Bremsstrahlung Beam Research Articles

Bremsstrahlung Beams from High-Power Electron Accelerators for Use in Radiation Processing

In this paper, the authors present the results of calculations of bremsstrahlung produced by a high power electron beam that is incident on a water cooled target. The calculation yields the efficiency of conversion of electron beam power to bremsstrahlung, the energy and angular distributions of the photons and the dose distribution of the resultant photon spectrum in a water phantom. The result is used to estimate radiation processing rates with bremsstrahlung photons and comparison is made with Co-60 radioisotope sources. In their analysis, 4 and 5 MeV electron beam sources are considered, since these machines are now available commercially at power levels up to 200 kW. Furthermore, while higher energy would provide higher conversion efficiency, 5 MeV is chosen as the maximum electron beam energy because it is likely that the U.S. Food and Drug Administration will place an upper limit of 5 MeV on bremsstrahlung sources for food irradiation. The bremsstrahlung converter is assumed to consist of a high-Z slab followed by a water filled copper channel, which provides the required cooling and reduces the transmission of electrons through the converter.

Threshold region photoproduction of neutralπmesons off complex nuclei

Photoproduction of ${\ensuremath{\pi}}^{0}$ mesons off targets of $^{6}\mathrm{Li}$, $^{12}\mathrm{C}$, $^{28}\mathrm{Si}$, $^{40}\mathrm{Ca}$, natural Cd, and natural Pb was studied using a bremsstrahlung beam with endpoint energies of 140, 145, 150, and 155 MeV. Photoproduction from a liquid hydrogen target was employed as a normalization. The measured yields were found to be in disagreement with published theoretical cross sections for $^{6}\mathrm{Li}(\ensuremath{\gamma}, {\ensuremath{\pi}}^{0})^{6}\mathrm{Li}$ and also in disagreement with a simple schematic model which assumed only coherent contributions from the ${M}_{1+}$ multipole. The schematic model, however, did approximately predict the relative magnitudes of the yield curves for the energy range 14-20 MeV over threshold.NUCLEAR REACTIONS $^{6}\mathrm{Li}$, $^{12}\mathrm{C}$, $^{28}\mathrm{Si}$, $^{40}\mathrm{Ca}$, Cd, Pb, ($\ensuremath{\gamma}, {\ensuremath{\pi}}^{0}$); ${E}_{\ensuremath{\gamma}}=140\ensuremath{-}155$ MeV; measured $\ensuremath{\sigma}$; test of reaction model.

Cross sections per equivalent quantum for (γ,n) reactions on C, N, O, F and Cl

Relative cross sections per equivalent quantum are determined for (γ,n) reactions on composite targets of C, N, O, F and Cl using bremsstrahlung beams generated by 25–70 MeV electrons in a 0.1 mm Au converter.

Evidence for a small component of circular polarization in relativistic electron bremsstrahlung

Using a transmission type polarimeter, an asymmetry has been found that can be ascribed to a circularly polarized component in the bremsstrahlung beam of unpolarized relativistic electrons from the Mainz electron linac, in the region of electron energiesEel between 140 and 400 MeV. The experiment is described in detail. Systematical errors are discussed. For a comparison with electroweak theory, an exact calculation for small photon emission angles (of the order ofm c2/Eel) is needed.

Proton beam therapy.

Conventional radiotherapy with high energy photons and electrons has been remarkably successful in treating many localized tumors. However, if tumors are large or are located very close to critical normal structures, it becomes very difficult to deliver a tumoricidal dose to the entire tumor volume without compromising the function of these adjacent organs. In such situations the local tumor control probability may be enhanced by improving the therapeutic ratio, defined as the ratio of the probability of tumor control to the probability of normal tissue injury. An increase in the therapeutic ratio may be' obtained by choosing a radiation modality that is more effective at killing tumor cells than normal cells or by employing a type of radiation that provides improved dose distribution relative to that achievable by conventional radiation modalities. Historically, great improvements in local control rates of deep-seated tumors were obtained through an increase in the energy of photon treatment beams. This occurred both at the time of the introduction of the 60 Cobalt teletherapy unit into radiation therapy, replacing X-ray machines with maximum energies of about 300 kVp, and with the introduction of high energy electron accelerators capable of producing bremsstrahlung beams with energies in the supervoltage range, i.e. above 2 MeV, such as Betatrons and linear accelerators (4). Since the introduc­ tion of high energy photon beams, physicists have continued to try to improve dose distributions through the use of sophisticated treatment plans, including multiportal and rotational techniques, beam modifiers such as compensating wedges, etc, and by the development of computer­ directed dynamic therapy techniques (8).

Use of a clinical accelerator in spectroscopic studies of the decay of 197Ir.

The 40-MeV direct beam bremsstrahlung irradiation facility of the Sagittaire clinical electron accelerator was used to produce 197Ir via the reaction 198Pt(lambda,p)197Ir. Gamma rays emitted in the Ir decay were detected with an intrinsic germanium system and multichannel analyzer. 106 of the spectrum peaks could be analyzed for energy, relative intensity, and half-life. Half-life results indicate that 197Ir decays from two isomers, one with half-life 6.1 +/- 0.1 min and another with half-life 8.9 +/- 0.1 min.

Analyse multielementaire instrumentale des phosphates marocains par photoactivation nucleaire

Photonuclear activation and high resolution gamma-ray spectrometry have been used to determine non-destructively the concentrations of 25 trace elements in Maroccan phosphate rock. The activation was simultaneously performed for phosphate samples and multielement standards in the bremsstrahlung beam of a linear electron accelerator at 18 and 30 MeV maximum energies. Gamma-ray spectra were recorded with a 79 cm3 Ge(Li) detector and a programmable analyzer IN 96. A detailed study of interferences was carried out. Limits of detection between 0.5 and 125 μg·g−1 were obtained.

Analyse multielementaire des cheveux par photoactivation nucleaire

Photonuclear activation with 18 and 35 MeV bremsstrahlung beams and gamma ray spectrometry has been used to determine the concentrations of 14 elements in human hair. A careful study of interferences made possible a nondestructive analysis with practical limits of detection between 0.1 and 100 μg·g.

Doserates during isotope production

Formulae are given for the calculations of dose rates delivered to a target during isotope production ( D dot in Gy s −1) and for saturation activities ( A in Bq kg −1) for irradiations with thermal and fast neurons, which charged particles and with bremsstrahlung beams. As an example D dot /D ratios are calculated for the production of 34 m Cl, 38Cl and 39Cl isotopes by several nuclear reactions.

Self-powered detector methods for precision monitoring of pulsed bremsstrahlung from linac sources

Further development of self-powered detecting devices for monitoring bremsstrahlung beams at the University of Toronto linac laboratory has provided the conceptual basis for a bremsstrahlung monitoring system offering reliable and precise information concerning the location, magnitude and distribution of the radiation dose as it is being administered. Experimental results indicate the feasibility of monitoring the radiation delivered in a typical bremsstrahlung burst with a precision of better than 2%, while the (centroid) position of each burst can be monitored to ±0.5 mm. These and other characteristics of the self-powered devices are expected to permit the design and implementation of monitoring systems which can eliminate the delivery of more than one faulty bremsstrahlung burst in a prescribed radiation treatment.

Threshold Photoneutron Angular Distribution and Polarization Studies of Nuclei

The photoneutron method was applied to the study of (i) deuteron photodisintegration, (ii) giant magnetic dipole resonances in heavy nuclei, (iii) mechanism of radiative capture in light nuclei and (iv) isospin splitting of the giant dipole resonance in 60Ni. These studies were performed with the pulsed bremsstrahlung beam and high-resolution spectrometer available at the Argonne high-current electron linac. A threshold photoneutron polarization method was developed in order to search for the giant Ml resonance in heavy nuclei. A surprisingly small amount of Ml strength was found in 208Pb. Furthermore, the Ml strength for the 5.08-MeV excitation in 17O, the best example of a single-particle Ml resonance in nuclei, was found to be strongly quenched. In addition, the 17O(?, no) 16O reaction was found to provide an ideal example of the Lane-Lynn theory of radiative capture. The interplay among the three components of the theory, internal, channel and potential capture, were evident from the data. An electron beam transport system was developed which allows the bremsstrahlung to impinge on the photoneutron target on an axis perpendicular to the usual reaction plane. This system provides an accurate method for the measurement of relative angular distributions in (?, n) reactions. This system was applied to a high-accuracy measurement of the relative angular distribution for the D(?, n)H reaction. The question of isospin-splitting of the giant dipole resonance in 60Ni was studied by using the unique pico-pulse from the accelerator and the newly installed 25-m, neutron flight paths.

Measurement of total photonuclear cross sections in the Δ-resonance region

Double differential cross sections for the photoemission of protons and charged pion photoproduction were investigated for a number of target nuclei (He, Be, C, O, Al, Ti, Cu, Sn, Pb) using the tagged bremsstrahlung beam at the Bonn 500 MeV-Synchrotron in the photon range k = (215–386)MeV. On the basis of these experimental results the total hadronic cross section was determined.

Determination de quelques elements traces dans le charbon, d'une maniere non destructive, par photoactivation nucleaire

Photonuclear activation and high resolution gamma spectrometry have been used to determine nondestructively 28 trace elements in coal. The samples and synthetic multielement standards were simultaneously irradiated in the bremsstrahlung beam at 18 and 30 MeV maximum energies. Taking into account the contribution of the main possible interferences, limits of detection between 0.2 and 125 μg·g−1 were obtained.

Measurements of subthreshold photofission angular distributions of238U and fission channel analysis

Measurements of photofission fragment angular distributions of238U between 5.1 and 6.0 MeV are presented. As γ-source the intense bremsstrahlung beam from the Catania University Microtron is used. The results are analysed within the framework of the double-humped barrier model with a potential constructed with three smoothly joined parabolae and a damping term in the second well. The values of the fission barrier parameters for the involved 2+0, 1−0 and 1−11 channels are extracted and a comparison with the theoretical predictions and experimental results in the literature is made. Delayed-fission contributions for the involved channels are taken into account in the 1−1 channel parameter determination.

Threshold detectors as monitors for bremsstrahlung beams

Simple formulae can be used for the production of isotopes with monoenergetic neutrons and charged particles, {~-s~ but problems arise if continuous [and often unknown) spectra are involved, as is the case for d/Be neutron beams and for thick converter bremsstrahlung beams. To determine continuous neutron spectra, threshold detectors are used at low energies { <20 MeV), besides time of flight and recoil proton techniques. In this note we present some experimental results using threshold detectors as monitors for bremsstrahlung beams, generated by 0.25 GeV electrons/-*.5~

High resolution, high energy telescope counters for (γ, charged particles) reaction studies

High resolution telescope counters which employ thick germanium and silicon surface barrier detectors have been constructed and used for experiments with bremsstrahlung beams at high energies. The system has been tested with ⪅160 MeV bremsstrahlung beams and Li targets. All charged particles up to E ≅ 100 MeV are clearly separated, and there is little background in the measured spectra except at very low energies. Data have been acquired for the 2D(γ, p) n reaction at 90° and for the 4He(γ, t τ) reaction angular distributions. The system may be constructed at about 1% of the cost of a magnetic spectrometer with a comparable energy resolution and analyzing capability.

Proton Beams for Imaging Applications

The idea of using protons for imaging purposes was suggested more than a decade ago. Since that time, several attempts to demonstrate the usefulness of this technique for medical imaging have been made. However, only recently have detailed analyses of the physical advantages between proton and x-ray imaging been made. These studies show an expected dose reduction of about a factor of 10 for proton compared to x-ray imaging. All of these studies assume that both proton and x-ray systems are working at the quantum statistical limit of sensitivity. However, the complexity of the bremsstrahlung beam, rapid variation of absorptive coefficient with energy, and complexity of making high spatial accuracy and high detection efficiency detectors could seriously limit the practical sensitivity with x-ray imaging.

Ion recombination and the response of a “P2”-quantameter

At high power densities ion recombination influences the response of an ion chamber to X-rays. This deviation from linearity is usually called “saturation”. In a National Bureau of Standards P2-type ionization chamber saturation effects have been investigated using an intense pulsed bremsstrahlung beam. The experimental results can be well represented by a general “saturation curve”, which enables the determination of the true primary ionization.

Resonant photon scattering fromPb206,207,208andBi209

Resonant photon scattering from /sup 206,207,208/Pb and /sup 209/Bi has been measured from 4 MeV to the neutron thresholds using enriched targets, Ge(Li) detectors and bremsstrahlung beams with end-point energies of 7.0, 7.5, 7.6, 8.0, 8.5, and 10.4 MeV. Energies and values of gGAMMA/sub 0/ /sup 2//GAMMA were obtained for many levels not observed in previous photon experiments. Spins of levels in /sup 206/Pb and /sup 208/Pb were determined from the angular distributions, and ground-state branching ratios were obtained from self-absorption measurements for seven transitions in /sup 208/Pb. The results are compared with earlier spectroscopic studies and with lower resolution average cross-section measurements. The spectra of /sup 207/Pb and /sup 209/Bi are discussed in terms of the excitations of the /sup 208/Pb core.

Statistical and pre-equilibrium (γ,α) cross sections ofZr90and their multipolarities via theZr90(e,α) reaction

The (e,..cap alpha..) cross section in /sup 90/Zr has been measured at incident electron energies from 13.5 MeV to 66.5 MeV for ..cap alpha.. particles between 6.9 and 16.8 MeV. The (..gamma..,..cap alpha..) cross section was deduced from it assuming both E1 interaction and E2 interaction. The angular distribution of the (..gamma..,..cap alpha../sub 0/) cross section and an experiment using the bremsstrahlung plus electron beam make it clear that the E1 interaction is dominant over all the present energy range. The (..gamma..,..cap alpha..) cross section extracted by using E1 virtual photon spectra has a large bump above the excitation energy of 30 MeV in addition to a bump in the giant dipole resonance region. The (..gamma..,..cap alpha../sub 0/) cross section also has a bump at the giant dipole resonance which exhausts most of the (..gamma..,..cap alpha..) cross section in that region. The compound nucleus model was used successfully to explain the bump at the giant dipole resonance. The cross section above 30 MeV is discussed in terms of the pre-equilibrium ..cap alpha.. emission process combined with the quasi-deuteron model.