MeV Γ-rays Research Articles

Relevance of thermally populated first excited state of 44Ti to the abundance problem of cassiopiea A: A model study

We examine the possible role of electron-capture on the thermally populated first 2+ excited state of 44Ti in hot astrophysical environments pertaining to post explosive nucleosynthesis supernova debris. We find in a simple schematic model that the astrophysical weak interaction rate for electron-capture decay of 44Ti can depend considerably on temperature and hence on time. We propose a time varying decay rate for the evolving supernova debris and demonstrate its consequence for the 44Ti mass yield of the supernova Cas A, observed through the measured 1.157 MeV γ-ray flux from the electron-capture decay of 44Ti.

Doppler broadening spectroscopy using the FAST-ComTec two-dimensional coincidence system: a case study

Positron annihilation Doppler broadening spectroscopy using the two-detector coincidence method turned out to be an important tool for the study and identification of defects and their chemical surrounding due to a strong reduction of the background. The lowest background (∼10 −5) is obtained by using two Ge-detectors measuring the energy of both annihilation γ-rays. We used the FAST-ComTec multiparameter system which may be considered as a standard for that purpose. A systematic study of measurement parameters influencing the results in such a system was done. The spectra were recorded in list mode (i.e., event by event) allowing off-line stabilization during a replay. The choice of optimum parameters for the stabilization is discussed. We found, however, stabilization in most cases not necessary for the observation of high momentum spectra. The use of 22 Na foil sources in this study implies a relatively high background due to 1.28 MeV γ-rays from the source. However, no significant influence of the source activity on the spectra was found. It is thus possible to use sources of high activity (up to 70 μCi) to obtain a sufficient count rate for coincidence measurements with high statistics.

The influence of high iron content in coal ash on measurement of 4.43 MeV carbon γ-rays

In the paper is considered the possibility of compensating the influence of anomalously high iron content in coal on its calorific value determination using 4.43 MeV carbon γ-ray from the 12 C (n, n'γ) 12 C reaction. The prominent 7.631 MeV and 7.645 MeV γ-rays from thermal neutron capture on iron have been used in the correction formula which gave significant improvement of the accuracy of carbon content and calorific value determination.

Origin of MeV gamma-ray emissions from blazars

We have developed a model for MeV emission from blazars that involves the production of MeV γ-rays through Inverse Compton (IC) scattering of electrons in the blob of a jet, with the UV photons of the accretion disk. The OSSE and the COMPTEL spectra of PKS 0528 + 134 have been fitted with the computed MeV γ-ray fluxes of the above model. A good fit was obtained with electrons of Lorentz factor of 10 1.5 to 10 3 and a jet with viewing angle of ≤ 15° and an accretion disk temperature in the range of 2.0 to 6.0 × 10 5 K.

Scattering of muonic hydrogen atoms

Our measurement compares the energy dependence of the scattering cross-sections of muonic deuterium and tritium on hydrogen molecules for collisions in the energy range 0.1–45 eV. A time-of-flight method was used to measure the scattering cross-section as a function of the muonic atom beam energy and shows clearly the Ramsauer–Townsend effect. The results are compared with theoretical calculations by using Monte Carlo simulations. The molecular pdµ and ptµ formation creates background processes. We measure the formation rates in solid hydrogen by detecting the 5.5 MeV (pdµ) and 19.8 MeV (ptµ) γ-rays emitted during the subsequent nuclear fusion processes.

Energy response of a novel compact Ge+BGO spectrometer for 4–20 MeV γ-rays

The energy response of a novel composite Ge+BGO spectrometer has been measured for high-energy (4–20 MeV) photons. The spectrometer consists of a cooled Ge solid state central detector surrounded by a single BGO scintillator crystal without having interfering materials in between. Good energy resolution (≈1.7%) combined with a considerable photopeak efficiency ( ε≈17%) have been obtained at 15.1 MeV in “adding-back” mode. GEANT Monte-Carlo code simulations reproduce the measured response rather well and show a very good suppression factor against neutrons (≥100) and also against cosmic rays (≥3000) for this composite spectrometer.

Energy and efficiency calibration of an array of six Euroball Cluster detectors used for beta-decay studies

The use of an array of six Euroball Cluster detectors, i.e. 42 large-volume germanium detectors, for beta-decay studies is described. The solid angle with respect to a source placed in the center of the array amounts to 65% of 4π sr. The total photo-peak efficiency for 1.33 MeV γ-rays is 10.2(5)%, without “adding back” the Compton-scattered events. For this γ-ray energy, the energy resolution of the entire setup is 2.8 keV. The performance of the array up to γ-ray energies of 8 MeV is discussed, and alternative ways of determining the photo-peak efficiency are presented.

Yield of nuclear reaction products from deuterium-saturated composite materials and layered structures

We have studied the nuclear reaction products emitted from layered structures and composite materials saturated with deuterium. As a rule, the neutron output from our samples was at the background level. The γ-ray output from the Ti+Si, Ti+Pd, and Ti+ceramic samples was 1.5–2 times the background level. The excess output of γ-rays is accompanied by an increase in acoustic emissions (AE). We measured the energy spectrum of the γ-rays for the Ti+ceramic sample, and found that near-3.0 MeV γ-rays were emitted with the highest probability.

Cosmic-Ray [TSUP]26[/TSUP]A[CLC]l[/CLC] and Its Decay in the Galaxy

Radioactive 26Al (τ1/2 = 8.7 × 105 yr) has been resolved from stable 27Al in the cosmic radiation for the first time using the high mass resolution, charged particle telescope on the Ulysses spacecraft. The average density of the interstellar medium through which the cosmic radiation propagates was determined within the framework of a homogeneous propagation model, where all 26Al is of cosmic-ray spallation origin. Heliospheric modulation of the two isotopes was included. The derived ratio 26Al/27Al = 0.061±0.007 yields an average interstellar medium density ρ = 0.26+0.05−0.04 atoms cm−3 and a Galactic confinement time of 19±3 million years. These values agree with the results from radioactive 10Be analysis and indicate that the cosmic radiation may spend part of its confinement time in a Galactic magnetic halo. The implications of the observed 1.809 MeV γ-rays from 26Al decay to 26Mg throughout the Galaxy are discussed with respect to 26Al, 27Al, and 26Mg in the cosmic radiation and their injection into the interstellar medium from nucleosynthesis in supernovae, novae, and red giant stars.

A Combined Model for the X‐Ray to Gamma‐Ray Emission of Cygnus X‐1

We use recent data obtained by three (OSSE, BATSE, and COMPTEL) of four instruments on board the Compton Gamma Ray Observatory (CGRO) to construct a model of Cyg X-1 that describes its emission in a broad energy range, from soft X-rays to MeV γ-rays, self-consistently. The γ-ray emission is interpreted to be the result of Comptonization, bremsstrahlung, and positron annihilation in a hot, optically thin, and spatially extended region surrounding the whole accretion disk. For the X-ray emission, a standard corona-disk model is applied. We show that the Cyg X-1 spectrum accumulated by the CGRO instruments during a ~4 year time period between 1991 and 1995, as well as the HEAO 3 γ1 and γ2 spectra, can be well represented by our model. The derived parameters match the observational results obtained from X-ray measurements.

Total cross section of the 14N(d,pγ) 15N nuclear reaction for analytical applications

In order to supply long-needed basic data for nitrogen determination in thin films by the deuteron induced gamma-ray emission method (DIGE, or d-PIGE), gamma yields of the reaction 14N(d,pγ) 15N were measured by bombarding thin Si 3N 4 films on silicon substrates. The γ-rays were detected by a high efficiency detection system (solid angle around 4π, photo-peak-efficiency 47% for 8 MeV γ-rays) including a 12 in. × 12 in. NaI(Tl) bore hole detector. Total cross section curves for the γ-rays of 7.3 and 8.3 MeV are presented in the deuteron energy range of 500–1500 keV. An example of its use in determining nitrogen content of an ultra-thin silicon oxynitride films is given.

The Influence of Grain Size on the Response of Emulsions to Ionizing Radiation

The relation between speed and emulsion grain size is examined for exposure of a grain size series of spherical emulsions to electrons in the range 25-100 keV, to X-rays in the range 20-290 kVp, and to cobalt(60) 1.2 MeV gamma (γ)-rays. II was observed for films processed in a black-and-white developer that the speed increased up to the largest size studied, which had a mean grain projective area of 10 pur, broadly in line with expet tations from simple theory. II was evident that the use of chemical sensitization had little impact on --peed for the fine grain emulsions of size 0.05 µm2 exposed to 20 kVp X-rays, but was crucial for all the emulsions exposed to 1.2 MeV γ-rays.The impact of cobalt(60) on tabular grain emulsions given colour development was studied also because of its relevance to the influence of natural background radiation, there was an increase in speed with grain size but the trend was identical for light and cobalt(bO) radiation. A single tabular emulsion was also examined after these two types of exposure in order to reveal the relation between speed and grain size for the different size classes within the emulsion. Speed increased with increasing grain size, but not as much as expected, and the trends for the two types of exposure were similar, iusl as in the case of the grain size series of tabular grain emulsions.

Gamma-ray detection with a 4π NaI spectrometer for material analysis

A γ-ray detection system with nearly 4π geometry for applications in materials science is described. The energy resolution of the NaI(Tl)-bore hole detector was found to be about 2% for 10 MeV γ-rays. The total detection efficiency has been determined to be above 80% for γ-energies between 0.5 and 20 MeV. A precision of the efficiency calibration of about 2% has been obtained for a wide range of γ-energies. The experimental results were found to be in excellent agreement with computer simulations using the GEANT computer code. The high efficiency of the system combined with its relatively high energy resolution opened a wide range of applications. Examples are the simultaneous detection of light isotopes in thin films by (d,pγ) nuclear reactions, high depth resolution profiling of isotopes by narrow, low-energy resonances in (p,γ) reactions, and hydrogen profiling using the 6.4 MeV resonance in the 1H(15N,αγ)12C reaction.

Development of the bolometer for the β+β+ decay experiment

We developed a bolometer with a 0.5 g CdTe absorber using a high-sensitivity NTD germanium thermistor. An energy resolution of 24 keV (FWHM) is obtained for 0.66 to 1.8 MeV γ-rays. It shows a good linearity of the response in the energy range between 0.66 and 5.5 MeV. The gain is found to be stable for a period of 72 days of continuous operation and relatively insensitive to temperature changes of the refrigerator. The temperature change of the refrigerator for this period is within ±1 mK. Since this material contains double positron emission ( β + β +) nuclides 106Cd, 108Cd and 120Te, a new series of double beta decay experiments may become possible with this new method.

Neutron activation analysis with PE moderated252Cf neutron source

The 1.369 MeV γ-rays emitted from24Na→24Mg+β−+γ after23Na(n,γ)24Na reaction were counted by high purity germanium (HPGe) detector and the half-life of24Na was derived. This process was simulated by Monte Carlo Neutron and Photon Transport Code (MCNP-4A).252Cf neutron source was moderated by a polyethylene (PE) cylinder to increase the cross section of neutron absorption reaction and to decrease the biological hazard. NaCl powder of 20 cm3 and 40 cm3 volume in cylindrical polypropylene capsules were irradiated by the neutrons passed through the moderator. MCNP-4A was used to determine the optimum size of PE moderator, to assume the realistic geometry of the HPGe detector, and to assume the absolute efficiencies of the detector. The count rates for 1.369 MeV γ-rays in the HPGe detector were calculated by MCNP-4A for 20 cm3 and 40 cm3 NaCl. The accumulated counts calculated tumed out to be higher than those actually measured by 31% with a relative error of 3%. The half-life of24Na measured within 4% and 1% for 20 cm3 and 40cm3 NaCl agrees with that of the reference. So, we can say that the result of MCNP-4A has about 30% of accuracy and 3% of precision in simulating the neutron activation analysis.

Gamma-Ray Variability of the BL Lacertae Object Markarian 421

We report on the γ-ray variability of Mrk 421 at Eγ > 300 GeV during the 1995 season, and concentrate on the results of an intense multiwavelength observing campaign in the period April 20 to May 5, which included >100 MeV γ-ray, X-ray, extreme-ultraviolet, optical, and radio observations, some of which show evidence for correlated behavior. Rapid variations in the TeV γ-ray light curve with doubling and decay times of 1 day require a compact emission region and significant Doppler boosting. The TeV data reveal that the γ-ray emission is best characterized by a succession of rapid flares with a relatively low baseline level of steady emission.

Intense Compton γ-ray source from the Duke storage ring FEL

Abstract A 1.1 GeV electron storage ring dedicated for UV-VUV FEL operation was commissioned last year at the Duke University Free Electron Laser Laboratory (DFELL) [1]. The OK-4 FEL was shipped to Duke from Novosibirsk earlier this year and is in the process of installation. The high average intracavity power and natural synchronization of electron and optical pulses in the OK-4 FEL will allow the production of intense beam of inverse Compton 5–150 MeV γ-rays. The projected brightness of this γ-ray source will allow the attainment of high energy resolution by simple geometrical collimation. The wide tunability of the OK-4 FEL will also provide for continuous tuning of the γ-rays energy. In this paper we discuss the influence of beam parameters on the γ-ray energy spread and projected performance of the Duke/OK-4 inverse Compton γ-ray source. A detailed description of this study could be found in Ref. [2].

Energy resolution measurements of benzene, toluene and cyclohexane scintillators for monoenergetic γ-rays

Energy resolution and difference of the maximum energy of Compton electrons from the half-height of the Compton edge was measured experimentally for benzene (C 6H 6), cyclohexane (C 6H 12) and toluene scintillators for monoenergetic γ-rays using a γ-γ coincidence technique. For 0.5–1.2 MeV γ-rays, the energy resolution for C 6H 6, C 6H 12 and toluene scintillators varies from 23.8−12.6%, 28.6−2.5% and 23.5−4.6%, respectively. The corresponding energy difference between the maximum Compton electron energy and the half height of the Compton edge for C 6H 6, C 6H 12 and toluene scintillators varies from 13.2−7.6%, 17.7−10.2% and 10.2−6.8%, respectively. The energy resolution of the C 6H 6 detector agrees within 5–10% of that obtained with an identical C 6D 6 scintillator.

Recent results from COMPTEL

The COMPTEL telescope aboard the Compton Gamma Ray Observatory has put MeV γ-rays into the midst of astronomy. Among recent highlights are the discovery of intense MeV emission from blazar-type active galactic nuclei, the surprising non-detection of any Seyfert galaxy at MeV energies, the first image of the Milky Way in the light of the26Al line at 1.809 MeV (possibly including a detection of the Vela supernova remnant), the discovery of 3–7 MeV emission from the Orion complex, which can be identified with nuclear interaction lines of12C and16O at 4.44 and 6.13 MeV, the detection of the44Ti line at 1.15 MeV from the supernova remnant Cas A, and the first results on the spectrum and propagation of low-energy (1–100 MeV) cosmic-ray electrons.

Determination of microquantities of Be in samples using α-sensitive track etch detectors

A new technique of determining Be content in samples using 4.6∸6 MeV γ-rays is proposed. This energy range is sufficient to knock out the neutron from the 9Be nucleaus and to excite the first 2.9 MeV level of the 8Be nuclei. 1.5 MeV alpha particles are registered by means of CR-39 or cellulose nitrate detectors. Alpha particles produced in the reaction of γ-rays with Li nuclei which are contained in samples may cause the background. The latter is excluded by heating of samples before irradiation for Li molecules to be evaporated. 11B nuclei do not produce alpha particles at the energy range of γ-rays presented above. The results obtained in the experiments of determining Be content in natural beryl crystal samples are in good agreement with their known chemical composition. Microparticles containing Beryllium were found in soil samples from the Ust-Kamenogorsk are deposit region, which contains 10 −3 − 10 −4 g/g of Be. The sensitivity of the method is 10 −6 g/g of Be in solids.