presented on the cross section for weakly interacting massive particle (WIMP) nucleon scattering in the KIMS CsI(Tℓ) detector array at the Yangyang Underground Laboratory. The exposure used for these results is 24 524.3 kg·days. Nuclei recoiling from WIMP interactions are identified by a pulse shape discrimination method. A low energy background due to alpha emitters on the crystal surfaces is identified and taken into account in the analysis. The detected numbers of nuclear recoils are consistent with zero and 90% confidence level upper limits on the WIMP interaction rates are set for electron equivalent energies from 3 to 11 keV. The 90% upper limit of the nuclear recoil event rate for 3.6-5.8 keV corresponding to 2-4 keV in NaI(Tℓ) is 0.0098 counts/kg/keV/day, which is below the annual modulation amplitude reported by DAMA. This is incompatible with interpretations that enhance the modulation amplitude such as inelastic dark matter models. We establish the most stringent cross section limits on spin-dependent WIMP-proton elastic scattering for the WIMP masses greater than 20 GeV/c2.

Performance measurements of a depth-encoding TraPET detector module

The new method to correct a parallax error and the loss of coincidence counts caused by the gap between modules was developed for a small animal PET scanner. We proposed the TraPET scanner composed of 6 dual-layer phoswich detector modules. Each detector module consists of a 5.0 mm-thick trapezoidal-shaped monolithic LSO crystal and a 23 x 23 array of GSO crystal. The layer of interaction is identified by the pulse shape discrimination method. One detector module was built and the algorithm for layer identification was optimized. The dual-layer crystals were optically coupled to a Hamamatsu H8500 position-sensitive PMT and a resistive charge divider was used to multiplex 64-channel anode outputs into 4-channel position signals. The 4 signals have been sampled continuously by 14-bit ADC at a sampling rate of 105 MHz and the pulse shape discrimination algorithm was achieved through FPGA programming. The detector module was irradiated with a Na-22 point source from the side of the crystals to obtain flood images of each layer and two layers were clearly identified, thus verifying the DOI capability. The TraPET detector proved to be a reliable design for correcting the parallax error and improving the sensitivity simultaneously in the small animal PET.

Fast neutron detection with 6Li-loaded liquid scintillator

We report on the development of a fast neutron detector using a liquid scintillator doped with enriched 6Li. The lithium was introduced in the form of an aqueous LiCl micro-emulsion with a di-isopropylnaphthalene-based liquid scintillator. A 6Li concentration of 0.15% by weight was obtained. A 125 mL glass cell was filled with the scintillator and irradiated with fission-source neutrons. Fast neutrons may produce recoil protons in the scintillator, and those neutrons that thermalize within the detector volume can be captured on the 6Li. The energy of the neutron may be determined by the light output from recoiling protons, and the capture of the delayed thermal neutron reduces background events. In this paper, we discuss the development of this 6Li-loaded liquid scintillator, demonstrate the operation of it in a detector, and compare its efficiency and capture lifetime with Monte Carlo simulations. Data from a boron-loaded plastic scintillator were acquired for comparison. We also present a pulse-shape discrimination method for differentiating between electronic and nuclear recoil events based on the Matusita distance between a normalized observed waveform and nuclear and electronic recoil template waveforms. The details of the measurements are discussed along with specifics of the data analysis and its comparison with the Monte Carlo simulation.

Signal modeling of high-purity Ge detectors with a small read-out electrode and application to neutrinoless double beta decay search in Ge-76

The GERDA experiment searches for the neutrinoless double beta decay of76Ge using high-purity germanium detectors enriched in76Ge. The analysis of the signal time structure provides a powerfultool to identify neutrinoless double beta decay events and to discriminate themfrom gamma-ray induced backgrounds. Enhanced pulse shape discriminationcapabilities of Broad Energy Germanium detectors with a small read-outelectrode have been recently reported. This paper describes the full simulationof the response of such a detector, including the Monte Carlo modeling ofradiation interaction and subsequent signal shape calculation. A pulse shapediscrimination method based on the ratio between the maximum current signalamplitude and the event energy applied to the simulated data shows quantitativeagreement with the experimental data acquired with calibration sources. Thesimulation has been used to study the survival probabilities of the decays whichoccur inside the detector volume and are difficult to assess experimentally.Such internal decay events are produced by the cosmogenic radio-isotopes68Ge and 60Co and the neutrinoless double beta decay of76Ge. Fixing the experimental acceptance of the double escape peakof the 2.614 MeV photon to 90%, the estimated survival probabilitiesat Qββ = 2.039 MeV are (86±3)% for 76Geneutrinoless double beta decays, (4.5±0.3)% for the 68Gedaughter 68Ga, and (0.9+0.4−0.2)% for 60Codecays.

A hybrid pulse shape discrimination technique with enhanced performance at neutron energies below 500 keV

A hybrid pulse shape discrimination (PSD) method is presented that combines a charge-integration PSD method with a reference-pulses PSD method. The reference-pulses PSD method uses detailed knowledge of the average, energy-dependent detector response to radiation. To obtain the reference-pulses, a large number of neutron and gamma-ray pulses was averaged in several pulse height regions. The reference neutron and gamma-ray pulses were then used in the new PSD method for the classification of a large number of measured pulses. The reference-pulses PSD method was applied below 70 keVee, whereas the standard charge-integration PSD method was used above 70 keVee. This new hybrid PSD method proves to be more accurate than the standard charge-integration PSD method for classification of neutrons and gamma rays. Specifically, the improvement is approximately 30% for neutrons in the smallest pulse height bin considered, which was between 20 and 30 keVee (corresponding to approximately 150 and 225 keV neutron energy deposited, respectively). For this pulse height bin, approximately 72% of the neutrons were correctly classified by the hybrid PSD method. The average number of correctly classified neutrons is approximately 88% for the hybrid PSD method between 20 and 100 keVee (corresponding to approximately 150 and 670 keV neutron energy deposited, respectively) as opposed to 83% for the charge-integration PSD method.

Neutron–gamma discrimination based on pulse shape discrimination in a Ce:LiCaAlF6 scintillator

Abstract We demonstrate neutron–gamma discrimination based on a pulse shape discrimination method in a Ce:LiCAF scintillator. We have tried neutron–gamma discrimination using a difference in the pulse shape or the decay time of the scintillation light pulse. The decay time is converted into the rise time through an integrating circuit. A 252Cf enclosed in a polyethylene container is used as the source of thermal neutrons and prompt gamma-rays. Obvious separation of neutron and gamma-ray events is achieved using the information of the rise time of the scintillation light pulse. In the separated neutron spectrum, the gamma-ray events are effectively suppressed with little loss of neutron events. The pulse shape discrimination is confirmed to be useful to detect neutrons with the Ce:LiCAF scintillator under an intense high-energy gamma-ray condition.

Suppression of gamma-ray sensitivity of liquid scintillators for neutron detection

Methods to reduce gamma-ray sensitivity of a liquid scintillator EJ309 have been studied. Zero-crossing pulse shape discrimination method was used to separate events generated by neutron and gamma radiation between 60− keVee and 4 MeVee. The measurements were carried out under irradiation from an intense 137Cs source, yielding dose rate of 10 mR/h at the detector. A Pu–Be source was used to establish neutron integration window. Pile-up rejection (PUR) circuit was used to reduce gamma-ray induced events under irradiation from an intense gamma-ray source. Further, application of lead, tin and copper shields was done in order to decrease intrinsic gamma-neutron detection efficiency.

Light Pulse Shapes in Liquid Scintillators Originating From Gamma-Rays and Neutrons

Liquid scintillators loaded with boron-10 or lithium-6 are capable to detect gamma rays, fast neutrons and also thermal neutrons. One of the popular methods applied in order to distinguish events originating from different particles is the pulse shape discrimination (PSD). The previously presented study of boron-10 loaded liquid scintillators using the PSD method showed different discrimination performance in scintillators such as BC523A, BC523A2, EJ339A2 and EJ309B5. It triggered a further study of the light pulse shapes in these scintillators originating from events related to gamma rays, fast and thermal neutrons. The light pulse shapes, measured using the single photon method, were recorded together with the 2-dimensional n/gamma discrimination data. Next, the recorded light pulses were gated using energy and the PSD information to extract pulses characteristic of the only one kind of particles. Finally, the analysis of the light pulse shapes with multi-exponential fits and calculation of decay time constants and intensities of components were performed. The results were compared with the data obtained for liquid scintillators not sensitive to thermal neutrons BC501A, EJ301 and EJ309.

Pulse shape classification in liquid scintillators using the fuzzy c-means algorithm

A new approach to pulse shape classification for neutron detectors of type BC501A has been investigated. The method is based on the fuzzy c-means (FCM) algorithm, which allows finding clusters of similar shapes in a set of digitized detector pulses. The aim of the method is to provide principal pulse shapes, which further can be used to apply a pulse shape based particle identification. Since the method is not adapted to the case of liquid scintillator signals it is of general use and can also be applied to signals of other detector types. A detailed study of the quality of the FCM method for the search of principal pulse shapes in BC501A liquid scintillators is presented using a 500Msample/s 12bit digitizer and a 252Cf neutron source. A comparison to principal pulse shapes extracted using time-of-flight information proves the applicability of the method. Finally, an example of a pulse shape discrimination method based on the extracted principal pulse shapes is presented and compared to the well-known integration method.

TraPET: High performance small animal PET with trapezoidal phoswich detector

Abstract In recent years, small-animal PET scanners with depth of interaction (DOI) capability have been developed for molecular imaging research. The aim of this study is to perform simulations to design a high performance small-animal PET, called TraPET. TraPET has an inner diameter of 76.21 mm with 6 dual-layer phoswich detector modules. Each module is composed of a 5.0-mm-thick trapezoidal-monolithic-LSO crystal with a front face (surface facing toward the inside of the scanner) of 44.0×44.0 mm2 and a back face of 50.0×50.0 mm2 and a 25×25 array of LuYAP crystals with a 2.0×2.0 mm2 sensitive area with a 15.0 mm thickness. DOI information is extracted by a pulse shape discrimination method. The ability of event positioning in the trapezoidal-monolithic-LSO was evaluated by modeling the light distribution in the crystal using DETECT2000 and a 16×16 array of silicon photo-multipliers (SiPMs), with a 3.0 mm pixel size, selected as the photo-sensor. Also, the sensitivity and gap filling effect between modules were simulated using the Monte Carlo code, GATE. The new detector showed higher and more uniform sensitivity, as compared to scanners with rectangular-shaped detectors, because the trapezoidal-monolithic-LSO minimizes the dead space within the detector ring. In conclusion, our new detector proved to be a reliable design for small-animal PET with high spatial resolution by DOI information, and high sensitivity by high filling fraction.

Measurement of cosmic-neutron energy spectrum at the summit of Mt. Fuji

We measured a cosmic-neutron energy spectrum for the energy range from 9 to 160 MeV at the summit of Mt. Fuji at 3776 m above sea level, using a phoswich-type scintillation detector to study high altitude cosmic-ray exposure. Neutron signals from phoswich-type scintillation detector were separated from other particle signals by means of a pulse shape discrimination method, and unfolded with the FERDO-U code. The measured unfolded cosmic-neutron energy spectrum showed two peaks, one at around 20 MeV and the other at 110 MeV. The 110 MeV peak had good energy resolution, and the peak energy and the spectral neutron fluence rate around the peak were in agreement with those of our calculation. On the contrary, the appearance of a 20 MeV peak did not agree with the previous cosmic-neutron spectrum measurements and calculations. We believe that it is due to the overlap of signals of different particles caused by insufficient particle identification.

A Digital Method for the Discrimination of Neutrons and $\gamma$ Rays With Organic Scintillation Detectors Using Frequency Gradient Analysis

A digital method for the discrimination of neutron and γ-ray events from an organic scintillator has been investigated by using frequency gradient analysis (FGA) based on the Fourier transform. Since the scintillation process and the photomultiplier tube (PMT) anode signal are often very noisy, most pulse-shape discrimination methods in a scintillation detection system (e.g., the charge comparison (CC) method or pulse gradient analysis (PGA)) using time-domain features of the signal depend greatly on the associated de-noising algorithm. In this research, the performance of the new FGA method and the PGA method have been studied and compared on a theoretical basis and then verified by time-of-flight (TOF). The frequency-domain features extracted by the FGA method exhibit a strong insensitivity to the variation in pulse response of the photomultiplier tube (PMT) and can be used to discriminate neutron and γ-ray events in a mixed radiation field. It is shown that the FGA method results in an increased figure of merit (FOM) which corresponds to a reduction in the area of overlap between neutron and γ-ray events. The FGA method has the potential to be implemented in current embedded electronic systems to provide real-time discrimination in standalone instruments.

Erratum: Scintillation time dependence and pulse shape discrimination in liquid argon [Phys. Rev. C78, 035801 (2008)

Using a single-phase liquid argon detector with a signal yield of 4.85 photoelectrons per keV of electronic-equivalent recoil energy (keVee), we measure the scintillation time dependence of both electronic and nuclear recoils in liquid argon down to 5 keVee. We develop two methods of pulse shape discrimination to distinguish between electronic and nuclear recoils. Using one of these methods, we measure a background and statistics-limited level of electronic recoil contamination to be $7.6\times10^{-7}$ between 60 and 128 keV of nuclear recoil energy (keVr) for a nuclear recoil acceptance of 50% with no nuclear recoil-like events above 72 keVr. Finally, we develop a maximum likelihood method of pulse shape discrimination using the measured scintillation time dependence and predict the sensitivity to WIMP-nucleon scattering in three configurations of a liquid argon dark matter detector.

Particle identification with time-of-flight and pulse-shape discrimination in neutron-transmutation-doped silicon detectors

A method for the identification of energetic charged particles has been investigated based on the employment of pulse-shape discrimination (PSD) in a silicon detector in addition to conventional time-of-flight (ToF) techniques. The method makes use of the fact that, at fixed energy, the particle's velocity, or ToF, is a measure of the particle's mass A while the time structure of the current pulse in a silicon energy detector, used as the ToF stop, permits identification of nuclear charges Z. In the measurements presented here, ToF and PSD methods were applied simultaneously. We used micro-channel plate (MCP) detectors as fast time pick-offs and surface-barrier (SB) n-type Si detectors made from homogeneously neutron-transmutation-doped (n-TD) silicon. As the particles, products from the reactions of a 400 MeV 20Ne beam impinging on 12C, 27Al and 208Pb targets were employed. With using fast current-sensitive pre-amplifiers for the 250 and 800 μ m SB detectors a major progress in particle identification with respect to both, mass A and charge Z was achieved. In addition, using a stack of two closely mounted n-TD SB detectors, a timing measurement between the two detectors permitted to study PSD in the rear detector with the time trigger derived from the foremost one. Finally, the internal PSD method was implemented which determines pulse rise-time in single detectors from analyzing differentiated preamplifier timing signals.

Neutron and gamma-ray cross-correlation measurements of plutonium oxide powder

For the first time, measurements of the time-dependent cross-correlation distributions of plutonium oxide have been made separately for neutrons and gamma rays. Six EJ-309 liquid scintillation detectors with a digital, offline pulse shape discrimination and pulse timing method were used to measure five different samples of varying mass and burnup. The number of (neutron, neutron) correlations were selectively analyzed versus plutonium mass and a clear, increasing trend was observed. Additionally, the measurement scenarios were modeled using the MCNP-PoliMi code and good agreement was observed between the measured and simulated cross-correlation functions.

Continuous depth-of-interaction encoding using phosphor-coated scintillators

We investigate a novel detector using a lutetium oxyorthosilicate (LSO) scintillator and YGG (yttrium–aluminum–gallium oxide:cerium, Y3(Al,Ga)5O12:Ce) phosphor to construct a detector with continuous depth-of-interaction (DOI) information. The far end of the LSO scintillator is coated with a thin layer of YGG phosphor powder which absorbs some fraction of the LSO scintillation light and emits wavelength-shifted photons with a characteristic decay time of ∼50 ns. The near end of the LSO scintillator is directly coupled to a photodetector. The photodetector detects a mixture of the LSO light and the light emitted by YGG. With appropriate placement of the coating, the ratio of the light converted from the YGG coating with respect to the unconverted LSO light can be made to depend on the interaction depth. DOI information can then be estimated by inspecting the overall light pulse decay time. Experiments were conducted to optimize the coating method. 19 ns decay time differences across the length of the detector were achieved experimentally when reading out a 1.5 × 1.5 × 20 mm3 LSO crystal with unpolished surfaces and half-coated with YGG phosphor. The same coating scheme was applied to a 4 × 4 LSO array. Pulse shape discrimination (PSD) methods were studied to extract DOI information from the pulse shape changes. The DOI full-width-half-maximum (FWHM) resolution was found to be ∼8 mm for this 2 cm thick array.

Boron-10 Loaded BC523A Liquid Scintillator for Neutron Detection in the Border Monitoring

A BC523A liquid scintillator loaded with boron-10 was tested as a detector for both fast and thermal neutrons. A pulse shape discrimination (PSD) method based on a zero-crossing principle was applied to distinguish between neutron and gamma radiation. High quantum efficiency Photonis XP5500B photomultiplier was used to enhance light detection from the scintillator. This allowed a good registration of the energy spectrum of neutron capture events on boron-10, corresponding to about 60 keV gamma-rays. The applied PSD method proved to be useful for n/gamma discrimination. A good resolving power of the method was achieved even without gating on neutron capture events. A comparison with a standard BC501A liquid scintillator was done to evaluate thermal neutrons detection efficiency in BC523A.

A wavelet-based pulse shape discrimination method for simultaneous beta and gamma spectroscopy

In this paper, a novel pulse shape discrimination method in scintillator detectors for simultaneous beta and gamma spectroscopy is presented. This method is based on using wavelets to discriminate between pulse shapes in the frequency (scale) domain rather than in the time domain. The proposed method shows a better discrimination performance compared to other time-domain methods in terms of robustness to noise and reliability. This method is also able to detect pile-up events which could not be detected using other time-domain methods. This method can be easily implemented with FPGA technology and digital signal processors.

Digital pulse shape analysis for the capture-gated liquid scintillator BC-523A

The BC-523A detector is a boron-10-loaded liquid scintillation detector that is sensitive to not only fast neutrons and gamma rays, but also to thermal neutrons. This detector has recently attracted the attention of several researchers and has shown promise for neutron spectroscopy. Several efforts have been made to accurately characterize this detector for several neutron and gamma-ray energies. In this paper, we show, for the first time, that neutron capture pulses can be visually distinguished from neutron scattering pulses by using an offline digital pulse shape discrimination method based on charge integration. In addition, new Monte Carlo simulation and measurement results for time to capture are compared and reasonable agreement is achieved. This work has applications in fields such as nuclear safeguards, nuclear nonproliferation, and national security.

Digital discrimination of neutrons and gamma-rays in liquid scintillators using wavelets

A novel algorithm for the digital discrimination of neutrons and gamma-rays in a mixed radiation field is presented. Most of the pulse shape discrimination methods in scintillation detection systems use time-domain features of the signal (e.g. charge comparison method or constant-time discrimination). However, there are no frequency-domain discrimination methods up to date in the literature. Our method employs the wavelet transform to extract frequency-domain features for discrimination. Compared to the pulse gradient analysis (PGA) algorithm, it provides an improvement in reducing the overlap area between neutron and gamma events and also in increasing the Figure of Merit (FoM). Another advantage of this method consists in the removal of the dependency of the discrimination method on timing features. Since the light output in the scintillation process is very noisy, this kind of dependency may degrade the performance of the algorithm.