Pulse Height Resolution Research Articles

Light output of EJ228 scintillation neutron detectors

The light output of neutron detectors based on the plastic scintillator EJ228 is studied as a function of neutron energy using a time tagged (252)Cf source. Calibration of the light output scale is performed by fitting the experimental distribution of Compton scattering events of photons from a (22)Na source with a response function obtained by Gaussian smearing of the predicted line-shape. The light output curve as well as the pulse height resolution for the EJ228 scintillators is very close (within 5%) to those recently reported for NE213 type organic liquid scintillators.

Characterization of Scintillators by Modern Photomultipliers—A New Source of Errors

The observed discrepancy in the light output, measured for a number of LSO, LYSO and BGO scintillators by different photomultipliers (PMTs), triggered studies to understand the problem. For that purpose the photoelectron number was measured by two different methods: the direct one based on a comparison of the full energy peak to that of the single photoelectron and by a method based on the pulse height resolution of the peak due to the light pulser. In this study, a significant number of different PMTs from Photonis and Hamamatsu were used. We concluded that the number of photoelectrons measured by means of the direct method was higher than the number of photoelectrons calculated from the pulse height resolution of the light pulser peak for all of the PMTs but XP2020Q. It leads to a large dispersion in the estimated light output for a given scintillator. In detail, the light output of BGO and LSO determined with the R6231 and R2059 PMTs is comparable to those measured with XP2020Q PMT and the S3590-18 pin photodiode, when photoelectron number is calculated from the pulse height resolution. Further in-depth studies of the photoelectron number at different bias voltages suggested that the effect is related to the space charge created in the dynode structure of the PMTs. Operation of PMTs at lower bias/gain minimizes this effect; thus, low noise electronics are recommended to determine the single photoelectron peak under these conditions. Moreover, the absolute light output of scintillators is affected by differences in the quantum efficiency calibrations by Photonis and Hamamatsu.

3D silicon pixel sensors: Recent test beam results

The 3D silicon sensors aimed for the ATLAS pixel detector upgrade have been tested with a high energy pion beam at the CERN SPS in 2009. Two types of sensor layouts were tested: full-3D assemblies fabricated in Stanford, where the electrodes penetrate the entire silicon wafer thickness, and modified-3D assemblies fabricated at FBK-irst with partially overlapping electrodes. In both cases three read-out electrodes are ganged together to form pixels of dimension 50 × 400 μ m 2 . Data on the pulse height distribution, tracking efficiency and resolution were collected for various particle incident angles, with and without a 1.6 T magnetic field. Data from a planar sensor of the type presently used in the ATLAS detector were used at the same time to give comparison.

Temperature characterization of scintillation detectors using solid-state photomultipliers for radiation monitoring applications

We have characterized two state-of-the-art solid-state photomultipliers, one by SensL, the other by Hamamatsu, coupled to scintillators by Saint-Gobain Crystals in the −25 to +50 °C temperature range. At room temperature, the energy resolution at 661.6 keV measured with both detectors is worse than the resolution obtained when the crystals are coupled to a regular photomultiplier tube. Both the pulse height and pulse height resolution of the 661.6 keV gamma rays in the 137Cs spectrum vary strongly with temperature. The noise threshold determined from the 22Na spectrum increases quadratically as the temperature is increased to well above 100 keV at +50 °C for both detectors.

BES III Time-of-Flight Readout System

A time-of-flight (TOF) readout system has been developed for a Beijing Spectrometer (BES III). This paper will present an overview of the entire TOF readout system design and its performance in the operation. In a BES III TOF detector, a total of 448 photomultiplier tubes (PMTs) have been installed in barrel and end-cap sections. The corresponding 448 channels' readout electronics aimed precisely to measure the arrival time of PMT signals from the TOF detector is mainly housed in two VME64xP crates, each crate containing 14 FEE modules, 14 FEE_rear modules, one Fast Control module (FastCtrl), one Clock module, and one Power-PC interface module. Due to the structure of the spectrometer, the two VME crates with 448 channels of readout electronics are placed in two locations; each is 18 m away from the TOF detector. In order to increase the capability of noise immunization and achieve better signal-to-noise ratio, each channel has a preamplifier equipped to PMT base. The preamplifier drives the differential signals over 18 m of shielded twisted-pair cables to the VME crates. The readout system achieved a timing resolution of 25 ps after time-walk and INL corrections. The amplitude-dependent time-walk correction is accomplished by measuring corresponding pulse height. Achieved pulse-height resolution is better than 10 mV in a dynamic range 180-5000 mV. The whole TOF readout system was installed in the spectrometer and has been in operation steadily since then, with the performance well meeting the requirements of the physics experiments.

Performance of proportional counters under high count rate, high gas gain and at high working gas pressure

The increase of radiation intensity measured by a proportional counter leads to the undesirable changes of its parameters, such as pulse height and energy resolution. It has been found that both the pulse height and the energy resolution significantly decreases with increasing radiation intensity. The spatial resolution measured using a charge dispersive readout technique and the charge centroid determination is also degraded. Possible overlapping of ion tails from successive hits in the detector leads to a continuous but fluctuating current through the counter and its front-end electronics. All those effects might significantly degrade the basic detector properties, especially at very high counting rates of up to 20 MHz as it is expected at LHC Experiments. For higher values of the gas gain and high count rate the total space-charge effect leads to significant non-linearities in the detector response, which in addition depend on the deposited energy, counter geometry, type of working gas and its pressure. The performance of proportional counters under high gas gain and high count rate at high working gas pressure was studied experimentally. The results obtained are presented.

Studies of timing properties for a TOF counter at an external target facility

Timing and amplitude properties of a prototype scintillator TOF counter at an external target facility are studied with a cosmic rays test. The dependence of signal pulse height and time resolution on the coordinate along the scintillator TOF counter is investigated with two different discriminators. A time resolution of 165 ps can be achieved at the center of the counter with a constant fraction discriminator. Time resolution better than 150 ps is obtained at the center with a leading edge discriminator after time walk correction is applied for off-line analysis.

Effects of radiation damage caused by proton irradiation on Multi-Pixel Photon Counters (MPPCs)

We have investigated the effects caused by proton-induced radiation damage on Multi-Pixel Photon Counter (MPPC), a pixelized photon detector developed by Hamamatsu Photonics. The leakage current of irradiated MPPC samples linearly increases with total irradiated doses due to radiation damage, which is not completely recovered even after a year from the irradiation. No significant change has been observed in the gains at least up to 8.0 Gy ( 9.1 × 10 7 n / mm 2 in 1 MeV neutron equivalent fluence, Φ eq ). The device has completely lost its photon-counting capability due to baseline fluctuations and noise pile-up after 21 Gy irradiation ( 2.4 × 10 8 n / mm 2 in Φ eq ), which might be problematic for some applications, such as ring-imaging Cherenkov detectors. We have found that the pulse-height resolution has been slightly deteriorated after 42 Gy irradiation ( 4.8 × 10 8 n / mm 2 in Φ eq ), where the measured sample has been illuminated with a few hundred photons. This effect should be considered in the case of energy-measurement applications.

A sectioned spectrometer of fast neutrons

Development of a capture gated spectrometer on the basis of a liquid organic scintillator doped with enriched 6Li is discussed. Particular interest is evoked by the good pulse height resolution of the spectrometer for 14-MeV neutrons, which is expected to be very high, ∼10–15%. This resolution is attained by compensating for the nonlinearity of the light yield in the scintillator owing to the use of separate optically isolated sections, which independently detect scintillations from each recoil proton. The detector is sensitive to fluence rates ranging from 10−4 to 102 cm−2 s−1 above a threshold of 500 keV under conditions of uncorrelated γ-ray background at a level of up to 102 s−1 (E > 100 keV). A pilot model of the detector based on a scintillator without a lithium dopant has been produced and tested. The detector efficiency is governed by the scintillator volume (∼1.2 l); for 3-MeV neutrons, its value is 0.2–0.5%. The response of the pilot detector to neutrons from a Pu-α-Be source with energies of up to 10 MeV has been measured. Initial testing indicates a low threshold at an ∼600-keV energy of a recoil proton. A good spectral response is obtained using the criterion that three optical sections of the detector operate at a time. This spectrometer can find application in low-background experiments in basic physics research, as well as in space research and nuclear medicine for measuring the parameters of the neutron flux.

Investigations for the use of the fast digitizers with [formula omitted] detectors for radiative capture measurements at GELINA

The relatively long dead time in conventional data acquisition systems that provide simultaneously the pulse height and the time information for the detected events hinders cross-section measurements with high count rates. This is the case for capture cross-section measurements at the time-of-flight facility GELINA using high radioactive samples or thick samples of materials having strong resonances. Either the high average count rate (e.g. due to the radioactivity of the sample) or the high instantaneous count rate for strong resonances results in a large dead time correction. One solution to reduce the impact of the dead time is the use of a data acquisition system based on fast digitizers. The performances of two commercial digitizers (CAEN N172B and Acqiris DC282), coupled to a C 6 D 6 scintillator, have been tested in terms of pulse height linearity and resolution, dead time and time resolution. The signal processing was done on-line obtaining simultaneously the pulse height and time information for each detected event. With both digitizers a comparable pulse height linearity and resolution has been obtained as with a conventional system. The total dead time of both digital systems is at least a factor 5 shorter than the one for the conventional system. The main difference in performance between the two digitizers is the time resolution. For a relatively large scintillator, a time resolution of about 2 ns has been achieved with the DC282 module and the conventional system while the time resolution was limited to 15 ns with the CAEN N1728B module. For most nuclei a 15 ns time resolution is sufficient to perform resonance shape analysis. Therefore, the CAEN N1728B module can be used for the majority of capture cross-section measurements at GELINA. However, for nuclei with low level density, for which the resolved resonance region extends to the keV-region, a better time resolution is required and the Acqiris DC282 module has to be used.

Modeling of TES X-Ray Microcalorimeters with a Novel Absorber Design

We have successfully modeled our new TES (transition-edge sensor) X-ray microcalorimeters with a novel X-ray absorber design that is suitable for close-packed array with high quantum efficiency. We have determined device parameters that reproduce complex impedance curves and noise spectra throughout the transition. Observed pulse height, decay time and baseline energy resolution were in good agreement with simulated results using the same parameters.

Effect of On-Chip Magnetic Shielding for TES Microcalorimeters

We investigated the magnet field dependence of the X-ray pulse height and the critical current of a Ti/Au bilayer TES micro-calorimeter. The pulse height was strongly affected by the magnetic field intensity applied perpendicularly to the TES surface. We found that the critical current at zero temperature, Ic0, decreased by a factor of two by applying a magnet field of ∼10 μT. Our data are consistent with a TES sensitivity proportional to (I/Ic0)−2/3, as predicted by the Ginzburg-Landau theory. This fact implies that the shape of the R–T curve of the TES is partly determined by the critical current of the superconductor. In order to make our TES microcalorimeters less sensitive to the external magnetic field, we fabricated devices equipped with on-chip magnetic shielding. One device has a turn-around style electrical lead made of Al, in which the return line is laid beneath the Ti/Au TES. Another device has a Nb layer deposited between a multi-layer membrane. We demonstrated that the devices were usable below 200 μT with small degradation of the pulse height and energy resolution.

Features of scintillation characteristics of CsI:Na and NaI:Tl crystals as the basis for soft gamma radiation detectors

Light collection processes for X-ray and soft gamma radiation detectors were simulated for various detector constructions, reflector types and roughness of detector surfaces. The experimental study of light output and pulse-height resolution of the CsI:Na and NaI:Tl detectors was carried out to check the simulation results and to find out optimal conditions that could maximize scintillation parameters.

The detector system of the BigSol spectrometer at Texas A & M

The detector system used at the first focus of the BigSol superconducting solenoid beam line at the Texas A & M superconducting cyclotron is presented. The system is composed of a position sensitive PPAC followed by an ionization chamber (IC) with a YAP(Ce) array in its back plane. The position sensitivity of the PPAC as well the energy resolution of the IC is investigated as a function of the counting rate. Pulse height, pulse height resolution and time resolution of the YAP(Ce) crystals are studied for a variety of heavy ion beams ranging from 20Ne to 197Au at energies from 15AMeV to 40AMeV. The pulse shape discrimination method using a Flash ADC is also explored, in order to identify decay products (alpha particles and electron/gamma) from the fragments implanted in the scintillator.

Uncertainty in relative energy resolution measurements

We suggest a new method for the determination of the detector relative energy resolution and its uncertainty based on spline approximation of experimental spectra and a statistical bootstrapping procedure. The proposed method is applied to the spectra obtained with NaI(Tl) scintillating detectors and 137Cs sources. The spectrum histogram with background subtracted channel-by-channel is modeled by cubic spline approximation. The relative energy resolution (which is also known as pulse height resolution and energy resolution), defined as the full-width at half-maximum (FWHM) divided by the value of peak centroid, is calculated using the intercepts of the spline curve with the line of the half peak height. The value of the peak height is determined as the point where the value of the derivative goes to zero. The residuals, which are normalized over the square root of counts in a given bin ( y-coordinate), obey the standard Gaussian distribution. The values of these residuals are randomly re-assigned to a different set of y-coordinates where a new “pseudo-experimental” data set is obtained after “de-normalization” of the old values. For this new data set a new spline approximation is found and the whole procedure is repeated several hundred times, until the standard deviation of relative energy resolution becomes stabilized. The standard deviation of relative energy resolutions calculated for each “pseudo-experimental” data set (bootstrap uncertainty) is considered to be an estimate for relative energy resolution uncertainty. It is also shown that the relative bootstrap uncertainty is proportional to, and generally only two to three times bigger than, 1 / N tot , which is the relative statistical count uncertainty ( N tot is the total number of counts under the peak). The newly suggested method is also applicable to other radiation and particle detectors, not only for relative energy resolution, but also for any of the other parameters in a measured spectrum, like peak position, shape parameters, etc.

Development of a segmented fast neutron spectrometer

We discuss the development of a spectrometer based on full energy absorption using liquid scintillator doped with enriched 6Li. Of specific interest, the spectrometer is expected to have good pulse height resolution, estimated to lie in the range 10–15% for 14-MeV neutrons. It should be sensitive to flux rates from 10−6 cm−2 s−1 to 106 cm−2 s−1 above a threshold of 500 keV in an uncorrelated γ background of up to 104 s−1. We have constructed a pilot version of the detector using undoped liquid scintillator, and we report its present status. The detector’s efficiency is determined by the volume of the scintillator (∼1.21) and is estimated to be 0.2–0.5% for 3-MeV neutrons. The good pulse height resolution is achieved by compensation of the nonlinear light yield of the scintillator due to the use of optically separated segments, which collect scintillations from each recoil proton separately. We demonstrate here the response of the detector to neutrons from a Pu-α-Be source, whose energies range up to 10 MeV. Initial testing indicates a low threshold (≈600 keV) and good spectral response after requiring a multiplicity of three segments. Such a spectrometer has applications for low-background experiments in fundamental physics research, characterizations of neutron flux in space, and the health physics community.

Response of YAP:Ce scintillators to energetic heavy ions

The response of YAP:Ce scintillators to energetic heavy ions in the mass range A = 20−200 has been studied. The pulse height and the pulse height resolution have been explored in details for a single YAP:Ce crystal, coupled to R4141 PMT, by using 20Ne, 40Ar, 84Kr, 129Xe and 197Au beams having energies ranging from 40 A MeV to 15 A MeV. Energy degraded beams have also been employed in order to cover a large energy range. Finally, timing properties were studied in the BIGSOL spectrometer at Texas A&M by measuring the heavy fragment time-of-flight against a PPAC detector.

Stability of spectrometric characteristics of CsI:Tl detectors depending on the surface treatment method

The influence of various mechanochemical methods used to treat the entrance surface of a CsI:Tl scintillator on the specific light output and the pulse-height resolution during detection of α particles with energies of 2 or 5 MeV is described. Nonlinearity between the light output and the energy of α particles increases from 7% for a polished surface to 35% after grinding, which leads to a degraded pulse-height resolution. A method for treating the CsI:Tl surface is proposed that minimizes the light-output nonlinearity and ensures stability of spectrometric characteristics in time.

Heteroepitaxial diamond detectors for heavy ion beam tracking

Heteroepitaxy has been shown to produce uniform crystals of single crystal diamond. An advantage of heteroepitaxy is its potential for creating materials suitable for energetic particle detectors that require large-area plates. At this point, there is little information on the electronic properties of heteroepitaxial material and its suitability for detector use is an open question. To address this issue, a heteroepitaxially grown single crystal was used for high-temperature DC transport measurements and for evaluation as a heavy ion particle detector. Using a transverse geometry with an as-grown Ir film as one electrode and a diffused Ti–Au top electrode, the sample showed no detectable current leakage at room temperature up to 100 V bias. At temperatures between 250 and 600 °C, the DC electrical conductivity exhibited thermally activated behavior with an activation energy of 1.4 eV. A 76Ge beam with an energy of 100 MeV/u produced at the Coupled Cyclotron Facility at Michigan State University was used to test detector performance. At typical beam fluences, single events were recorded with instrumentally limited rise times of 0.5 ns and pulse width 1 ns. Single pulses corresponded to an energy loss Δ E = 46 MeV. A pulse-height resolution of the energy loss signal of Δ E / E = 18% was measured. Of particular importance is the fast performance at high beam intensities. No degradation of timing properties and energy resolution could be observed with count rates up to 10 6 particles/s. These measurements reflect the properties of the as-grown heteroepitaxial diamond crystal to which no subsequent processing was applied. It is anticipated that by using thicker crystals that allow the defective initial growth region to be removed mechanically, greatly improved energy resolution can be observed.

A new HPGe setup at Gelina for measurement of gamma-ray production cross-sections from inelastic neutron scattering

A new setup was developed at the Geel Electron LINear Accelerator (Gelina) pulsed white neutron source for the measurement of continuous energy excitation functions of gamma-ray production cross-sections in neutron-induced inelastic scattering. Characteristic gamma-rays are identified, background-free, using the good pulse height resolution of two large volume High-Purity Germanium (HPGe) detectors. High incident-energy resolution is achieved by the time-of-flight method with a flight-path length of 200 m and a time resolution of 8 ns full-width at half-maximum. A neutron energy resolution of 1.1 keV fwhm is obtained at 1 MeV (36 keV at 10 MeV) . Absolute cross-sections are obtained by measuring the neutron flux with a 235 U fission chamber. The setup was successfully applied to the determination of excitation curves of gamma-ray production cross-sections, level inelastic cross-sections and total inelastic cross-sections for 52 Cr and 58 Ni .