Volume 2, Issue 4, July 1975
Index of content:
2(1975); http://dx.doi.org/10.1118/1.594175View Description Hide Description
The current status of DT targets for cancer therapy is reviewed with regard to beam composition and energy as well as target configuration, lifetime, and neutron output. The advantages and shortcomings of the various target designs are discussed. It is concluded that several approaches can provide the necessary dose rate and target lifetime for clinical therapy purposes.
2(1975); http://dx.doi.org/10.1118/1.594177View Description Hide Description
Nuclear magnetic resonance(NMR) techniques have been used to study several i n v i v o and stabilized tissue samples. The results show that a multicomponent behavior characterizes the magnetization relaxation of all investigated samples. Various specimens were allowed to undergo necrotic processes and then they were examined with the same techniques used in i n v i v o tissue. A marked approach toward a single‐exponential magnetization‐decay behavior was observed in these necrotic samples. In addition, large and sometimes irreversible temperature variations have been observed in the relaxation parameters of several preserved and stabilized tissue samples. The most dramatic changes observed throughout these experiments relate to the long‐lived magnetization components, and are not observed in i n v i t r o experiments performed solely on necrotic tissues. Meaningful differentiation between normal and neoplastic tissue by NMR techniques is virtually useless unless the roles of the temperature and necrosis on the different magnetization‐decay components are well understood.
2(1975); http://dx.doi.org/10.1118/1.594178View Description Hide Description
Neutrondosimetry intercomparison visits have been made by physicists from the M. D. Anderson Hospital–Texas A&M University Project to the Naval Research Laboratory, the University of Washington, and the MRC Cyclotron at Hammersmith Hospital. The Naval Research Laboratory and University of Washington physicists have made dosimetry intercomparisons at the Texas A&M Variable‐Energy Cyclotron (TAMVEC). The parameters that are usually measured during these visits are tissue kerma in air, tissuedose at depth of dose maximum, relative central‐axis depth dose, neutron/gamma ratios in air and in phantom, and photoncalibrations of ionization chambers. In addition, beam profiles and dose buildup curves are sometimes measured. Other parameters that are compared are values of W, stopping power ratios, kerma corrections, and calculations that lead to the statement of tumordoses for patients. This paper presents some of the results of the intercomparisons and discusses the implications of the findings.
2(1975); http://dx.doi.org/10.1118/1.594179View Description Hide Description
A mathematical analysis of the shape of x‐ray attenuation curves is presented that suggests a new method of determining the kVp of bremsstrahlung beams. The main result is a simple analytic formula which gives an excellent approximation to the attenuation curve for large thickness of high‐Z absorber. From this, a graphical procedure is derived which yields a straight line whose slope is related to the mass attenuation coefficient evaluated at the peak energy. Included are corrections for aluminum filtration, ion‐chamber energy dependence, and coherent scattering. The method is limited to beams generated by a constant potential and with negligible characteristicphotons in the filtered spectrum. Numerical data are given for using copper absorber below 70 kVcp and tin above 90 kVcp.
2(1975); http://dx.doi.org/10.1118/1.594180View Description Hide Description
We present the results of an experimental test of a new method for measuring the kVp of a diagnostic x‐ray beam. The method involves measuring x‐ray attenuation in copper or tin absorbers and plotting the attenuation curve in a new way. Our measurements were done at 60, 90, 100, and 120 kVp on a 3‐phase, 12‐pulse diagnostic generator. The kVp was measured with a calibrated voltage divider‐oscilloscope system. Except at 90 kVp, excellent agreement was found, with precisions on the order of ±1%. The 90‐kVp point appeared to be somewhat affected by characteristic tungsten radiation and gave agreement with a lower value of precision.
2(1975); http://dx.doi.org/10.1118/1.594181View Description Hide Description
An inexpensive recoil‐proton counter telescope has been designed for absolute fluence measurements of a gas‐target neutron source for radiation therapy. The detector has an absolute efficiency of 1.1×10−9 at 20 cm from an isotropic source and is useful for production rates of 109–1013neutrons per second. The telescope consists of a thin hydrogenous irradiator foil and a surface‐barrier detector to count recoil protons within a defined solid angle. The telescope provides n‐γ discrimination as well as discrimination against scatteredneutrons. Initial tests of the counter telescope were performed using the DD reaction employed in development stages of the gas target. A clear separation of full‐energy recoil protons from background and scatteredneutron events was evident in the pulse‐height spectra without the use of coincidence gating techniques.
2(1975); http://dx.doi.org/10.1118/1.594182View Description Hide Description
A systematic survey of the thermoluminescence (TL) of rare earth activated sulfates of the form MeSO4:RE was made, where Me denotes a divalent metal and RE the rare earth. SrSO4:Tb3+ and BaSO4:Eu2+ exhibit very high TL efficiency following gamma irradiation, comparable to that of the most sensitive phosphors currently available for TL dosimetry. Due to their relatively high Z, these phosphors could prove useful as quality indicators for personnel dosimetry or for detecting very small exposures of low‐energy x rays of a known quality.
2(1975); http://dx.doi.org/10.1118/1.594183View Description Hide Description
A simple optical mechanism is presented for the fine adjustment of sidelights. The characteristics of the device are that large angular displacements are required to achieve small lateral motions of a light beam. For this reason, the mounting mechanism may be made to quite coarse tolerances.