Margarete Ehrlich

Influence of Irradiation Rate on the Production of F Centers in LiF (TLD Grade)

Optically polished samples of extruded plaques of LiF (TLD grade) were given 60Co gamma‐ray exposures ranging from about 35 000–450 000 R, at exposure rates between about 1.5×103 and 5×106 R/h. Over this range of exposures and exposure rates, the resulting absorption in the F band was found to be independent of exposure rate, and to increase with exposure, even for exposures for which the thermoluminescence exhibited saturation effects. After thermoluminescence readout at about 235°C, practically all the absorption in the F band created by the 60Co gamma‐ray exposures had disappeared. These results are compatible with the dominant role given F centers in some of the current models for the thermoluminescence process in LiF (TLD grade).

Uniformity of high-energy electron-beam calibrations

A survey is made of the first year of operation of a new service offered by the National Bureau of Standards (NBS). Dosemeter units, consisting of polystyrene blocks holding stoppered quartz spectrophotometer cells filled with Fricke solution, are being shipped periodically to groups requesting assistance with absorbed-dose measurements in high-energy electron beams. As a check on their stability, all dosemeters are pre-exposed to 60Co γ-rays. The participants irradiate a portion of the dosemeters with electrons, using energies between about 5 and 50 Mev, and doses between 4000 and 8000 rads in water. The exposed dosemeters are returned to NBS for evaluation. During the first year of operation, slightly more than one-half of the doses reported by the participants were within ±5% of the NBS dose interpretation, but some differed by as much as 30% or more. Little correlation was found between a participants method of beam calibration and agreement with NBS dose interpretation.

RECIPROCITY LAW FOR X-RAYS. PART II. FAILURE IN THE REVERSAL REGION

Experimental data are presented that show the reversal behavior of the characteristic curves for one commercial x-ray film as a function of x-ray exposure rate and establish the presence of reciprocity-law failure and intermittency effect in the reversal region. Under the present experimental conditions, lower exposure rates of x-radiation are seen to favor reversal while the opposite is shown to be the case for visible light. Earlier work with different types of emulsions and under different developing conditions mentions an effect with visible light similar to the one here obtained with x-rays; however, a satisfactory interpretation of the basic mechanisms involved is still lacking and further experiments are required to establish the connection between the effects of x-rays and visible light.

Reciprocity Law for X-Rays.* Part I: Validity for High-Intensity Exposures in the Negative Region

A 50-kv beryllium window x-ray tube was used to confirm the validity of the reciprocity law for x-radiation over an exposure rate range from approximately 0.033 r/sec to approximately 1100 r/sec for the ascending branches of the characteristic curves of two widely used radiation monitoring films. A fast rotary shutter producing pulses of ten microseconds in duration facilitated the coverage of this 1:30 000 range. Conventional x-ray development as well as surface and internal development were employed. From the absence of reciprocity-law failure in the ascending branches of the characteristic curves the conclusion is drawn that, under the present experimental conditions, luminescence does not compete with the photographic effect.

Difficulties encountered with some intermediate-atomic-number radiation-protection dosimeters irradiated on-phantom in low-energy photon beams.

During a measurement-assurance study done with a particular type of radiation-protection dosimeter, we obtained unexpectedly poor results when several of the dosimeters were irradiated simultaneously on-phantom in a low-energy bremsstrahlung beam. The dosimeters incorporated low- and intermediate-atomic-number (Z) thermoluminescence dosimeter elements with the intermediate-Z elements under Pb filters. The response of the low-Z elements was in good agreement with the predicted response regardless of dosimeter location on the phantom, while agreement for the intermediate-Z elements was poor for some locations not coinciding with the phantom center. We have since studied the behavior of this dosimeter type and of four other types in detail by irradiating them, one at a time, on- and off-phantom, with their geometric centers displaced up to 10 cm along the vertical and horizontal axes through the center of the beam cross section. Two low-energy bremsstrahlung beams and a 137Cs gamma-ray beam were used. Variation of response to low-energy photons with on-phantom location was observed to some degree in all intermediate-Z dosimeter elements covered with high-Z filters. The reason for the observed dependence of response upon on-phantom location seems to be insufficient side shielding against phantom and filter albedo, as proved in one case, in which added shielding removed this dependence for the element immediately adjacent to the added shield. Since the effective dose equivalent to the human body is lower for lateral incidence of low-energy photons than for the same photons incident from the front or back, dosimeters incorporating high-Z filters without side shielding (and thus expected to exhibit the phenomenon) do not lend themselves to the determination of the effective dose equivalent or any quantity proportional to it by currently employed methods. A detailed study of the angular dependence of dosimeter response performed on a suitable phantom should aid in eliminating such inadequate dosimeters from practical use.

A system for electron therapy dosimetry surveys with thermoluminescence dosimeters.

Radiation-therapy dosimetry surveys employing thermoluminescence dosimeters (TLDs) are now being considered for high-energy electron beams. Using a system of individually calibrated pressed LiF TLDs in a water and a polystyrene phantom, we established that the distortions of depth-dose distributions in non-conducting materials previously observed at high absorbed doses and high dose rates were not detectable in the present geometry at doses and dose rates as much as 40 times higher than those employed in radiation therapy. The system was then used to measure TLD response in water and in polystyrene in the nominal electron-energy range from 7 to 18 MeV. In the water phantom, the well-known trend for TLD response to decrease with increasing electron energy was observed. In the polystyrene phantom, TLD response was found to be independent of electron energy.

Rate Dependence in a Solarizing Commercial X-Ray Emulsion: Sensitometric Properties, Number, and Structure of Developed Grains*

A comparison was made of optical density, mass of developed silver, number, projected area, and structure of the grains of a solarizing radiographic bromo-iodide emulsion exposed to Co60 gamma radiation at two different intensities, and developed in a surface developer. In contrast to the optical density and the mass of silver developed for a given exposure, which show only little dependence on intensity, before solarization sets in, the dependence of the number of developed grains on intensity of exposure is quite marked. Another anomaly is observed in the re-reversal region for exposure to high-intensity radiation. There, mass of silver does not vary with exposure in the same direction as optical density.Associated with the changes in photometric behavior are marked changes in grain structure, while the changes in the projected area of developed silver are small. The photometric properties cannot be correlated uniquely with grain structure over the entire exposure range.

Concerning Li2B4O7 thermoluminescence dosimeters

This letter reports the comparison of two commercially available types of lithium borate thermoluminesence dosimeters—one in which crystalline lithium borate was pressed into chips, and the other in which lithium borate was dispersed in a glass matrix. When irradiated with cobalt 60 gamma radiation, the response of a single sample of each type was reproducible to within 1%. However, differences between the two samples were apparent in their long term storage characteristics. (AIP)

PROPOSED NATIONAL BUREAU OF STANDARDS PROGRAM FOR THE CALIBRATION OF INSTRUMENTS USED IN HIGH-ENERGY ELECTRON AND X-RAY BEAMS.

The National Bureau of Standards (NBS) recently started a program that is t o lead to the establishment of an absorbed-dose calibration service for high-energy x-rays and electrons. It is the purpose of this paper to acquaint the reader with the two phases of this program: ( I ) the uniformity check of the calibrations performed by the users of high-energy electron beams and (2) the long-range research and development plans connected with the future absorbed-dose instrument-calibration service for high-energy x-rays and electrons.

THERMOLUMINESCENCE RESPONSE OF LiF TO X- AND GAMMA RAYS; A STUDY OF RATE AND ENERGY DEPENDENCE OVER A WIDE RANGE OF EXPOSURES

Abstract The total thermoluminescence light emission (“response”) of LiF (TLD grade) was studied as a function of exposure and exposure rate, and as a function of photon energy. 60 Co gamma rays and a broad spectrum of low-energy bremsstrahlung were employed. No rate dependence of the response was detected over the entire range of exposures and exposure rates employed (from about 10 2 R to 2 × 10 7 R, and from about 10 2 R/h to 7 × 10 6 R/h, respectively). This represents further evidence that centers other than F centers are involved in the thermoluminescence of LiF (TLD grade). A comparison of the curves of response versus exposure for the two photon spectra confirms Naylors findings that the superlinearity region is steeper for 60 Co gamma radiation than for low-energy X-rays, and reveals that the effect is indeed dependent on photon energy rather than on exposure rate. These findings are compatible with an explanation of superlinearity as being due to the formation of additional traps by the radiation proper. For exposures above those causing superlinearity, the difference in curve shape again disappears. Also, there is no dependence on energy of the location and height of the response maximum, which suggests that the inhibiting mechanism is independent of photon energy.