Monoenergetic Gamma-ray Source Research Articles

Application of the MCNP 5 code to simulate the shielding features of concrete samples with different aggregates

The gamma ray attenuation parameters for three different series of concretes containing different ratios of basalt, magnetite, and bismite mineral aggregates were studied using the Monte Carlo simulation MCNP 5 code. The simulation was performed using a monoenergetic gamma ray source at an energy range from 0.02 MeV to 10 MeV. The simulation agreed with results obtained via XCOM at all energies. The highest mass attenuation coefficient was found in the sample containing 50% bismite aggregate (Bi 50), while the lowest mass attenuation coefficient was found in the 100% basalt aggregate (OD). The results revealed that the concrete samples with bismite and mixed magnetite-bismite mineral aggregates had lower half value layers than the concrete samples with magnetite and basalt aggregates. The concrete encoded OD had the highest half value layer values, varying from 0.01 to 13.75 cm, while the Bi 50 concrete sample had the lowest HVT, varying from 0.004 to 6.15 cm at the investigated energies. The effective atomic numbers of the concrete samples were also evaluated. The effective atomic number values increased as the magnetite ratios in the magnetite and mixed magnetite-bismite concrete samples increased and decreased as the bismite ratios decreased.

Modeling an HPGe detector response to gamma-rays using MCNP5 code

The response function is the important information for the precise interpretation of experimental data and also for characterizing the developing nuclear instruments. Measurement of the response function normally requires a number of mono-energetic gamma-ray sources, a long acquisition time and an appropriate experimental setup. The Monte Carlo method, as an alternative to response function measurement, has widely been used and recommended. In this study, a computational model of an HPGe detector has been developed by using the MCNP5 code. To validate the simulated model, the simulations from mono-energetic sources have been compared to the corresponding measured data. Any deviation from the measurement could be attributed to the unmodeled details of the detector crystal, so they needed adjustment. Moreover, an analysis has been undertaken on the dependency of detection efficiency on the dead layer thickness of the germanium crystal. Having developed a computational model of the crystal, a set of correction factors was extracted to take into account the gamma-ray self-absorption within the source volume. The simulated model of the HPGe detector in this study can be used to calculate the detection efficiency when the samples are not of the standard geometry which require self-absorption considerations.

Ultrafast gamma-ray line emission driven by laser-accelerated ion beams

The Monte Carlo code Geant4 is used to demonstrate ultrafast gamma-ray line emission from the 12C +12C reaction driven by laser-accelerated ion beams. With carbon ions of the order of tens of MeV accelerated by laser-bombarding the carbon target, characteristic gamma-ray line emission is generated with durations as short as picoseconds through ultrafast gamma-ray transitions. Because the relative bandwidths of such gamma-ray lines can be reduced to less than 1% and the yield rate of gamma photons is around 10−5 for an incident carbon ion, this gamma-ray line emission can be used as an ultrafast monoenergetic gamma-ray source with a flux of 1.2 × 106 photons/J into 4π. We also show that the bremsstrahlung background induced by electrons accelerated simultaneously with the carbon ions can be mitigated by choosing a proper detection angle and an optimized target thickness.

Ambient dose equivalent conversion coefficients for radionuclides exponentially distributed in the ground

Conversion coefficients of radionuclide deposition density to the ambient dose equivalent rate at 1 m height above ground were calculated for exponentially distributed sources in the ground. First, Monte Carlo transport simulations assuming exponential distributions in the ground were performed to obtain ambient dose equivalent for mono-energetic gamma-ray sources having different relaxation depths; next, on the basis of the simulated data, conversion coefficients for radionuclides were composed considering recent nuclear decay data. The ambient dose equivalent rates were then compared to the effective dose rates for reference adults and a new-born baby as well as to air kerma rates quoted from previous studies. It was confirmed that the ambient dose equivalent sufficiently overestimates effective doses, independently of age, for sources exponentially distributed in the ground. Furthermore, the air kerma was found to also overestimate the effective doses for all ages in the same conditions. In order to verify the computed conversion coefficients, the ratio of ambient dose equivalent to air kerma obtained by simulation was compared to the ratios measured at hundreds of locations in Japan which have been contaminated with radioactive cesium after the accident at the nuclear power plant in Fukushima Prefecture, Japan, in 2011; a good agreement was observed.

Monte Carlo calculations of the response features for NaI detectors

The response features for sodium iodide detector for a monoenergetic gamma-ray source are calculated because of the importance of this type of detector in industry and research. The response features calculated are: full energy peak, single scattered escape, single escape peak, double scattered escape, double escape peak, besides single scatter, double scatter and triple scatter continua. These features are compared for different sizes and geometric efficiencies of the detector. The Monte Carlo simulation provides the weighting factors of the previously mentioned eight features of the response for the Nal detector that are necessary for response function generation for that type of detectors.

Gamma-ray pulse-height response of an NE-213 scintillation detector

Gamma-ray response functions were measured for an NE-213 scintillation detector using a superconducting electron microtron with a bremsstrahlung monochromator which provided over 100 monoenergetic gamma-ray sources between 2 and 11.5 MeV. The absolute measurements were used to construct a gamma-ray response matrix to unfold NE-213 gamma-ray pulse-height data. The energy spectra of gamma rays from 24Na, PuBe, and 252Cf sources were measured and unfolded as a test of the response matrix.

Measurements of primary spectra from a kilocurie 60Co unit and a 6 MeV linear accelerator.

Spectral measurements have been made of gamma radiation from a T.E.M. Stabilatron kilocurie cobalt 60 unit and of roentgen radiation from a Varian Clinac 6 MeV linear accelerator with a NaI crystal spectrometer, both units normally used by us for radiation therapy. The high photon flux density from these two types of radiation sources, especially the pulsating radiation from the accelerator, necessitates a reduction in the intensity. This is performed by spreading in A1 foil (SCRIMGER & CORMACK 1963, BENTLEY et coll. 1967, LOFROTH 1970), which reduces both intensity and energy, and by collimation. Multiple counts are reduced by means of a dual delay line amplifier (CHASE & SVELTO 1961 ) . A response matrix (SAM et coll. 1968) for the detector was constructed from the monoenergetic gamma ray source spectra and applied to obtain the gamma ray and roentgen spectra from the measured pulse height distribution. An iterative unfolding procedure ( SKARSGARD et coll. 1961 ) solved the matrix equation:

Moments Method Calculation of Buildup Factors for Point Isotropic Monoenergetic Gamma-Ray Sources at Depths Greater than 20 Mean-Free-Paths

A method of extending moments method calculations for monoenergetic, point isotropic gamma ray sources to depths of approximately 50 mean-free-paths is described. Sample calculations are given for water and aluminum. A method of error analysis recently developed by Spencer suggests that for the range from 10 to ∼50 mean-free-paths, the errors associated with the reconstruction of the flux from moments are generally <1%. Comparisons are made between the asymptotic, deep penetration trend of the calculated results and the trend predicted by a theory developed by Fano.

The General Atomic Coupled Electron Linear Accelerator

The General Atomic Linear Accelerator Facility consists of two L-band linear accelerators which can be operated independently or coupled together to form one machine. The first accelerator is a single section powered by a 20-MW klystron and is capable of operating over an energy range of one MeV to 17 MeV. Peak currents of 16 A at 12 MeV and 40 ns pulse length have been obtained with the first Linac. The second Linac, which is built in line with the first machine, consists of three sections powered by one 10-MW and two 20-MW klystrons. The second Linac is capable of producing a 35-MeV electron beam with an intensity of 2 A at pulse length less than 0.1 μs. By rotating the injector of the second Linac it accepts the beam from the first machine, thereby creating a higher energy machine capable of delivering energies of up to 80 MeV. Other features of the facility include six independent irradiated cells, 15 target positions, and a high power positron converter; as well as monoenergetic gamma ray sources, high intensity neutron targets, and a single particle per pulse capability. In addition, a special cell contains the APFA-III accelerator-pulsed fast burst reactor, which is operated at a multiplication of 1000 to obtain neutron fluences of 1012 - 1013 n/cm2 in 5 μs.

The Single-Scattering Approximation to the Solution of the Gamma-Ray Air-Scattering Problem

The results of a calculation using the single-scattering approximation, with no exponential attenuation or buildup, is compared tc results of a Monte Carlo calculation of the dose rate and number flux in air from a monoenergetic gamma-ray source. The comparison shows that the simple approximation is generally adequate for unshielded detectors. (auth)