An experimental study on cross-section ratio of coherent to incoherent scattering for 145 keV incident gamma photons

The coherent (Rayleigh) to incoherent (Compton) scattering cross-section ratio of elements, in the range 6 ≤ Z ≤ 82, are determined experimentally for 145 keV incident gamma photons. An HPGe (High purity germanium) semiconductor detector is employed, at scattering angle of 50°, 70° and 90°, to record the spectra originating from interactions of incident gamma photons with the target under investigation. The intensity ratio of Rayleigh to Compton scattered peaks observed in the recorded spectra, and corrected for photo-peak efficiency of gamma detector and absorption of photons in the target and air, along with the other required parameters provides the differential cross-section ratio. The measured values of cross-section ratio are found to agree with theoretical predictions (corresponding to 4.939, 6.704 and 8.264 Å⁻¹ photon momentum transfer) based upon non-relativistic form factor, relativistic form factor, modified form factor and S-matrix theory.     

Measurement of the energy distribution of bremsstrahlung using Compton scattering

The GEANT 4.9.2 software package is applied for the simulation of primary and Compton scattered bremsstrahlung spectra. The possibility of restoring primary γ-radiation spectra with the energy 0.02–1200 MeV using Compton scattering on secondary scattering targets is studied. The advantages of light targets using are demonstrated. An additional bremsstrahlung contribution from e^±-pairs is calculated for high-energy γ quanta; the influence of CXR on the form of scattered X-ray quantum spectra is estimated. The spectra which have a multipeak structure are restored. For high-energy γ quanta there occurs significant spread of energies in the Compton scattered radiation spectrum, which imposes stringent conditions on collimation and detector resolution capability. The possibility of measuring the bremsstrahlung spectra from a primary target during high-angle Compton scattering is estimated.     

Measurement of photon polarization from the three-photon annihilation of orthopositronium

Using a Compton polarization analyzer the linear polarization of one photon from the three-photon decay of orthopositronium under several geometrical conditions has been measured. For α₁=45°, 75°, 105° and 120°, the ration of the number of photons Compton-scattered parallel to the plane of the three photons to the number scattered perpendicular to that plane was found to be 1.20±0.03, 1.53±0.03, 1.76±0.06 and 1.86±0.06, respectively. The linear polarization of the photons along the plane of the three photons was determined as 0.15±0.04, 0.33±0.04, 0.49±0.04 and 0.51±0.05, respectively. This result is in agreement with theoretical predictions.     

Measurement of the effective atomic number of some transition and rare earth compounds using the Rayleigh to Compton scattering ratio of gamma radiation

The effective atomic numbers of some transition and rare earth compounds have been determined by measuring the ratio of Rayleigh to Compton scattering signal using 59.5?keV gamma radiation from americium-241 radioactive source. The scattered gamma photons from the elements and compounds at an angle of 90° were detected using an ORTEC high-purity germanium detector coupled with 16K multi-channel analyzer. By measuring the ratio of Rayleigh to Compton scattered signal (momentum transfer 3.38?Å^?1), the effective atomic numbers of the transition and rare earth compounds have been determined and compared with theoretical values predicted by AutoZeff, power law, and direct method.     

Compton scattering of 59.54 keV γ‐rays from Fe and p‐Si samples in an external magnetic field

We report on the Compton scattering of photons from samples whose surface charge density distributions are changed by an external magnetic field. We performed a Compton scattering experiment known to be particularly sensitive to the behavior of the relatively slower moving outer electrons (valence electrons) involved in bonding in condensed matter. The external magnetic field was used to change the surface charge density distributions of Fe and p‐Si samples. Samples were located in the external magnetic field of intensity 215 G and in a direction which was perpendicular both to the current and surface of the samples bombarded by 59.5 keV γ‐photons emitted from an Am‐241 point source. Currents in the ranges 0–8.5 A and 0–300 µA were applied to the Fe and p‐Si samples, respectively. The Compton scattered photons at an angle of 100° were detected by an Si(Li) detector. It was observed that the counts acquired under the Compton peaks tended to decrease linearly with increasing current in a magnetic field. The results show that positive charge carriers behave like negative charge carriers and electrons are more effective than holes in the Compton scattering of γ‐rays. Copyright © 2003 John Wiley & Sons, Ltd.     

Non-destructive evaluation of scientific and biological samples by scattering of 145 keV gamma rays

The objective of present experiment is to assign effective atomic number (Z_eff) to samples of scientific interest (oxides of lanthanoids, also called rare earths, and alloys of lead and tin of known composition) and to measure stable iodine content of tissue (biological sample). An HPGe semiconductor detector, placed at 70° to the incident beam, detects gamma photons scattered from the sample under investigation. The experiment is performed on various elements with atomic number satisfying, 6 ≤ Z ≤ 82, for 145 keV incident photons. The intensity ratio of Rayleigh to Compton scattered peaks, corrected for photo-peak efficiency of gamma detector and absorption of photons in the sample and air, is plotted as a function of atomic number and constituted a fit curve. From this fit curve, the respective effective atomic numbers of the scientific samples are determined. The agreement of measured values of Z_eff with theoretical calculations is found to be quite satisfactory. The measured intensity ratio from phantom (KI solutions, simulating thyroid content of stable iodine) varies linearly with KI concentration and provides stable iodine content of tissue.     

Components of detector response function: Monte Carlo simulations and experiment

Components of the response function of an HPGe detector for 32 keV incident photons (Ba Kα x‐rays) were studied using a Monte Carlo program. Physical mechanisms and the role of incident photons, detector x‐rays, photoelectrons and Compton recoil and Auger electrons for each component were investigated. The position, intensity and shape of the components, particularly of the photoelectrons, were studied in detail. Two distinct components for photoelectron escape were identified by considering the fate of photoelectrons, Ge x‐rays and Auger electrons produced in the same interaction. In contrast to the often‐cited shelflike structure, it was found that both components have a slope. The contribution of recoil electrons to the spectrum for single, double and multiple Compton scattering followed by photoelectric absorption of the scattered photon was investigated. The results of the Monte Carlo simulations are presented along with the measured Ba x‐ray spectrum. Copyright © 2006 John Wiley & Sons, Ltd.     

Accurate standard‐based quantification of X‐ray fluorescence data using metal‐contaminated plant tissue

Quantitative X‐ray fluorescence (XRF) measurements have been conducted on naturally lead‐contaminated samples. The calibration procedure using the ratio of fluorescence to Compton scattered radiation was investigated using Monte Carlo simulation. Experimental results with low‐energy photons (14 keV) and simulations show a very good linearity of the fluorescence to Compton ratio as a function of metal concentration. Lead (Pb), iron (Fe) and zinc (Zn) are measured in samples of Phaseolus vulgaris (bean seeds) that have been grown using a nutritive solution with different Pb dopings. Naturally contaminated samples are thus obtained. The calibration must be done for fixed conditions of X‐ray energy and scattering angle, while X‐ray beam intensity and detector to sample distance can change from one sample to another. Simulation allows to evaluate the matrix effect on the calibration curve, and shows that linearity is preserved even in the presence of other heavy elements in the fluorescence spectrum. However, calibration must be done using samples with similar matrix as it affects the slope of the curve. Copyright © 2010 John Wiley & Sons, Ltd.     

Relativistic double differential cross sections for Compton scattering by bound electrons: Test at low photon energies and predictions at intermediate photon energies

Cross section profiles d²σ/dΩdω′ for Compton scattering of photons by bound electrons are calculated for all subshells of the atom. Results obtained from the form factor approximation and from a relativistic version of the impulse approximation are compared with experimental data for Cu and Pb at a scattering angle ofθ=145° and a photon energy of 662 keV. The impulse approximation proves to be superior to the form factor approximation and is used to predict cross section profiles for a primary energy of 50 MeV and different scattering angles and charge numbers. It is shown that only for the heaviest atoms and scattering angles belowθ=5° there is a non-negligible contribution of Compton scattering to the elastic peak.     

A new way of measuring the contribution of scattered photons to x‐ray fluorescence yields in secondary excitation mode

The characteristic K x‐rays from a secondary exciter in conjunction with an x‐ray tube or a radioisotope as primary source are frequently used for target excitation in x‐ray fluorescence measurements. In this experimental arrangement, the exciting photon spectrum consists of two parts: (i) characteristic K x‐rays of the secondary exciter and (ii) photons from the primary source, which are scattered from the secondary exciter. A new method is proposed for correcting the observed target x‐ray yield due to unwanted excitation by the scattered photons. This method involves measurement of x‐ray yield after replacing the secondary exciter by an equivalent exciter consisting of a low‐Z element foil with a high‐Z element backing. Copyright © 2004 John Wiley & Sons, Ltd.     

Compton scattering of 59.5 keV gamma rays from p-Si sample in an external electric field

This study is related to Compton scattering of photons from a p-Si sample whose surface charge density distributions are changed by an external electric field. The external electric field intensity in the range 0-75 kV/m was used to change the surface charge density distributions of the sample. The sample surface perpendicular to the electric field was selected as the scattering surface. The p-Si sample was bombarded by 59.5 keV γ-photons emitting from an Am-241 point source. The Compton scattered photons at an angle of 90^o were detected by an Si(Li) detector. The Compton scattering intensity suddenly increased with the application of the electric field since the applied electric field distorts both the negatively charged scattering center (free electron, bound electron, ionized acceptor) and the positively charged scattering center (hole) and their momentum distribution in the sample. There is a good third-order polynominal relation between the Compton scattering intensity and the increasing (or decreasing) electric field intensity. The results show that the positively charged scattering centers behave like negatively charged scattering centers, but the latter are slightly more effective than the former in the Compton scattering of γ-rays from the sample in the electric field.     

Compton scattering in ignited thermonuclear plasmas

Inertially confined, ignited thermonuclear D-T plasmas will produce intense blackbody radiation at temperatures T greater, similar20 keV; it is shown that the injection of GeV electrons into the burning core can efficiently generate high-energy Compton scattering photons. Moreover, the spectrum scattered in a small solid angle can be remarkably monochromatic, due to kinematic pileup; a peak brightness in excess of 10;{30} photons/(mm(2) mrad(2) s 0.1% bandwidth) is predicted. These results are discussed within the context of the Schwinger field and the Sunyaev-Zel'dovich effect.     

Escape of photons and electrons from an HPGe detector at 81 keV

Escape of Ge K x‐rays, Compton‐scattered incident radiation and photoelectrons from an HPGe detector was investigated for 81 keV incident photons. All three escape mechanisms were observed in the same experiment. Experimental escape fractions were compared with the results from Monte Carlo simulations. Good agreement was obtained for the escape of photons. However, the simulations underestimated the escape of photoelectrons. Copyright © 2005 John Wiley & Sons, Ltd.     

Untersuchungen über den Doppel-Compton-Effekt

The differential cross section for the double Compton effect has been measured by analysing the energy of the scattered photons. The γ rays are detected by sodium iodid scintillation crystals and photomultipliers, fixed at 90° to each other and to the incident beam. For detecting low energy photons down to 7.2 keV one photomultiplier was replaced by a proportional counter. The experimental results obtained in this work are over the total range of the measured spectrum in good agreement with the theoretical predictions.     

Inelastic and elastic scattering differential cross‐sections of 59.5 keV photons for Cu and Zn targets

Differential cross‐sections for incoherent and coherent scattering of 59.54 keV photons were measured for Cu and Zn targets at scattering angles of 40–135° in a reflection geometry setup with a graded shielding arrangement. The ratio of these cross‐sections, which can be determined with higher precision than absolute values, is given. A method was used to determine differential incoherent and coherent scattering cross‐sections of elements. This method is based on simultaneous measurement of fluorescence radiation and scattered radiation, thus avoiding problems with measuring source strength and source‐to‐detector solid angle. The measurements were performed using a point source of Am‐241 radioisotope and an Si(Li) detector. The experimental results, which complement earlier results, were compared with the theories of incoherent scattering function and form factor. Copyright © 2004 John Wiley & Sons, Ltd.     

Escape of photoelectrons and Compton‐scattered photons from an HPGe detector

The response function of a planar HPGe detector due to escape of photoelectrons and Compton‐scattered photons was studied for a point source with 59.5 keV energy. It was shown that both mechanisms, in addition to Ge x‐ray escape, leading to partial deposition of energy, could be observed in the same experiment. A Monte Carlo program was used to investigate these components of the response function. The results indicate that although the escape of scattered photons and Ge x‐rays are of the same magnitude, the escape of photoelectrons plays a more important role in the efficiency of a detector. Copyright © 2003 John Wiley & Sons, Ltd.     

Improved Rayleigh to Compton scattering ratio curves for mass attenuation coefficients determination for X‐ray fluorescence analysis

Nondestructive assays are essentials for certain types of sample materials, and, among those, the X‐ray fluorescence technique enables the determinations of stable elements, and there is an increasing effort on the development of equipment to suit the various needs. Nevertheless, a great difficulty on the analysis of unknown materials' composition is to account for self‐absorption of the fluorescence photons that must be considered in the elemental concentration calculation. The correlation between the Rayleigh to Compton scattering ratio to the mass attenuation coefficient has proved to follow a single polynomial function for the first 20 elements of the periodical table with a correlation factor higher than of 0.998 for the sixth order function. The Rayleigh to Compton scattering ratios for pure elements and the 22.16 keV photons, the main energy from an X‐ray tube with silver anode, were determined with the MCNP6 Monte Carlo computer code. Two scattering angles were considered. Reference samples were measured, and the calculated results were compared to the literature values of the mass attenuation coefficient for some known samples and showed to be within 20% for de 90° scattering angle. Only Lucite was slightly above 20%. Curve fit coefficients are also presented for the 7.11‐ and 17.40‐keV photon energies.     

Compton energy-absorption scattering cross-sections for H,C, N,O, P,Ca and assessment of Doppler broadening

Total Compton, individual shell and Compton energy-absorption scattering cross-sections are evaluated in the energy region 0.005 to 10 MeV for H, C, N, O, P and Ca. Compton energy absorption cross-sections deviate numerically with available values. The cause of the numerical discrepancies are not fully understood but can be attributed to Doppler broadening of the Compton scattered photons through a given angle. Received 25 June 2002 / Received in final form 27 September 2002 Published online 3rd April 2003 RID="a" ID="a"e-mail: dvrao@ssmain.uniss.it     

Energy and intensity distributions of 279 keV multiply scattered photons in bronze — an inverse response matrix approach

An inverse response matrix converts the observed pulse-height distribution of a NaI(Tl) scintillation detector to a true photon spectrum. This also results in extraction of intensity and energy distributions of multiply scattered events originating from interactions of 279 keV photons with thick targets of bronze. The observed pulse-height distributions are a composite of singly and multiply scattered events in addition to bremmstrahlung originating from slowing down of Compton and photo-electrons in thick targets. To evaluate the contribution of multiply scattered events, the spectrum of singly scattered events contributing to inelastic Compton peak is reconstructed analytically. The optimum thickness (saturation depth), at which the number of multiply scattered events saturate, has been evaluated in different energy bin meshes chosen for scintillation detector response unfolding. Monte Carlo calculations based upon the package developed by Bauer and Pattison (Compton scattering experiments at the HMI (1981), HMI-B 364, pp. 1–106) supports the present experimental results.      

A three‐crystal spectrometer for high‐energy resolution fluorescence‐detected X‐ray absorption spectroscopy and X‐ray emission spectroscopy at SSRF

A Johann‐type spectrometer for the study of high‐energy resolution fluorescence‐detected X‐ray absorption spectroscopy, X‐ray emission spectroscopy and resonant inelastic X‐ray scattering has been developed at BL14W1 X‐ray absorption fine structure spectroscopy beamline of Shanghai Synchrotron Radiation Facility. The spectrometer consists of three crystal analyzers mounted on a vertical motion stage. The instrument is scanned vertically and covers the Bragg angle range of 71.5–88°. The energy resolution of the spectrometer ranges from sub‐eV to a few eV. The spectrometer has a solid angle of about 1.87 × 0^?3 of 4π sr, and the overall photons acquired by the detector could be 10⁵ counts per second for the standard sample. The performances of the spectrometer are illustrated by the three experiments that are difficult to perform with the conventional absorption or emission spectroscopy. Copyright © 2016 John Wiley & Sons, Ltd.