Gamma-ray Energy Spectra Research Articles

Detection of Gamma Rays of up to 50 T[CLC]e[/CLC]V from the Crab Nebula

Gamma rays with energies greater than 7 TeV from the Crab pulsar/nebula have been observed at large zenith angles, using the Imaging Atmospheric Technique from Woomera, South Australia. CANGAROO data taken in 1992, 1993 and 1995 indicate that the energy spectrum extends up to at least 50 TeV, without a change of the index of the power law spectrum. The observed differential spectrum is \noindent $(2.01\pm 0.36)\times 10^{-13}(E/{7 TeV})^{-2.53 \pm 0.18} TeV^{-1}cm^{-2}s^{-1}$ between 7 TeV and 50 TeV. There is no apparent cut-off. The spectrum for photon energies above $\sim$10 TeV allows the maximum particle acceleration energy to be inferred, and implies that this unpulsed emission does not originate near the light cylinder of the pulsar, but in the nebula where the magnetic field is not strong enough to allow pair creation from the TeV photons. The hard gamma-ray energy spectrum above 10 TeV also provides information about the varying role of seed photons for the inverse Compton process at these high energies, as well as a possible contribution of $\pi ^{\circ}$-gamma rays from proton collisions.

Improvement of energy spectrum characteristics of CdZnTe semiconductor detector by means of digital waveform processing with neural network algorithm

We have studied the improvement of gamma-ray energy spectrum characteristics of a CdZnTe semiconductor detector by means of a neural network algorithm. The neural network recognizes the pulse shapes and determines the corrective magnification factors of pulse heights. After the corrective procedure with the network, the gamma-ray energy spectra of some radioisotope sources become more ideal with better energy resolution.

Determination of Multiply Scattered Gamma Ray Energy Spectra in Water by the Monte-Carlo Method

The spectral distribution of multiply scattered gamma ray energy in water is determined by the Monte Carlo sampling technique in the energy range 0.5 - 1.6 MeV. It is found that the energy for which the scattered photon number is a maximum, is related to the source energy and this is in agreement with studies made by Minato for a point source in different media.

Gamma-ray burst energy spectra: theoretical models, old and new

The modelling of gamma-ray burst (GRB) spectra has considerable potential for increasing the understanding of these enigmatic sources. A diversity of ideas and analyses has been generated over the last two decades to explain line features and continuum shapes, encompassing both older galactic neutron star and “new age” cosmological source models. This paper reviews some of the highlights of these studies, discussing the merits and limitations of various ideas, and in particular their compatibility with the observational data. The first focus will be on continuum models for GRBs, which include optically thin synchrotron emission and resonant Compton upscattering near galactic neutron stars, while the synchrotron and non-magnetic inverse Compton scattering mechanisms are prominent in the less well-developed cosmological scenarios. Line formation scenarios will then be discussed, in particular the scattering model for producing cyclotron features, which remains the only viable explanation for the Ginga observations of double lines. Absorption-like line production in cosmological burst models is generally difficult, though interesting notions such as femtolensing interference patterns have been proffered.

Approximating Model for Multilayer Gamma-Ray Buildup Factors by Transmission Matrix Method: Application to Point Isotropic Source Geometry

An approximating formula recently proposed by the authors for gamma-ray buildup factors of multilayered shields is applied to point isotropic source problems.The formula, which is formulated in vector form with a four-group approximation, handles the gamma-ray energy spectrum directly and uses the transmission and albedo matrices to take gamma-ray transmission and back-scattering effects into consideration. The gamma-ray transmission and back-scattering probabilities through a 1-mean-free-path-(mfp-) thick shell depend on the shell curvature. This phenomenon plays an important role in simulating the gamma-ray buildup factor in point isotropic source geometry. In this model, the dependence is described by simplified expressions. The feasibility of the formula for systematically describing the point isotropic buildup factors was tested by using buildup factors calculated by the Monte Carlo method as reference data. The materials used in the tests were water, iron, and lead, and the source energies assumed were 0.5, 1, and 10MeV. Through the tests, the method was found to reproduce the reference data of double-layered shields of these materials very well. With the same parameters, the buildup factors of three-layered shields are also reproducible. Buildup factors computed with two different group structures were examined to test the adequacy of the energy group structure adopted. The group structure previously adopted was found to be adequate in the energy range of 0.5 to 10 MeV

On the accuracy of determination of fluxes and energy spectra of primary gamma rays by a single imaging atmospheric Cherenkov telescope

The strong energy dependence of the detection efficiency of gamma -rays by imaging atmospheric Cherenkov telescopes (IACT) both at the hardware and software levels, leads to significant uncertainties in the estimate of primary gamma -ray fluxes. Nevertheless, detailed Monte Carlo calculations of the principal characteristics of IACTS, in particular of collection areas and of the gamma /proton separation efficiency, allow a rather high accuracy in the evaluation of the absolute gamma -ray fluxes and in the determination of the spectral index of power-law spectra. However, the poor energy resolution of a single IACT does not permit us to distinguish features such as sharp cutoffs in primary gamma -ray spectra unambiguously. Extended sources cause additional problems in the separation and determination of the gamma -ray flux.

A computer-controlled system for slow positron implantation spectroscopy

The design and construction of a computer-controlled beam transport system and its application to slow positron implantation spectroscopy is described. The control features include the facility to vary the positron beam energy, observe visually the associated spatial movement of the beam profile, and change the beam position so as to hit the sample centrally at all energies selected. This procedure allows the automatic collection of annihilation gamma ray energy spectra and calculation of lineshape parameters as a function of incident positron energy. Examples of such measurements for metallic and semiconductor samples are used to illustrate the performance of the system.

A New Approximating Model for Gamma-Ray Buildup Factors of Stratified Shields

A new approximate expression for gamma-ray buildup factors of multilayered shields is proposed. The expression is formulated based on the vector form and considers the gamma-ray energy spectrum directly. It treats the gamma-ray transmission by a transmission matrix and the backscattering by an albedo matrix. Its capability of reproducing the buildup factors for multilayered shields is demonstrate by using double-layered shields composed of two materials of water, iron, and lead at 1 and 10 MeV. The data of three-layered shields of these materials are also very well reproduced. The mechanism of the density effect arising, which appears in the buildup factor for a point isotropic source, is clearly interpreted by the current method to be a geometrical effect. A correction factor for incorporating the density effect into the current expression is derived. The modified expression is successfully applied to buildup factors for a 0.5-MeV point isotropic source for two-layered shields of water and iron.

Saturation of the width of the giant dipole resonance at high excitation energies

Gamma-ray energy spectra have been measured for the 16O + 118Sn reaction at incident beam energies of 200 and 280 MeV. Comparing the experimentally obtained spectra with statistical-model calculations shows that the width of the isovector giant dipole resonance saturates beyond excitation energies of 165 MeV in the compound nucleus.

Preliminary investigation to test extragalactic hypothesis for gamma-ray bursts

Recent observations of CGRO satellite challenged the almost universally accepted galactic origin of gamma-ray bursts and have revived the extragalactic, cosmological models. This hypothesis should imply some variations of the morphological and energetic characteristics of the weaker events to which we think the farthest ones might belong. The existence of such variations due to the cosmological red-shift should be tested. In this paper we examine the possible correlations between temporal morphology of the gamma-ray bursts, peak intensity and photon energy spectra when the red-shift effects do not yet occur. We examine the peak intensities and the energy spectral-hardness distributionversus rise times for events observed by previous space missions, with a logN-logS shape free of cosmological effects. The results of this analysis suggest that a wide range of values of the temporal and spectral characteristic parameters is an intrinsic property of the sources.

Defects in electron-irradiated 3C-SiC epilayers observed by positron annihilation

Positron annihilation has been used to study defects induced by 1 MeV electron irradiation in cubic silicon carbide (3C-SiC) epitaxially grown on Si substrates by chemical vapor deposition. Narrowing of the Doppler-broadened energy spectrum of annihilation gamma-rays by the electron irradiation was observed in the fluence range above 5 × 1016 e/cm2. The electron fluence dependence of the narrowing is accounted for by the introduction of monovacancies and divacancies in 3C-SiC by the irradiation.

Search for 100 TeV gamma-ray emission from the Galactic disk

Upper limits have been obtained to the diffuse gamma-ray emission from the Galactic disk. The limits I(gamma)/I(p) less than 0.2 percent, i.e., I(gamma)(greater than 130 TeV) less than 3.2 x 10 exp -13/q cm/s/rad provide the first measurement obtained in the angular window corresponding to the distribution of the ISM. It is still, however, a factor of about 30 higher than the flux calculated from the cosmic ray interactions by Berezinsky and Kudryavtsev (1990). A lower limit of gamma greater than 2.3 is obtained to the power-law index of the differential energy spectrum of Galactic gamma rays from 100 MeV to 100 TeV, setting a limit to the possibility of a hard extrapolation of the Galactic gamma-ray spectrum to the 100 TeV region. 21 refs.

Positron Annihilation and Electron Spin Resonance of Electron-Irradiated 3C-SiC

ABSTRACTPositron annihilation and electron spin resonance (ESR) have been used to study defects introduced by lMeV electron irradiation in n-type cubic silicon carbide (3C-SiC) epitaxially grown on Si by chemical vapor deposition. Positron annihilation measurements by using variable-energy positron beams indicated the narrowing of the Doppler-broadened energy spectrum of annihilation gamma-rays and the decrease in the effective diffusion length of positrons with increasing the electron fluence. These results show the formation of vacancy-type defects in 3C-SiC. An ESR spectrum labeled T1, which has an isotropie g-value of 2.0029±0.0001, was observed in electron irradiated 3C-SiC. The T1 spectrum is interpreted by hyperfine interactions of paramagnetic electrons with 13C at four carbon sites and 29Si at twelve silicon sites, leads that the Tl center results from a point defect at a silicon sublattice site. The production rate of the Tl center was in good agreement with the carrier removal rate, indicating that the Tl center captures an electron from the conduction band. All these results are accounted for by the introduction of negatively charged vacancies at silicon sublattice sites in 3C-SiC by the irradiation.

Search for photons from the decay of the delta resonance in heavy-ion collisions.

Results are presented from a search for high-energy gamma rays (${\mathit{E}}_{\ensuremath{\gamma}}$g140 MeV) resulting from the decay of the delta resonance produced in heavy-ion collisions. The energy spectrum of gamma rays in the energy range 15${\mathit{E}}_{\ensuremath{\gamma}}$180 MeV was measured at a laboratory angle of 90\ifmmode^\circ\else\textdegree\fi{} for the reaction $^{14}\mathrm{N}$+Zn. The incident beam energy was E/A=75 MeV. The energy spectrum in the region 50${\mathit{E}}_{\ensuremath{\gamma}}$180 MeV is exponential with an inverse slope parameter of 26 MeV. There is no indication of an enhancement in the spectrum above 140 MeV due to the decay of the delta resonance.

GSAP: Fortran code for gamma-spectrum analysis

The program GSAP performs fully automatic evaluation of gamma-ray energy spectra measured with semiconductor detectors. After inputting data comprising the experimental spectrum, energy and FWHM calibrations, and parameters controlling the peak search are supplied. The program starts peak searching from the spectrum beginning. The detected peaks are arranged into multiplets, which are unfolded by standard non-linear least-squares fits assuming Gaussian peak shape and linear background. The program proceeds until all multiplets are processed. The determined peak parameters are printed and the result of each particular fit is shown in graphical form.

Measurement of underground gamma ray spectra for solar neutrino studies

The energy spectrum of underground gamma rays is measured up to 20 MeV, using a large sodium iodide crystal assembly at a depth of 1230 m in a silver, lead and zinc mine. The results are compared with other experiments and their significance discussed. The effect of this gamma ray background is assessed in terms of the Sydney solar neutrino detector, SUNLAB, that is currently being constructed. In particular, this background defines an acceptable range of threshold energies. Beyond this range, the solar neutrino signal cannot be established within a reasonable running time.

Mass dependence of high-energy gamma-ray production in heavy-ion reactions.

Energy spectra and angular distributions were measured for high-energy gamma rays (E/sub ..gamma../>25 MeV) from the reactions /sup 7/Li+Li, /sup 7/Li+Pb, /sup 20/Ne+Mg, /sup 40/Ar+Ca, and /sup 40/Ar+Pb at incident beam energies of E/A = 30 MeV. The gamma-ray energy spectra for E/sub ..gamma../>25 MeV are roughly exponential and were observable to E/sub ..gamma..//similar to/100 MeV. The angular distributions of the gamma rays are slightly forward peaked in the laboratory frame and are consistent with emission from a single source traveling at the nucleon-nucleon center-of-mass velocity. The angular distribution in the nucleon-nucleon frame shows a substantial dipole component. The gamma-ray production cross section is roughly proportional to (A/sub proj/A/sub target/)/sup 2/3/ or to the calculated mean number of first neutron-proton collisions.

High-energy gamma-ray production in light-ion induced reactions.

Results are presented for high-energy gamma-ray production in light-ion induced reactions. Energy spectra and angular distributions were measured for high-energy gamma rays (E/sub /gamma//>20 MeV) from the reactions /sup 2/H on C, Zn, and Pb at E/A = 53 MeV, and /sup 4/He on C, Zn, and Pb at E/A = 25 and 53 MeV. The gamma-ray energy spectra are roughly exponential, and for /sup 2/H at E/A = 53 MeV extend out to the full available energy (/similar to/00 MeV). The angular distributions of the gamma rays are slightly forward peaked in the laboratory frame, but are symmetric about 90/sup 0/ when transformed to the nucleon-nucleon center-of-mass frame. For the carbon target data the angular distribution in the nucleon-nucleon frame shows a substantial dipole component. This suggests that the major high-energy gamma-ray production mechanism is bremsstrahlung from first collisions of projectile and target nucleons.

Gamma ray measurements during deuterium and 3He discharges on TFTR

Gamma ray count rates and energy spectra have been measured in TFTR deuterium plasmas during ohmic heating and during injection of deuterium neutral beams for total neutron source strengths up to 6 × 10 15 neutrons per second. The gamma ray measurements for the deuterium plasmas are in general agreement with predictions obtained using simplified transport models. The 16.6 MeV fusion gamma ray from the direct capture reaction D( 3He, γ) 5Li was observed during deuterium neutral beam injection into 3He plasmas for beam powers up to 7 MW. The measured yield of the 16.6 MeV gamma ray is consistent with the predicted yield. The observation of this capture gamma ray establishes the spectroscopy of the fusion gamma rays from the D- 3He reactions as a viable diagnostic of total fusion reaction rates and benchmarks the modeling for extension of the technique to D-T plasmas.

A Borehole Correction Model for Capture Gamma Ray Spectroscopy Logging Tools

Summary A borehole correction model, based onmore than 1,000 laboratory measurements, has been developed for logging tools that measure gamma ray energy spectra induced by the capture of thermal-energy neutrons. The measurements were made with a field-worthy tool incorporating a pulsed source of 14-MeV neutrons and a NaI(TI) gamma ray detector. In addition to the obvious geometric dependencies on borehole and casing sizes, the sensitivity of the measurement to elements within the borehole was found to depend just as strongly on the porosity and salinity of the borehole and formation fluids. The model decouples the geometric effects from most of the nuclear physics effects by explicitly including the time decay of the neutron flux in each homogeneous region.