Monoenergetic Spectra Research Articles

Physics of solar neutron production: Questionable detection of neutrons from the 31 December 2007 flare

Spacecraft observations in the inner heliosphere offer the first opportunity to measure 1-10 MeV solar neutrons. We discuss the physics of low-energy neutron production in solar flares and show that, even at interacting-particle energies of 2 MeV/nucleon, neutrons with energies >10 MeV are produced. On the other hand, a significant fraction of 1-10 MeV neutrons result from interactions of >10 MeV/nucleon ions in typical flare spectra. We calculate the escaping neutron spectra for mono-energetic and power-law particle spectra at the Sun for the location and observation angle of MESSENGER at the time of its reported detection of low-energy neutrons associated with the 2007 December 31 solar flare. We detail concerns about this questionable observation of solar neutrons: 1. the inferred number of accelerated protons at the Sun for this modest M2-class flare was 10X larger than any flare observed to date, 2. the onset and duration of the 'solar' neutron count rate was similar to that of the solar energetic particles (SEPs), and 3. the authors' argument that the SEPs were dominated by electrons and so could not have produced the neutron counts locally in the spacecraft. In contrast we argue that solar energetic protons and alpha particles, through local neutron production and accidental coincidences, were the source of most of the reported 'solar-neutron' counts.

Efficiency measurements of deuterated liquid scintillators using d(d,n)3He coincidence events

Abstract Deuterated liquid scintillators are promising from the perspective of n / γ separation and neutron energy measurement without time-of-flight. In order to turn these into precision instruments for nuclear physics, however, the absolute efficiency must be known. The efficiency is typically a function of energy and is strongly dependent on the electronics thresholds used with the photomultiplier detectors. This study involves the extraction of this efficiency using the d(d,n)3He reaction (Ed=16.0 MeV) and n-3He coincidence events. The detection and identification of the ions are very efficient and allows one to project pure monoenergetic neutron spectra all the way down to the electronics threshold. Timing information was also recorded in these measurements to provide further constraints and a consistency check.

A Monte Carlo study of primary electron production inside photoconductors for digital mammography and indications of material suitability

Materials like a-Se, a-As2Se3, GaSe, GaAs, Ge, CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbO, TlBr, PbI2 and HgI2 are possible photoconductors for direct conversion digital mammography detectors. The physical characteristics of primary electrons, such as their number, energies, direction angles and spatial distributions, strongly affect the characteristics of the final signal and hence image quality. In previous work, a Monte Carlo model has been developed that simulates the generation of primary electrons inside these materials for x-ray spectra in the mammographic energy range. Using this model the energy, angular and spatial distributions of primary electrons have been studied. For the case of CdTe, CdZnTe, Cd0.8Zn0.2Te and ZnTe, an investigation was also made concerning the dependence of the primary electron production on the incident x-ray energy. In this paper, this investigation has been extended to include the rest of the photoconducting materials. The investigation is realized studying the number of primary electrons produced along with the escaping of photons (both incident and fluorescent) and the number of fluorescent photons emitted for 39 monoenergetic x-ray spectra with energies between 2 and 40 keV. The information obtained from the overall investigation of the primary signal in the various photoconductors gives some good indications of the suitability of PbI2 and HgI2.

The cross-section data from neutron activation experiments on niobium in the NPI p-7Li quasi-monoenergetic neutron field

The reaction of protons on 7 Li target produces the high-energy quasi- monoenergetic neutron spectrum with the tail to lower energies. Proton energies of 19.8, 25.1, 27.6, 30.1, 32.6, 35.0 and 37.4 MeV were used to obtain quasi-monoenergetic neutrons with energies of 18, 21.6, 24.8, 27.6, 30.3, 32.9 and 35.6 MeV, respectively. Nb cross-section data for neutron energies higher than 22.5 MeV do not exist in the literature. Nb is the important material for fusion applications (IFMIF) as well. The variable-energy proton beam of NPI cyclotron is utilized for the production of neutron field using thin lithium target. The carbon backing serves as the beam stopper. The system permits to produce neutron flux density about 109 n/cm2 /s in peak at 30 MeV neutron energy. The niobium foils of 15 mm in diameter and approx. 0.75 g weight were activated. The nuclear spectroscopy methods with HPGe detector technique were used to obtain the activities of produced isotopes. The large set of neutron energies used in the experiment allows us to make the complex study of the cross-section values. The reactions (n,2n), (n,3n), (n,4n), (n,He3), (n,α) and (n,2nα) are studied. The cross-sections data of the (n,4n) and (n,2nα) are obtained for the first time. The cross-sections of (n,2n) and (n,α) reactions for higher neutron energies are strongly influenced by low energy tail of neutron spectra. This effect is discussed. The results are compared with the EAF-2007 library.

Performance of fluorescence nuclear track detectors in mono-energetic and broad spectrum neutron fields

Properties of fluorescent nuclear track detectors (FNTDs) made of Al 2O 3:C, Mg crystals are investigated after irradiation in mono-energetic and broad spectrum neutron fields. Two types of neutron converters were investigated, high density polyethylene and natural LiF. Imaging in fluorescence contrast is performed using laser scanning confocal detection. Individual tracks induced by recoil protons, alpha particles and tritium ions are identified and counted. Two approaches to median neutron energy estimation using track density as a function of scanning depth and the ratio of track densities for two different converters are discussed.

Development of an algorithm for evaluating personal doses due to photon fields in terms of operational quantities for TLD badge system in India

In order to evaluate and report the personal doses in terms of personal dose equivalent, the performance of the CaSO(4):Dy based thermoluminescence dosemeter (TLD) badge used for countrywide personnel monitoring in India is investigated using monoenergetic and narrow spectrum radiation qualities equivalent to those given in ISO standards. Algorithms suitable for evaluating H(p)(10) and H(p)(0.07) within +/- 30 % are developed from the responses of dosemeter elements/discs under different filters for normal as well as angular irradiation conditions using these beams. The algorithm is tested for TLD badges irradiated to mixtures of low- and high-energy ((137)Cs) beams in various proportions. The paper concludes with the results of test of algorithm by evaluation of badges used in the IAEA/RCA intercomparison studies and discussion on inherent limitations.

The propagation of ultra-high energy tau leptons and neutrinos while skimming the Earth

The detection of Earth-skimming tau neutrinos has turned into a very promising strategy for the observation of UHE cosmic neutrinos. The sensitivity of this channel crucially depends on the parameters of the propagation of the tau neutrino (and the tau lepton) through the terrestrial crust, which governs the flux of emerging tau leptons that can be detected. This propagation problem is usually treated in a simplified framework where several effects are neglected, e.g. the possibility of multiple regenerations of the tau neutrino, the weak interactions of the tau lepton, as well as the stochastic nature of its energy losses. We discuss here the validity of such approximations by studying the propagation in standard rock of tau leptons and neutrinos with both mono-energetic and power-law spectra. We also investigate the impact of such simplifications in non-standard scenarios for the neutrino-nucleon interactions as well as for the tau energy losses.

Simulated and measured spectral characteristics of 137Cs reference radiation beams

137Cs sources are widely used in calibrations of radiation protection dosimeters and doserate meters. In these calibrations, conversion coefficients from the air kerma to various dose equivalent quantities are needed. These conversion quantities are functions of photon energy. In this work, Monte Carlo simulations and measurements are used for quantifying the energy distributions of the photon fluence from two 137Cs sources used for calibration purposes. The results indicate that the standard set of conversion coefficients, obtained from a monoenergetic spectrum, do not sufficiently take into account the scattered radiation (most of which is scattered by the source itself): For the large (14x15.6 mm) and small (3.5x3 mm) 137Cs sources, the simulated values for the conversion coefficients H'(0.07)/Ka and H*(10)/Ka (which were nearly equal for the both quantities) showed around 1.0 and 1.5% deviations from the ISO standard value 1.20 (for the large and small sources, respectively). Similarly, the conversion coefficient Hp(10)/Ka showed around 1.5 and 2.5% deviations from the ISO standard value 1.21 for the large and small 137Cs sources, respectively, whereas Hp(0.07)/Ka had the values 1.22 and 1.23. The amplitude of the variation of the conversion coefficients due to varying 137Cs source size did not exceed 1%. Thus, the overall uncertainty (with coverage factor k=2) due to varying source size for the average values of the conversion coefficients can be estimated as low as 0.6%.

Energy Resolution of Plastic Scintillation Detector for Beta Rays

Many experiments on neutrinoless double beta decays have been proposed recently for determining the effective mass of neutrinos. Some of them use a plastic scintillator for measuring the beta-ray energy. Achieving a high energy resolution is critical for neutrinoless double beta decay experiments (e.g., MOON and superNEMO). The motivation of this study is to investigate the energy resolution of a plastic scintillation detector in terms of ldquocomponentsrdquo in the MeV-electron region. It is known that the total energy resolution of a plastic scintillation detector for a monoenergetic spectrum consists of two components: statistical and intrinsic energy resolution. In this paper, these two components were investigated for electrons. Experiments on protons were also performed for the purpose of comparison. Three additional experiments were performed to determine (1) the uniformity of the plastic scintillator materials by scanning a 2.8-MeV proton microbeam, (2) the energy spread of electrons and protons, and (3) the effect of the electron-beam size on the set-up. These additional experiments were performed to ensure that these factors did not affect measurements of the two above-mentioned components.

Reconstruction of X-ray spectra with the energy sensitive photon counting detector Medipix2

A hybrid photon counting detector like the Medipix can be used to measure the energy deposited by X-ray photons in a sensor pixel. We calculate the expected energy deposition distribution with the help of the Monte-Carlo simulation tool ROSI and thus determine the energy response function. These response functions are used in two deconvolution methods to determine X-ray spectra from measured energy deposition spectra. We present reconstruction results for monoenergetic and polychromatic X-ray spectra.

An empirical model of electronic portal imager response implemented within a commercial treatment planning system for verification of intensity‐modulated radiation therapy fields

Quality assurance (QA) of an intensity‐modulated radiation therapy (IMRT) plan is more complex than that of a conventional plan. To improve the efficiency of QA, electronic portal imaging devices (EPIDs) can be used. The major objective of the present work was to use a commercial treatment planning system to model EPID response for the purpose of pre‐treatment IMRT dose verification.Images were acquired with an amorphous silicon flat panel portal imager (aS500: Varian Medical Systems, Palo Alto, CA) directly irradiated with a 6‐MV photon beam from a Clinac 21EX linear accelerator (Varian Medical Systems). Portal images were acquired for a variety of rectangular fields, from which profiles and relative output factors were extracted. A dedicated machine model was created using the physics tools of the Pinnacle3 (Philips Medical Systems, Madison, WI) treatment planning system to model the data. Starting with the known photon spectrum and assuming an effective depth of 7 cm, machine model parameters were adjusted to best fit measured profile and output factors. The machine parameters of a second model, which assumed a 0.8 MeV monoenergetic photon spectrum and an effective depth in water of 3 cm, were also optimized. The second EPID machine model was used to calculate planar dose maps of simple geometric IMRT fields as well as a 9‐field IMRT plan developed for clinical trials credentialing purposes.The choice of energy and depth for an EPID machine model influenced the best achievable fit of the optimized machine model to the measured data. When both energy and depth were reduced by a significant amount, a better overall fit was achieved. In either case, the secondary source size and strength could be adjusted to give reasonable agreement with measured data. The gamma evaluation method was used to compare planar dose maps calculated using the second EPID machine model with the EPID images of small IMRT fields. In each case, more than 95% of points fell within 3% of the maximum dose or 3 mm distance to agreement. These results are slightly poorer than those obtained using an ion chamber array, which confirms agreement to within 2% of the maximum dose or 2 mm distance to agreement for all points within these fields.PACS numbers: 87.55Qr, 87.56.Fc

Nanocluster explosions and quasimonoenergetic spectra by homogeneously distributed impurity ions

A plasma expansion into vacuum and the resultant ion acceleration are studied analytically and numerically. The expansion of an initially uniform spherical plasma (consisting of a nanocluster or microdroplet) with radius Ru0 and electron density nu0 is driven by the explosion of hot electrons having an initial temperature Te0. A self-similar solution describes the nonrelativistic expansion of a finite plasma mass with a full account of charge separation effects. Such key features as the energy spectrum, maximum ion energy, and energy transfer efficiency from the electrons to the ions are given by simple analytic formulas as a function of the normalized droplet radius, Λu=Ru0∕λD, where λD=Te0∕4πnu0e2 is the Debye length. The solution predicts that impurity ions doped homogeneously in a droplet plasma are accelerated quasimonoenergetically by the electrostatic field generated by the charge separation. The prediction is confirmed by N-body particle simulations. The origin of the monoenergetic spectrum is attributed to the spherical geometry.

SU‐GG‐I‐68: Dosimetryc Study to Voxel Model Applied to Carciac Procedures in Nuclear Medicine

Purpose: The objective of this study is to realize a computational dosimetry on a simplified model of miocardic perfusion, considering the usual different ways to describe the spectra emitted for Tc‐99m. Method and Materials: The GEANT4 code was used to simulate two geometries: the radial dose distribution from an isotropic point source described by the three spectra, modeled with photon emission from the center of a 1.3‐m‐diam sphere of muscle tissue; and a second considering an uniform distribution of Tc‐99m in the heart muscle of the adult male voxel model MAX, to take the organs dose distribution. The different ways to describe the spectra emitted for Tc‐99m was: monoenergetic spectrum of 140 keV; three photons emissions spectrum (2.1, 141 and 143 keV) and total spectrum (including characteristics x‐rays and Auger electrons). Results: For distance low than 1 cm from the point source the radial dose distribution is higher for total spectrum. The radiation dose in a sphere with 0.01 mm defined at the center of the sphere of muscle was 0.369 mGy Bq1 h1 (total spectrum), 0.276 mGy Bq1 h1 (three photon spectrum), and 0.005 mGy Bq1 h1 (monoenergetic spectrum). This data shown that include Auger electrons, characteristic x‐rays, and low energy gamma give a significant contribution to total energy deposition. This results corroborates the simulations realized using the voxel model. The data variation shown that monoenergetic and three gamma spectra, comparing to total spectra simulated, produce a decrease on absorbed dose on cardiac tissue of 19.2% and 7.1%, respectively. Conclusion: The results shown that the combined transport of electrons Auger and characteristics x‐rays of the Tc‐99m increase the radiation dose, especially on organs/tissues closer to those have had absorbed the radiopharmaceuticals. This study indicates the importance on describe the complete radiopharmaceuticals spectrum on dosimetric simulations in Nuclear Medicine.

Tau energy losses at ultrahigh energy: Continuous versus stochastic treatment

We study the energy losses of the tau lepton in matter through electromagnetic processes at ultrahigh energy. We use both a stochastic and a continuous framework to treat these interactions and compare the flux of tau leptons propagated after some amount of matter. We discuss the accuracy of the approximation of continuous energy losses by studying the propagation in standard rock of taus with both monoenergetic and power-law injection spectra. Despite fluctuations introduced by the stochastic nature of the radiative processes, we show that the continuous approximation leads to the same results for power-law injection spectra.

Calibration procedure for a neutron monitor at energies below 20 MeV

A liquid scintillation detector aimed for neutron energy and fluence measurements in the energy region below 20MeV has been calibrated using monoenergetic and white spectrum neutron fields. Careful measurements of the proton light output function and the response matrix have been performed allowing for the application of unfolding techniques using existing codes. The response matrix is used to characterize monoenergetic neutron fields produced by the T(d,n) reaction at low deuteron energies.

Neutron and γ-radiation field characteristics for 14 MeV neutron generators used with a stilbene-crystal spectrometer

Neutron and γ-ray field characteristics have been examined for two 14 MeV neutron generators by the use of spheres and special cylindrical systems composed of various materials. Measurements have been made on the energy resolution of the spectrometer fitted with a stilbene scintillation crystal in relation to monoenergetic 14 MeV neutrons and the broadening of the monoenergetic neutron spectrum due to interaction with a target material block.

Spectral shaping of laser generated proton beams

The rapid progress in the field of laser particle acceleration has stimulated a debate about the promising perspectives of laser based ion beam sources. For a long time, the beams produced exhibited quasi-thermal spectra. Recent proof-of-principle experiments demonstrated that ion beams with narrow energy distribution can be generated from special target geometries. However, the achieved spectra were strongly limited in terms of monochromacity and reproducibility. We show that microstructured targets can be used to reliably produce protons with monoenergetic spectra above 2 MeV with less than 10% energy spread. Detailed investigations of the effects of laser ablation on the target resulted in a significant improvement of the reproducibility. Based on statistical analysis, we derive a scaling law between proton peak position and laser energy, underlining the suitability of this method for future applications. Both the quality of the spectra and the scaling law are well reproduced by numerical simulations.

Energy, angular and spatial distributions of primary electrons inside photoconducting materials for digital mammography: Monte Carlo simulation studies

Materials such as a-Se, a-As2Se3, GaSe, GaAs, Ge, CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbO, TlBr, PbI2 and HgI2 are potential candidates as photoconductors in direct detectors for digital mammography. The x-ray induced primary electrons inside a photoconductor's bulk comprise the initial signal that propagates and forms the final signal (image) on the detector's electrodes. An already developed model for a-Se has been properly extended to simulate the primary electron production in the materials mentioned. Primary electron characteristics, such as their energy, angular and spatial distributions that strongly influence the characteristics of the final image, were studied for both monoenergetic and polyenergetic x-ray spectra in the mammographic energy range. The characteristic feature in the electron energy distributions for PbI2 and HgI2 is the atomic deexcitation peaks, whereas for the rest of the materials their shape can also be influenced by the electrons produced from primary photons. The electrons have a small tendency to be forward ejected whereas they prefer to be ejected perpendicular (θ = π/2) to the incident beam's axis and at two lobes around φ = 0 and φ = π. At practical mammographic energies (15–40 keV) a-Se, a-As2Se3 and Ge have the minimum azimuthal uniformity whereas CdZnTe, Cd0.8Zn0.2Te and CdTe the maximum one. The spatial distributions for a-Se, a-As2Se3, GaSe, GaAs, Ge, PbO and TlBr are almost independent of the polyenergetic spectrum, while those for CdTe, CdZnTe, Cd0.8Zn0.2Te, ZnTe, PbI2 and HgI2 have a spectrum dependence. In the practical mammographic energy range and at this primitive stage of primary electron production, a-Se has the best inherent spatial resolution as compared to the rest of the photoconductors. PbO has the minimum bulk space in which electrons can be produced whereas CdTe has the maximum one.

MONO-ENERGETIC ELECTRON GENERATION AND PLASMA DIAGNOSIS EXPERIMENTS IN A LASER PLASMA CATHODE

Acceleration of plasma electrons by laser wake-filed is the most promising process to produce the next generation of compact accelerators because of its ultrahigh acceleration gradient. To achieve the efficient and stable mono-energetic acceleration of plasma electrons, we investigated the laser-plasma interaction and its correlation to electron acceleration through single shot measurements. We observed a thin laser channel when a mono-energetic spectrum was generated.

A monitor for neutron flux measurements up to 20 MeV

A liquid scintillation detector aimed for neutron energy and fluence measurements in the energy region <20 MeV has been calibrated using monoenergetic and white spectrum neutron fields. Careful measurements of the proton light output function and the response matrix have been performed allowing for the application of unfolding techniques using existing codes. The response matrix is used to characterise monoenergetic neutron fields produced by the T(d,n) at a low-energy deuteron accelerator installed at the Swedish Defense Research Agency (FOI).