H. Roos

Absence of A Dose-rate Effect in the Transformation of C3H 10T1/2 Cells by α-particles

The findings of Hill et al. (1984) on the greatly enhanced transformation frequencies at very low dose rates of fission neutrons induced us to perform an analogous study with alpha-particles at comparable dose rates. Transformation frequencies were determined with gamma-rays at high dose rate (0.5 Gy/min), and with alpha-particles at high (0.2 Gy/min) and at low dose rates (0.83-2.5 mGy/min) in the C3H 10T1/2 cell system. alpha-particles were substantially more effective than gamma-rays, both for cell inactivation and for neoplastic transformation at high and low dose rates. The relative biological effectiveness (RBE) for cell inactivation and for neoplastic transformation was of similar magnitude, and ranged from about 3 at an alpha-particle dose of 2 Gy to values of the order of 10 at 0.25 Gy. In contrast to the experiments of Hill et al. (1984) with fission neutrons, no increased transformation frequencies were observed when the alpha-particle dose was protracted over several hours.

The depth-dependence of the biological effectiveness of 60Co gamma rays in a large absorber determined by dicentric chromosomes in human lymphocytes

Radiobiological evidence is shown concerning a significant depth-dependence of the maximum relative biological effectiveness at limiting low doses (RBE(M)) of (60)Co gamma rays in a cubic polymethylmethacrylate (PMMA) phantom of 30 cm edge length. Using the dose-response curve for the dicentric data in human lymphocytes obtained in the present experiment at a depth of 20 cm, together with the comprehensive and consistent data set determined earlier at smaller depths of the PMMA phantom, there is an increase in the RBE(M) value by a factor of 2.18 +/- 1.25 at a depth of 20 cm relative to 1 cm in the phantom. All the dicentric data are based on identical exposure durations and irradiation temperatures as well as identical culture and evaluation conditions, with blood from the same donor.

Enhanced response of the Salmonella mutagenicity test to ionizing radiations

Gamma-ray-induced reversions in the Ames Salmonella tester strain TA2638 have been studied for their dependence on a number of experimental parameters. It is shown that exposure to ionizing radiations soon after plating is not the procedure that yields results which correspond to those obtained in the standard utilization of the test with chemical mutagens. The ability to detect mutants is improved by irradiation 6 hr after the beginning of the incubation of the plated bacteria. This procedure has the double advantage of a markedly increased ratio of radiation-induced to spontaneous revertants and of resulting in substantial insensitivity to fluctuations in the number of bacteria initially plated. The reversion-doubling dose so obtained is 1.3 Gy; i.e., it is sufficiently small to disregard inactivation of the bacteria.

The variance-covariance method: Microdosimetry in time-varying low dose-rate radiation fields

SummaryThe variance-covariance method is employed at low doses and in radiation fields of low dose rates from an241Am (4 nGy/s) and a90Sr (300 nGy/s) source. The preliminary applications and results illustrate some of the potential of the method, and show that the dose average of lineal energy or energy imparted can be determined over a wide range of doses and dose rates. The dose averages obtained with the variance-covariance method in time-varying fields, for which the conventional variance method is not suitable, agree well with results obtained under the condition of constant dose rate. The results are compared to data obtained in terms of the conventional single-event measurements. The method has evident advantages, such as facility and speed of measurement.

His+ reversions caused in Salmonella typhimurium by different types of ionizing radiation.

The yield of his+ reversions in the Ames Salmonella tester strain TA2638 has been determined for 60Co gamma rays, 140 kV X rays, 5.4 keV characteristic X rays, 2.2 MeV protons, 3.1 MeV alpha particles, and 18 MeV/U Fe ions. Inactivation studies were performed with the same radiations. For both mutation and inactivation, the maximum effectiveness per unit absorbed dose was obtained for the characteristic X rays, which have a dose averaged linear energy transfer (LET) of roughly 10 keV/micron. The ratio of the effectiveness of this radiation to gamma rays was 2 for inactivation and about 1.4 for the his+ reversion. For both end points the effectiveness decreases substantially at high LET, i.e., for the alpha particles and the Fe ions. The composition of the bottom and the top agar was the one recommended by Maron and Ames [Mutat. Res. 113, 173-215 (1983)] for application in chemical mutagenicity tests. The experiments with the less penetrating radiations differed from the usual protocol by utilization of a technique of plating the bacteria on the surface of the top agar. As in an earlier study [Roos et al., Radiat. Res. 104, 102-108 (1985)] greatly enhanced yields of mutations, relative to the spontaneous reversion rate, were obtained in these experiments by performing the irradiations 6 h after plating, which differs from the conventional procedure to irradiate the bacteria shortly after plating.

Calibrating and testing tissue equivalent proportional counters with 37Ar

A method for testing and calibrating tissue equivalent proportional counters with37Ar is described.37Ar is produced by exposure of argon in its normal isotope composition to thermal neutrons. It is shown that - up to volume ratios of 0.01 of argon to the tissue equivalent gas - there is no appreciable effect of the argon admixture on the function of the proportional counter. Conventional calibration methods with characteristic x-rays or with α-particles require modifications of the detectors, and they test only small sub-volumes in the counters. In contrast, argon permits calibrations and tests of the resolution that are representative for the entire counter volume and that do not require changes in detector construction. The method is equally applicable to multi-element proportional counters; it is here exemplified by its application to a long cylindrical counter of simplified design that is part of such a multi-element configuration.

Effect of wire eccentricity on the performance of cylindrical proportional counters

A theoretical and experimental investigation of the influence of eccentricity of the multiplication wire on the performance of cylindrical proportional counters is presented. The electric field in the counter is calculated by the method of images, and the Townsend formalism is used to derive the gas gain. The experimental determination of detector performance is carried out with37Ar. The dependence of the gas gain and of the counter resolution on eccentricity is discussed, and it is shown that eccentricities up to 0.2 are of no concern in microdosimetric measurements with cylindrical proportional counters.

Criteria of applicability for autoradiography of tritium.

Autoradiography is an effective tool for the imaging of radionuclide distributions in various samples. In sophisticated applications with special preparation and development of sample-emulsion combinations and subsequent grain counts it can be highly quantitative, but it requires carefully controlled conditions and a variety of counter-checks, for example through scintillation spectroscopy. Less refined applications use X-ray films as detectors, and their seeming simplicity tends to invite artefacts and misinterpretations. Particular care needs to be taken, if one deals, or presumes to deal, with the low-energy ß-emitter tritium. Because of the short electron ranges the film must be in intimate contact with the sample, which tends to produce chemographic artefacts; without added spectroscopic measurements it is impossible to discriminate the spurious signals from a blackening of the film due to tritium. Recent statements concerning autoradiographic tritium measurements in tree samples have created considerable public concern and have demonstrated the pitfalls of uncritical use. This paper presents order-of-magnitude criteria for the detection threshold in the autoradiography of tritium; they can serve as an exclusion principle for some of the more extravagant misinterpretations.

Scheimpflug Attachment for Ophthalmological Slitlamps

Scheimpflug imaging is a technique suited for the quantification of minor opacities of the human eye lens. In the past, several Scheimpflug systems have been developed but being fairly complex and expensive, they have not been widely used. At present, only one instrument is on the market. For these reasons, a new Scheimpflug attachment-to be operated in connection with a slitlamp-was developed. The system is of compact, light-weight and modular design and consists of commercially available components. The Scheimpflug image, recorded by a CCD camera, is digitized in real time by a PC. It is displayed on a video monitor for on-line alignment. The data obtained are permanently stored on optical disk and are available for further analysis. The design of a prototype instrument is described. The practical applicability of the instrument and the high quality of the images is demonstrated.

Findings in human lenses 40 years after injection of Ra-224

Shortly before and after the end of World War II, about 900 patients in Germany were injected with known dosages of Ra-224 for the intended treatment of ankylosing spondylitis and tuberculosis. The pr