Measured Pulse Height Distributions Research Articles

Radionuclide identification algorithm for organic scintillator-based radiation portal monitor

We have developed an algorithm for on-the-fly radionuclide identification for radiation portal monitors using organic scintillation detectors. The algorithm was demonstrated on experimental data acquired with our pedestrian portal monitor on moving special nuclear material and industrial sources at a purpose-built radiation portal monitor testing facility. The experimental data also included common medical isotopes. The algorithm takes the power spectral density of the cumulative distribution function of the measured pulse height distributions and matches these to reference spectra using a spectral angle mapper.F-score analysis showed that the new algorithm exhibited significant performance improvements over previously implemented radionuclide identification algorithms for organic scintillators. Reliable on-the-fly radionuclide identification would help portal monitor operators more effectively screen out the hundreds of thousands of nuisance alarms they encounter annually due to recent nuclear-medicine patients and cargo containing naturally occurring radioactive material. Portal monitor operators could instead focus on the rare but potentially high impact incidents of nuclear and radiological material smuggling detection for which portal monitors are intended.

Organic liquid scintillation detectors for on-the-fly neutron/gamma alarming and radionuclide identification in a pedestrian radiation portal monitor

We present new experimental results from a radiation portal monitor based on the use of organic liquid scintillators. The system was tested as part of a 3He-free radiation portal monitor testing campaign at the European Commission’s Joint Research Centre in Ispra, Italy, in February 2014. The radiation portal monitor was subjected to a wide range of test conditions described in ANSI N42.35, including a variety of gamma-ray sources and a 20,000n/s 252Cf source. A false alarm test tested whether radiation portal monitors ever alarmed in the presence of only natural background. The University of Michigan Detection for Nuclear Nonproliferation Group’s system triggered zero false alarms in 2739 trials. It consistently alarmed on a variety of gamma-ray sources travelling at 1.2m/s at a 70cm source to detector distance. The neutron source was detected at speeds up to 3m/s and in configurations with up to 8cm of high density polyethylene shielding. The success of on-the-fly radionuclide identification varied with the gamma-ray source measured as well as with which of two radionuclide identification methods was used. Both methods used a least squares comparison between the measured pulse height distributions to library spectra to pick the best match. The methods varied in how the pulse height distributions were modified prior to the least squares comparison. Correct identification rates were as high as 100% for highly enriched uranium, but as low as 50% for 241Am. Both radionuclide identification algorithms produced mixed results, but the concept of using liquid scintillation detectors for gamma-ray and neutron alarming in radiation portal monitor was validated.

Convergence of unfolded spectrum with response function for single radiation based on Bayes' theorem

A Bayesian unfolding method has previously been proposed to infer the source spectrum from measured pulse height distribution data. In contrast to the conventional unfolding methods using the Bayes' theory, the pulse height response function for a single radiation is used as the likelihood function. In the present work, the convergence of the unfolded to the source spectrum by this method is proven analytically from the viewpoint of the Kullback–Leibler divergence when a flat-type spectrum is assigned to the initial prior spectrum. In addition, the unfolding capability of this method is demonstrated using a numerical simulation for the ideal case where the statistical error in the measured pulse height distribution can be neglected.

A Linear Mode Avalanche Photodiode for Ion Detection in the Energy Range 5–250 keV

We investigated the response linearity and the energy resolution of a silicon based reach-through avalanche photodiode (APD) for low energy ions ranging from 5 keV to 250 keV. We found that space charge affects the APD gain, and that this could be compensated by applying a simple correction formula to the measured pulse height distributions. The comparison between former theoretical or experimental studies and our results showed that there was no evidence of additional performance degradation within the tested energy range in terms of the linearity and the energy resolution compared to conventional non-avalanching silicon detectors. Thanks to the low noise level (equivalent to 0.9 keV in silicon) at the room temperature even for a large detection area (15 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), APDs would be potentially used for detection of low-energy ions in various environments such as space plasma physics, particle physics, and many industrial applications.

Analysis of a measured neutron background below 6 MeV for fast-neutron imaging systems

Detailed and accurate information on the neutron background is relevant for many applications that involve radiation detection, both for non-coincidence and coincidence countings. In particular, for the purpose of developing advanced neutron-detection techniques for nuclear non-proliferation and nuclear safeguards, the energy-dependent, ground-level, neutron-background information is needed. There are only a few previous studies available about the neutron background below 10MeV, which is a typical neutron energy range of interest for nuclear non-proliferation and nuclear-safeguards applications. Thus, there is a potential for further investigation in this energy range. In this paper, neutron-background measurement results using organic-liquid scintillation detectors are described and discussed, with a direct application in optimization simulations of a fast-neutron imager based on liquid scintillators. The measurement was performed in summer 2011 in Ann Arbor, Michigan, USA, and the measurement setup consisted of several EJ-309 liquid scintillators and a fast waveform digitizer. The average neutron flux below 6MeV was measured to be approximately 4e−4counts/cm2/s. In addition, the relationship between the neutron-background count rate and various environmental quantities, such as humidity, at Earth's ground level was investigated and the results did not reveal any straightforward dependences. The measured pulse height distribution (PHD) was unfolded to determine the energy spectrum of the background neutrons. The unfolded neutron-background spectrum was implemented to a previously-created MCNPX-PoliMi model of the neutron-scatter camera and simple-backprojection images of the background neutrons were acquired. Furthermore, a simulated PHD was obtained with the MCNPX-PoliMi code using the “Cosmic-Ray Shower Library” (CRY) source sub-routine which returns various types of radiation, including neutrons and photons at a surface, and accounts for solar cycle, altitude, and geomagnetic deflection. Comparison between the measured and simulated PHDs shows very good agreement.

Electronic measurement of microchannel plate pulse height distributions

Microchannel plates are a central component of the x-ray framing cameras used as analog imagers in many plasma experiment diagnostic systems. The microchannel plate serves as an amplifying element, increasing the electronic signal from incident radiation by factors of 10(3)-10(5), with a broad pulse-height distribution. Seeking to optimize the photon-to-electron conversion efficiency and noise distribution of x-ray cameras, we will characterize the pulse-height distribution of the electron output from a single microchannel plate. Replacing the framing camera's phosphor-coated fiber optic screen with a charge-collection plate and coupling to a low-noise multichannel analyzer, we quantified the distribution in the total charge generated per photon event. The electronically measured pulse height distribution is used to estimate the signal-to-noise ratio of radiographic images from framing cameras.

A Measurement of the Scintillation Light Yield in ${\rm CF}_{4}$ Using a Photosensitive GEM Detector

The absolute photon yield of scintillation light produced by highly ionizing particles in pure CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> has been measured using a photosensitive Gas Electron Multiplier (GEM) detector. The detector consists of two standard GEMs and a CsI coated GEM which acts as a photocathode that is sensitive to the 160 nm scintillation light produced in CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> . The light yield was determined in terms of the number of scintillation photons emitted into a 4π solid angle produced per MeV of energy deposited in the gas by a 5.5 MeV alpha particle and found to be 314 ±15 photons per MeV. The quantum yield was determined using a fitting method to determine the number of photoelectrons from the measured pulse height distribution, and by an independent method using the measured gain of the GEM detector. The effect of scintillation light in CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> on the performance of Cherenkov detectors, such as the PHENIX Hadron Blind Detector (HBD) at RHIC, is also discussed.

Identification of shielded neutron sources with the liquid scintillator BC-501A using a digital pulse shape discrimination method

Fast and unambiguous identification of shielded neutron sources is of paramount importance in nuclear nonproliferation and international safeguards applications. Usually, the identification of neutron spectra relies on unfolding procedures that are unstable and time consuming. In this paper, we present a new application of an existing technique, which can be used for neutron source identification by direct analysis of the pulse height spectrum, i.e., without resorting to energy spectrum unfolding. This technique is based on the acquisition of neutron and γ-ray pulses by using a liquid scintillation detector and a fast waveform digitizer. An optimized digital pulse shape discrimination method based on standard charge integration is used to discriminate neutrons from γ-rays. The neutron pulses are then analyzed to generate a pulse height distribution. The accuracy of the neutron source identification technique was tested on three neutron sources: Cf-252, Am–Be, and Am–Li. Several source–shielding configurations were tested to assess the influence of potential shielding of the source and the sensitivity of the technique. For this investigation, lead and polyethylene shielding blocks were used. The measured pulse height distributions were compared with the distributions simulated with the MCNP-PoliMi code, and very good agreement was obtained. The results show that for both shielded and unshielded configurations the identification of Cf-252, Am–Be, and Am–Li shielded sources is easily achievable by direct analysis of the measured pulse height distributions, without the need of subsequent neutron energy spectrum unfolding.

Application of Unfolding Technique to HPGe Detector Using Response Functions Calculated with the EGS4 Monte Carlo Code

The EGS4 Monte Carlo simulation technique was used to obtain the energy spectra of photons arriving at a detector from the pulse height distributions measured by the same detector. First, the measured pulse height distribution for incident photons from several radiation sources such as 60Co, 137Cs, 152Eu and 207Bi with a collimator are compared with those calculated using the EGS4 code to investigate the feasibility of the simulation. The comparison showed good agreement of 98.7% for 60Co, 92.5% for 207Bi on the total counts. Second, the pulse height distributions were measured in the open space and then unfolded. The measurement of the distributions was done with changing the source to detector distance (SDD) from 10 cm to 100 cm for 60Co and 137Cs respectively. In the unfolding process, response functions of a high purity Ge (HPGe) detector were calculated using the EGS4 code. The calculated pulse height distributions were then normalized to the measured ones at the peaks of the incident photon energies. The ratio of the sum of counts of the main peaks to the total count in the unfolded spectra for 60Co varied from 5.4 to 5.7 times greater than those in the measured pulse height distributions, while from 2.5 to 2.9 times for 137Cs. Electron contribution to the unfolded spectra for 137Cs decreased as the source to detector distance increased, becoming negligible above 50 cm. The pulse height distributions at the center of the reference plane at 100 cm from the 60Co and 137Cs dummy sources located inside each irradiator were also measured and unfolded to obtain the real pulse height distribution. In the unfolded spectra, the photons scattered from the surrounding materials were reduced to approximately one fourth of those measured in the open space due to the small size of apertures of the irradiators. The ratio of the sum of counts for the main peaks to the total count was larger than those in the measured pulse height distributions by the factor of 5.0 for 60Co and 3.4 for 137Cs. The uncertainties estimated in the unfolding processes were around 0.1% for 60Co and 0.07% for 137Cs.

Experimental study on new hybrid approach to electron multiplication

An experiment to study new hybrid method for multiplying electrons based on a microchannel plate (MCP) and a silicon diode is described. The multiplication gain of the new method is jointly provided by the repetitive multiplication in the MCP operated in linear mode and the impact ionization in the silicon diode. The measured pulse height distribution from the MCP excited by UV light exhibits a peak and a negative-exponential tail regardless of MCP operating voltage above a minimum. Based on the Monte Carlo simulation, it is concluded that the characteristic shape of the measured pulse height distribution is primarily due to the spread of the first collision depth of a UV photon in the channel of the MCP and the bias angle of the MCP.

Propagation of uncertainties in unfolding procedures

A review of methods currently used to unfold particle spectra from measured pulse height distributions or other detector readings is given. It is pointed out that most of the measurements in particle spectrometry reveal ill-conditioned or ill-posed problems. The presentation which is given here for examples of such inverse problems is focussed on the algorithms used in the HEPRO unfolding program package of PTB. The question of uncertainty propagation is discussed for least-squares algorithms as well as for those based on maximum entropy. A first attempt has been made to quantify generally the “ambiguity” in ill-posed unfolding problems. The maximum entropy algorithm realized in the MIEKE code allows a clear distinction to be made between two parts of uncertainty, one part coming from ambiguity and one part coming from the usual uncertainty propagation. The resulting uncertainty matrix of the MIEKE code provides these two parts.

Inversion of ultrafine condensation nucleus counter pulse height distributions to obtain nanoparticle (∼3–10 nm) size distributions

Previous work ( Ahn and Liu (1990) J. Aerosol. Sci. 21, 249–261; Brockmann (1981) Ph.D. Thesis, University of Minnesota; Rebours et al. (1992) J. Aerosol. Sci. 23, S189–S192; Stolzenburg (1988) Ph.D. thesis, University of Minnesota) has shown that for particles smaller than about 15 nm, pulse heights produced by the optical detector in a white-light ultrafine condensation nucleus counter (UCNC; Stolzenburg and McMurry (1991) Aerosol. Sci. Technol. 14, 48–65) decrease with initial particle size. We have previously reported on the use of pulse heights from this instrument to determine the concentrations of freshly nucleated atmospheric nanoparticles in the 3–4 nm diameter range ( Weber et al. (1995) J. Atm. Sci. 52, 2242–2257; Weber et al. (1997) J. Geophys. Res. 102, 4375–4385). In this paper we report on the inversion of measured pulse-height distributions to obtain size distributions of particles in the 3–10 nm diameter range. Using methods developed by Stolzenburg ( Stolzenburg (1988) Ph.D. Thesis, University of Minnesota) the effect of diffusional broadening is taken into account so as to obtain monodisperse kernel functions from measured pulse-height distributions produced by DMA-generated calibration aerosols in the 3–50 nm diameter range. These kernel functions are then used with the MICRON algorithm described by Wolfenbarger and Seinfeld (1990, J. Aerosol. Sci. 21, 227–247) to obtain size distributions of nanoparticle aerosols from measured pulse height distributions. Calculations were done to ensure that assumed pulse-height data generated from selected known size distributions can be inverted to recover the original size distribution. Results from these validation studies are discussed. Applications of the inversion algorithm to data acquired in studies of homogeneous nucleation in the atmosphere are also presented.

Determination of Diagnostic X Ray Spectra Scattered by a Phantom

Photon spectra are reported that result from the scatter of diagnostic X rays at an appropriate water phantom that represents a patient. The tube voltages considered are between 52 kV and 110 kV, the scatter angles range from 10° to 142° to the normal radiation incidence direction. All spectral measurements were performed with a high-purity germanium detector. Spectral photon fluences are computed from the measured pulse height distribution by using an unfolding procedure. The required response functions of the detection system were obtained by using Monte Carlo methods. Reference is made to a catalogue compiling 35 spectra of scattered X rays in diagnostics resulting from a human substitute, together with information on relevant primary field parameters as well as air kerma for the scattered X rays.

Unfolding the response of a Ge detector used for in-situ gamma-ray spectrometry

In environmental radiation protection portable Ge detectors are used for in-situ gamma-ray spectrometry. In order to determine the complete photon fluence rate spectra including the continuum component due to photons scattered in the environment from measured pulse height distributions one needs to apply unfolding methods. A prerequisite of the unfolding is the knowledge of the response functions of the detector to monoenergetic photons. In this work it is investigated whether response functions for modern portable high purity Ge detectors can be calculated by Monte Carlo simulation of photon histories with sufficient accuracy for a reliable determination of the incident photon fluence spectrum. The codes MCNP and EGS4 were used for calculating detector response functions for photon energies ranging from 20 keV to 2.6 MeV. It is demonstrated that a detector response to photons from a 152Eu source can be unfolded with good precision. A comparison of measured and calculated responses shows that for applications in in-situ spectrometry response functions calculated by Monte Carlo simulations are more appropriate for unfolding than those determined experimentally since there is no need to separate the contribution to the response of photons scattered in the laboratory. The dependence of the response functions on the angular distribution of the incident photons was investigated by calculations for various irradiation geometries. Systematic errors in the photon fluence spectra resulting from applying for the unfolding response functions which do not correspond to the real irradiation geometry are estimated.

Measurement and simulation of spectral photon fluence rates in a small building exposed to a 60Co source

With the purpose of validating Monte Carlo simulation methods used for the computation of external gamma exposures in urban environments photon fluence rate spectra in a small house exposed to gamma radiation from an external 60Co source have been measured by in situ spectrometry with a portable high purity Ge detector and, on the other hand, calculated by Monte Carlo simulations of the house/environment geometry using two different photon transport codes. An unfolding procedure based on response functions of the detector, calculated by Monte Carlo simulation, was applied to the measured pulse height distributions in order to obtain the photon spectra. The respective photon fluence rate spectra as well as the derived kerma rates obtained are compared for several positions inside and outside of the house. In general a good agreement between measurements and calculations is found, for both the shape as well as the magnitude of the spectra.

Characteristics of a silicon detector for industrial X-ray computed tomography

We have evaluated a silicon solid state detector (Si-SSD) as a detector for Industrial X-ray Computed Tomography (IXCT) from the viewpoints of sensitivity and radiation damage. The detector which we used in this work, is a diffused junction type Si-SSD (43 × 3.8 × 0.5 mm) fabricated on high purity n-type silicon. We measured pulse height distributions for narrow pencil beam 60Co gamma rays parallelly incident to the detector. The expected deposit energy was 0.146 ± 0.001 MeV (11.7% of the incident photon energy). The calculated pulse height distribution using the general purpose EGS4 code was confirmed to be in good agreement with the measured one. The radiation damage was evaluated using the 60Co gamma ray source. Exposure of up to 88 kGy(Si) was achieved over a period of 20 hours, with an exposure rate of 4.4 kGy(Si)/h (36 mC/kg s). It was confirmed that the Si-SSDs could be used at least up to 88 kGy(Si) with X-rays equivalent to 60Co gamma rays.

Neutron response functions improved by taking into consideration the measured edge effect of the /sup 3/He proportional counter

Relative gas multiplication along the cylindrical axis of a /sup 3/He proportional counter was measured by using a collimated beam of thermal neutrons produced in a graphite pile. The gas multiplication changed gradually in the vicinity of both edges of the sensitive region. The total transition region of the gas multiplication was found to correspond to 23% of the nominal sensitive region. Response functions for thermal neutrons and 2.413 MeV neutrons were calculated with consideration of the measured edge effect by a modified NRESP Monte Carlo code. In both cases, the results showed good agreement with the measured pulse height distributions. >

Statistics of secondary electrons from W induced by He ions and atoms at 0° –90° incidence

Secondary electrons from a W target, induced by 2 KeV He 0 and He + projectiles at 0° -90° angles of incidence have been measured for the first time using the statistics of secondary electron emission. The statistics were determined from the analysis of measured pulse height distribution spectra for 8–32 KeV He + ions. The pulse height response of the detector is found to be sensitive to the applied bias to the detector. Its consequences in the interpretation of data are investigated. It was established that the number of electrons emitted per incident ion satisfy a Poisson distribution. Some new suggestions are made on the possible use of these results, in particular, on the role of the zeroth and the first Poisson coefficients, for surface studies.

Response of coaxial Ge(Li) detectors to narrow beams of photons for stripping of X-ray bremsstrahlung spectra

A Monte Carlo program is written for the calculation of the response of a coaxial Ge(Li) detector to a pencil beam of photons with energy up to 400 keV. The small effect of the inclusion of electron energy loss straggling and the binding effects in incoherent scattering are shown. The results of the calculations are compared with the full energy peak efficiency, determined experimentally with radioactive low scatter sources, and with the measured pulse height distributions of single γ emitting sources in narrow beam geometry. The calculated response function is used for the derivation of the photon fluence spectra produced by X-ray generators by stripping of measured pulse height distributions.

Probability density distribution of primary ionization calculated by means of an iterative deconvolution process

Abstract The probability density distribution of primary ionization in a proportional counter has been obtained by an iterative deconvolution process of the measured pulse height distribution using the single electron distribution. The Polya distribution function gave the best fit to measured single electron spectra and was used to describe the shape of single avalanches in gases. Results of distributions of primary ionization in methane and propane are presented.