Bremsstrahlung Photons Research Articles

Measurement of the flux-weighted average cross section for the \(^{186}\)W\((\gamma,p)^{185}\)Ta reaction at the bremsstrahlung end-point energy of 70 MeV

Abstract. The flux-weighted average cross section for the 186W(γ,p)185Ta reaction induced with 70 MeV bremsstrahlung end-point energy was measured using the activation method in combination with off-line gamma activity measurement. The 27Al(γ,2pn)24Na monitor reaction was used for the determination of the bremsstrahlung flux. The bremsstrahlung photons used to produce the photonuclear reactions were generated from a thin tungsten (W) converter by bombardment with a 70 MeV electron beam. The experimental flux-weighted average cross section is compared with the theoretical prediction, for which the absolute cross section was calculated using the TALYS 1.95 code and the bremsstrahlung spectrum was simulated using the computer program MCNPX. The present experiment was carried out at the 100 MeV electron linac of the Pohang Accelerator Laboratory, POSTECH, Pohang, Korea.

FORMATION AND FENOMENOLOGICAL DESCRIPTION OF BREMSSTRAHLUNG BEAM FOR PHOTONUCLEAR PROCESSES

Within the framework of the developed analytical model for description of the isotope production by bremsstrahlung radiation, the three key phenomenological parameters of the process have been established. Those parameters determine the total yield and volumetric distribution of the products of a photonuclear reaction with dominating giant dipole resonance (GDR) in a target. The applicability conditions as well as the accuracy of the phenomenological description of a flux of the above-threshold bremsstrahlung photons are studied. The form of description of a reference bremsstrahlung spectrum for analytical calculation of the photonuclear processes is proposed. The conditions and accuracy of its reproduction at an electron accelerator are investigated

Experimental observation of excitation and deexcitation of an as-yet unknown isomer during the irradiation of the 178m2Hf isomer by low energy bremsstrahlung photons

The whole set of spectra accumulated during the 178m2Hf isomer triggering experiments in 2015 has been analyzed in detail. It looks like an as-yet unknown isomer of 178Hf is excited during irradiations. The half-life of this isomer could to be between one and five days. As a result, the efficiency of triggering, which was recorded in 2015, appears to be underestimated at least by a factor of two. Additionally, the obtained results allow suggesting why other experimental groups failed to confirm the possibility of 178m2Hf isomer triggering.

Evaluation of radiation dose caused by bremsstrahlung photons generated by high-energy beta rays using the PHITS and GEANT4 simulation codes

ABSTRACT High-energy beta-ray sources generate bremsstrahlung photons in the building materials, generated by the radioactive nuclides inside the nuclear reactor buildings emitted by the Fukushima Daiichi nuclear reactor accident. Therefore, evaluating the bremsstrahlung dose subjected to the workers in the building is crucial for radiation protection. The precision for assessing the bremsstrahlung dose was investigated by comparing the Particle and Heavy Ion Transport code System (PHITS) and the GEometry ANd Tracking (GEANT4) simulation code results. Behind various shielding plates (lead, copper, aluminum, glass, and polyethylene with multiple thicknesses), the water cylinder was set to obtain the absorbed dose and the deposited energy spectrum by the bremsstrahlung photons. In comparing the deposited energy spectrum, the spectral shapes have consistent trends. Below several tens of keV, a peak is observed in the PHITS spectrum for lead. While comparing the absorbed dose, most of the results from both codes correlate within an ~10% difference for 2.280 MeV beta-ray sources and an ~20% difference for 0.5459 MeV beta-ray sources, except for ~30% for 20 mm thick lead. Although some differences occurred, the evaluation results of both codes correlated well with the above precision.

Re-utilization of long lived 99Tc radio isotope via Photon induced nuclear reactions

Technetium-99 is a radioactive waste produced primarily in nuclear reactors. It is also left as radioactive waste in hospitals, directly from 99mTc isomeric state. To bring down the quantity of technetium-99 radioactive waste, the nuclear reactions using photon beam is explored. The integral cross section of the reaction 99Tc(γ,γ′)99mTc has been determined using the photo-nuclear activation method. The experiment was done using bremsstrahlung photons having endpoint energies viz. 6, 9, 12, 16, and 20 MeV. 115In(γ,γ′)115mIn reaction has been used as a monitor reaction, for the flux normalization of the bremsstrahlung spectrum. Theoretical model calculations have been done using the nuclear reaction code Talys 1.9. Theoretical parameter values are optimized with the presently obtained data. Total cross sections are estimated and investigated the feasibility of re-utilization of the technetium-99 radioactive isotope.

Modelling atomic relaxation and bremsstrahlung in the deterministic code STREAM

STREAM, developed by the Computational Reactor Physics and Experiment laboratory (CORE) of the Ulsan National Institute of Science and Technology (UNIST), is a deterministic neutron- and photon-transport code primarily designed for light water reactor (LWR) analysis. Initially, the photon module in STREAM did not account for fluorescence and bremsstrahlung photons. This article presents recent developments regarding the integration of atomic relaxation and bremsstrahlung models into the existing photon module, thus allowing for the transport of secondary photons. The photon flux and photon heating computed with the newly incorporated models is compared to results obtained with the Monte Carlo code MCS. The incorporation of secondary photons has substantially improved the accuracy of photon flux calculations, particularly in scenarios involving strong gamma emitters. However, it is essential to note that despite the consideration of secondary photon sources, there is no noticeable improvement in the photon heating for LWR problems when compared to the photon heating obtained with the previous version of STREAM.

Development of a method for the on-line spectral characterization of Bremsstrahlung photons from an electron accelerator

The use of irradiators based on electron accelerator technologies coupled with an electron–photon conversion target requires an accurate and continuous characterization of the Bremsstrahlung spectrum produced. The indirect methods developed so far are either unsuitable for the constraints of on-line measurement or they call for excessively invasive devices. We present here a method for on-line reconstruction of the photon spectrum from the measurement of the energy deposited in a scintillator irradiated on its edge, using a simple, compact and robust device.The approach to reconstructing the Bremsstrahlung spectrum from the measurement of its energy deposited in a scintillator, translates mathematically into the solution of a linear inverse problem. The classical resolution of the problem is disrupted by its very nature and by the presence of noise in the experimental measurement of the deposited energy in the scintillator, but it is possible to apply methods compatible with the on-line constraint. In this paper, a method is implemented on the basis of Tikhonov regularization and, with the use of an effective tool, allows a multi-parametric study to be carried out in order to meet the on-line measurement constraint. In particular, the aim is to optimize the choice of scintillator and the properties of the acquisition system to be used.

Background studies and normalization of signal events in the Mu2e experiment

The Mu2e experiment is currently being constructed at Fermilab to search for the neutrino-less conversion of negative muons into electrons in the field of an aluminum nucleus. The experiment aims at a sensitivity of four orders of magnitude higher than previous related experiments, which implies highly demanding accuracy requirements both in the design and during the operation. To achieve such a goal, two important tasks should be accomplished. First, it is essential to estimate precisely the particle yields and all the backgrounds that could mimic the monoenergetic conversion electron signal. Second, it is necessary to normalize the signal events accurately. The normalization of the signal events is planned to be done using a detector system made of an HPGe detector and a Lanthanum Bromide detector, which will measure the rate of muons stopped on the aluminum target by looking at the emitted characteristic X-and γ-rays of energies up to 1809 keV. Therefore, it is essential to evaluate the detector system's performance before the start of the actual experiment. In this study, the first task was addressed by an extensive campaign of Monte Carlo simulations to investigate the relevant parameters and their impact on the experiment's sensitivity. The second task was handled by taking advantage of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) pulsed Bremsstrahlung photon beam at the ELBE facility. The detector system was tested at the ELBE facility under timing and background conditions similar to the ones expected at the Mu2e experiment. The study presents and discusses the simulation results and the detector system testing campaign.

Comparison of End-Point Energies of Bremsstrahlung Photons from 28- GHz ECR Ion Source in Three Measurement Locations

Bremsstrahlung photons measurement show elevated energy emission beyond critical energy in 28-GHz ECR and its nature is not revealed. Thus, the aim of this study is to unveil the nature of high end-point energies at the three measurement locations. Bremsstrahlung photons from a 28-GHz ECR ion source were measured at Busan center of Korean Basic Science Institute (KBSI). The gamma-ray detection system consists of three round type NaI(Tl) scintillation detectors placed 62 cm radially from the beam axis at the three measurement locations namely injection, centre and extraction sides of an ECR ion source, and a NaI(Tl) scintillation detector placed at the view port for monitoring photon intensity along the beam axis. Such design of the experiment was based on the inner structure of the ECR ion source and the shape of the ECR plasma. Also, bremsstrahlung photons were measured by exchanging the detector positions for studying systematic uncertainties. Monte Carlo simulation based on the Geant4 package was performed to take geometrical acceptance and the energydependent detection efficiency into account due to large non-uniformity in the material budget. True bremsstrahlung energy spectra from the 28-GHz ECR ion source were extracted using an inverse matrix unfolding method. The measured high end-point energies at the three measurement locations were 1.68 MeV, 1.72 MeV and 2.04 MeV for the injection, centre and extraction sides, respectively. The high end-point energy values of the measured bremsstrahlung photons beyond the theoretical energy value of 1.33 MeV were found to be correlated with the structure of the ECR ion source and the shape of the ECR plasma. Furthermore, the high end-point energy values resulting from the bremsstrahlung photons measurements in the 28-GHz ECR plasma are due to the higher number of unconfined electrons arriving at the chamber wall easily, and producing higher bremsstrahlung photons. The unconfined electrons are the results of imperfect magnetic confinement that cause electrons to escape from the plasma volume.

A Bayesian approach for measuring nuclear material in radioactive waste using photofission

Photofission provides a mean for measuring nuclear material in large and dense radioactive waste packages. High-energy interrogating bremsstrahlung photons produced by a LINAC are indeed able to induce fissions in the depth of the waste matrix. In addition, a scintillation detector can detect high-energy delayed gamma rays that are able to escape the waste package. Scintillation detectors also allow performing measurements during photon irradiation, between LINAC pulses, to measure delayed gamma rays from short-lived fission products. Photofission measurement with a LaBr3 scintillation detector can help determining, in a Bayesian frame, the mass and position of actinides in radioactive waste packages. In addition to counting statistics, the model takes into account photofission yields and waste matrix density uncertainties. The model performances are numerically illustrated in the case of a 235 g238U lump placed inside a 50 cm radius drum homogeneously filled with 2.35 g/cm3 density concrete. In the most unfavourable angular position, with 32 drum rotations, when the 238U radial position is 10% and 90% of the waste drum radius, the expected relative uncertainty on the 238U mass is respectively 32% and 45%. The 238U angular position uncertainty is 10 mrad and its radial position can be determined with an expected uncertainty of 2 cm and 3 cm, when the 238U is respectively placed at 10% and 90% of the waste drum radius.

Radiative corrections to neutron and nuclear β-decays: a serious kinematics problem in the literature

We report a serious kinematics problem in the bremsstrahlung photon part of the order-α outer (model independent) radiative correction calculations for those neutron (and nuclear beta) decay observables (like electron-neutrino correlation parameter measurement) where the proton (recoil particle) is detected. The so-called neutrino-type radiative correction calculations, which fix the neutrino direction in the bremsstrahlung photon integrals, use 3-body decay kinematics to connect the unobserved neutrino direction with the observed electron and proton (recoil particle) momenta. But the presence of the bremsstrahlung photon changes the kinematics from 3-body to 4-body one, and the accurate information about the recoil particle momentum is lost due to the integration with respect to the photon momentum. Therefore the application of the abovementioned 3-body decay kinematics connection for the radiative correction calculations, rather prevalent in the literature, is not acceptable. We show that the correct, so-called recoil-type radiative correction calculations, which fix the proton (recoil particle) momentum instead of the neutrino direction and use rather involved analytical, semianalytical or Monte Carlo bremsstrahlung integration methods, result usually in much larger corrections than the incorrect neutrino-type analytical methods.

Photo-response of the N=Z nucleus ^{24}Mg

The electric E1 and magnetic M1 dipole responses of the N=Z nucleus ^{24}Mg were investigated in an inelastic photon scattering experiment. The 13.0 MeV electrons, which were used to produce the unpolarised bremsstrahlung in the entrance channel of the ^{24}Mg(gamma ,gamma ^{prime }) reaction, were delivered by the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf. The collimated bremsstrahlung photons excited one J^{pi }=1^-, four J^{pi }=1^+, and six J^{pi }=2^+ states in ^{24}Mg. De-excitation gamma rays were detected using the four high-purity germanium detectors of the gamma ELBE setup, which is dedicated to nuclear resonance fluorescence experiments. In the energy region up to 13.0 MeV a total B(M1)uparrow = 2.7(3)~mu _N^2 is observed, but this N=Z nucleus exhibits only marginal E1 strength of less than sum B(E1)uparrow le 0.61 times 10^{-3} e^2 , fm^2. The B(varPi 1, 1^{pi }_i rightarrow 2^+_1)/B(varPi 1, 1^{pi }_i rightarrow 0^+_{gs}) branching ratios in combination with the expected results from the Alaga rules demonstrate that K is a good approximative quantum number for ^{24}Mg. The use of the known rho ^2(E0, 0^+_2 rightarrow 0^+_{gs}) strength and the measured B(M1, 1^+ rightarrow 0^+_2)/B(M1, 1^+ rightarrow 0^+_{gs}) branching ratio of the 10.712 MeV 1^+ level allows, in a two-state mixing model, an extraction of the difference varDelta beta _2^2 between the prolate ground-state structure and shape-coexisting superdeformed structure built upon the 6432-keV 0^+_2 level.

Effect of crystal structure on K shell binding energy of lead, mercury and tungsten using bremsstrahlung radiation

The K shell binding energies of lead, mercury, and tungsten in their pure elemental form and in some of their compounds have been measured for the first time by adopting a novel method. The method involves a weak beta source, an external bremsstrahlung converter, element and compound targets, and a high-resolution, high-purity germanium detector coupled to 16K multichannel analyzer. Continuous external bremsstrahlung photons produced by the interaction of beta particles from a 90Sr-90Y radioactive source with an iron foil are allowed to pass through the elemental and compound targets of lead, mercury, and tungsten. The spectrum of transmitted external bremsstrahlung photons is measured with a high-resolution high purity germanium detector spectrometer. The transmitted spectrum shows a sudden drop in intensity at K shell binding energy of the target. Such a sudden drop, which is due to the onset of the K shell photoelectric effect, has been used to determine the K shell binding energies. It has been investigated that the shifts in the K shell binding energies due to chemical environment of lead, mercury, and tungsten atoms is attributed to crystal structure.

Associated production of a top-quark pair with two isolated photons at the LHC through NLO in QCD

We report on the computation of NLO QCD corrections to top-quark pair production in association with two photons at the LHC. Higher-order effects and photon bremsstrahlung are taken into account in the production and decays of the top-quark pair. Top-quark and W-boson decays are treated in the Narrow Width Approximation conserving spin correlations up to NLO in QCD. This is the first time that the complete set of NLO QCD corrections to the ppto toverline{t}gamma gamma process including top-quark decays is calculated. We present results at the integrated and differential cross-section level in the di-lepton and lepton + jet channel. In addition, we investigate the effect of photon bremsstrahlung in toverline{t} production and top-quark decays, as well as the mixed contribution. The latter contribution, in which two photons occur simultaneously in the production and decay of the toverline{t} pair, proved to be significant at both the integrated and differential cross-section level.

Measurements of photoneutron dose rate for 15 MV photon beam from medical linear accelerator using neutron survey meter

High-energy medical linear accelerators (>10MV) are increasingly used in the medical field to treat cancer patients depending on the treatment organ and patient physic. Medical linear accelerators (Linacs) operating above 10 MV photon beam produce unwanted neutrons (photoneutron) by means of photonuclear reaction (γ, n) between bremsstrahlung photon beams and constituent materials of Linac head. This study involved the measurement of the neutron dose rate from photon beam of medical linear accelerator (Elekta Synergy) operated at 15 MV photon. For the measurement of dose rate, a neutron survey meter LB 6411 probe was used. Neutron dose rate was obtained as a function of delivered dose, field size and detector position. In the study neutron dose rate was found 13.14 mSv/h at a field size of 10 × 10 cm2 at position (0,90,0) cm for 350 cGy and 9.26 mSv/h at 5 × 5 cm2 field size at position (0,80,0) cm for 200 cGy photon dose delivery. The dose rate varied with respect to detector position in an irregular manner; 2.95 mSv/h at position (-100,0,0) cm -on the side of maze entrance and 4.85 mSv/h at position (100,0,0) cm -off side of maze entrance.

Similarity and distinctions of <sup>58, 60</sup>Ni photodisintegration processes

Reliability of (γ, 1n) and (γ, 2n) reactions cross sections for 58, 60Ni obtained in experiments carried out on the beams of bremsstrahlung and quasimonoenergetic annihilation photons was investigated using objective physical criteria. It was found that data of the first type experiment do not satisfy those criteria and definitely are not reliable. At the same time there are serious doubts in reliability of the data obtained in the second type experiment. New partial photoneutron reaction cross sections on 58, 60Ni satisfying physical criteria of data reliability were used for analysis of characteristics of both isotope photodisintegration processes. It was obtained that for neighboring (differ in two neutrons) isotopes because of various type systematic uncertainties there are significant disagreements between evaluated and experimental cross sections of various reactions which for isotopes 58Ni and 60Ni are of significant different character. It was shown that the reasons are not only the differences in energy thresholds and absolute values of cross sections of the (γ, 1n), (γ, 1n1p) and (γ, 2n) reactions on 58Ni and 60Ni but significant differences in specific features of neighboring isotopes shell structure also.

Real-time monitoring of ultra-high dose rate electron beams using bremsstrahlung photons

Recently, as the clinically positive biological effects of ultra-high dose rate (UHDR) radiation beams have been revealed, interest in flash radiation therapy has increased. Generally, FLASH preclinical experiments are performed using UHDR electron beams generated by linear accelerators. Real-time monitoring of UHDR beams is required to deliver the correct dose to a sample. However, it is difficult to use typical transmission-type ionization chambers for primary beam monitoring because there is no suitable electrometer capable of reading high pulsed currents, and collection efficiency is drastically reduced in pulsed radiation beams with ultra-high doses. In this study, a monitoring method using bremsstrahlung photons generated by irradiation devices and a water phantom was proposed. Charges collected in an ionization chamber located at the back of a water phantom were analyzed using the bremsstrahlung tail on electron depth dose curves obtained using radiochromic films. The dose conversion factor for converting a monitored charge into a delivered dose was determined analytically for the Advanced Markus® chamber and compared with experimentally determined values. It is anticipated that the method proposed in this study can be useful for monitoring sample doses in UHDR electron beam irradiation.

Shielding Analysis of a Preclinical Bremsstrahlung X-ray FLASH Radiotherapy System within a Clinical Radiation Therapy Vault.

A preclinical radiotherapy system producing FLASH dose rates with 12 MV bremsstrahlung x rays is being developed at Stanford University and SLAC National Accelerator Laboratory. Because of the high expected workload of 6,800 Gy w -1 at the isocenter, an efficient shielding methodology is needed to protect operators and the public while the preclinical system is operated in a radiation therapy vault designed for 6 MV x rays. In this study, an analysis is performed to assess the shielding of the local treatment head and radiation vault using the Monte Carlo code FLUKA and the empirical methodology given in the National Council on Radiation Protection and Measurements Report 151. Two different treatment head shielding designs were created to compare single-layer and multilayer shielding methodologies using high-Z and low-Z materials. The multilayered shielding methodology produced designs with a 17% reduction in neutron fluence leaking from the treatment head compared to the single layered design of the same size, resulting in a decreased effective dose to operators and the public. The conservative assumptions used in the empirical methods can lead to over-shielding when treatment heads use polyethylene or multilayered shielding. High-Z/Low-Z multilayered shielding optimized via Monte Carlo is shown to be effective in the case of treatment head shielding and provide more effective shielding design for external beam radiotherapy systems that use 12 MV bremsstrahlung photons. Modifications to empirical methods used in the assessment of MV radiotherapy systems may be warranted to capture the effects of polyethylene in treatment head shielding.

Status of experimental photonuclear results

Absolute majority of data on photonuclear reaction cross sections was obtained in experiments using bremsstrahlung and quasimonoenergetic photons produced at annihilation in flight of relativistic positrons. There are significant disagreements both in absolute value and shape between the results of experiments of both types. So, the pronounced structural features are clearly observed in the cross sections obtained using bremsstrahlung but those are practically absent in the cross sections obtained on the beams of annihilation photons. The disagreements in absolute values are definitely systematic: they are directly dependent on the data retrieval method and significantly exceed the statistical errors achieved in experiments. There are also the significant disagreements between the data obtained in experiments of the same type. In combined systematic investigations of many data on various photonuclear (primarily, photoneutron) reaction, both total and partial, cross sections the possible reasons for disagreements between the results of photonuclear experiments were found out, the status of those experiments and methods of accounting and elimination of observed disagreements are discussed.

Multiparticle natSe([formula omitted]) reactions induced with bremsstrahlung end-point energies of 20–80 MeV

The flux weighted average cross sections 〈σ〉 and cross sections per equivalent photon σq were first measured for the photonuclear multiparticle reactions natSe(γ, xnyp) at end-point bremsstrahlung gamma energies ranging from 20 to 80 MeV. The experiments were performed with the beam from the electron linear accelerator LINAC-200 with the use of the γ-activation technique. The bremsstrahlung photon flux was calculated with the program Geant4. The theoretical values of the flux weighted average cross sections 〈σ〉 and the cross sections per equivalent photon σq were calculated using the partial cross section σ(E) computed with the TALYS package code and the combined model of photonuclear reactions.