Frank H. Attix

Composition of A‐150 tissue‐equivalent plastic

In recent years, the use of tissue-equivalent materials has become quite common in fast-neutron dosimetry, with the A-150 plastic developed by Shonka et al. probably the most popular. Information on this specific plastic is scantily reported in the literature and as a consequence a preponderance of authors unknowingly reference an article by Shonka describing an early version of a tissue substitute plastic but having a different elemental composition than the present A-150 formulation. We have reviewed the results of 21 chemical analyses which have occurred over a time span of four years on a total of 14 samples of A-150 plastic and based on these data and the formulation of the plastic, have arrived at a suggested composition for A-150 tissue-equivalent plastic. The ambiguities of water absorption by nylon, one of the components of the plastic, and the uncertainty this reflects in the composition of the plastic were evaluated.

Further consideration of the track−interaction model for thermoluminescence in LiF(TLD−100)

At the Second International Conference on Luminescence Dosimetry in Gatlinburg in 1968 Claffy, Klick, and Attix introduced the track−interaction model in an attempt to explain the phenomena of supralinearity and sensitization generally observed in the thermoluminescence of LiF(TLD−100). Their proposal in general terms was that the electron−hole traps are located near the paths of the energetic charged particles (primary or secondary) which traversed the crystal during ionizing irradiation, and that this nonrandom spatial distribution affects the thermoluminescence process both with respect to its dependence upon dose and upon linear energy transfer. Moreover the observed sensitization of the phosphor, after receiving a large radiation dose followed by a partial annealing, was attributed to the survival of luminescence centers. In the present work a very simplistic model has been adopted for purposes of estimating how the average distance between luminescence centers along a charged−particle track compares...

Thermoluminescence and Color Centers in LiF:Mg

A variety of optical absorption, emission and luminescence excitation spectra have been measured in an attempt to identify the centers involved in the thermoluminescence of commercial LiF:Mg. It is concluded that the principal trapping centers consist of a hole trapped near various groupings of Mg2+ ions and vacancies. The optical absorption bands of these centers occur in the 3100–3800 A region which contains several absorption bands corresponding to different geometries of the centers. It is suggested that the 2200‐A band arises from Mg2+ ion‐vacancy complexes which have captured two holes. During thermoluminescence, holes are transported from traps to emitting centers. The luminescent center appears to be the F center both in an isolated position and when adjacent to a complex involving Mg2+ ions.

New Thermoluminescent Dosimeter

A simple dosimeter design is described in which a thermoluminescent phosphor is mounted on an electrically heatable support in an evacuated or gas‐filled envelope. With CaF2:Mn as the phosphor, the device detects gamma‐ray doses in the milliroentgen range and is linear in response up to at least 2×105 r. Dose readings can be made in less than a minute with simple instrumentation requiring no darkroom facilities. The dosimeter may be reused many times. The response is independent of dose rate at least over the range 10 mr/min to 7000 r/min. With suitable tin shields the response is independent of energy over the range 40 kev to 1.25 Mev. The advantages of this device for monitoring of personnel in health physics operations are pointed out.

Dosimetry intercomparisons between fast-neutron radiotherapy facilities.

Neutron dosimetry 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, tissue dose at depth of dose maximum, relative central-axis depth dose, neutron/gamma ratios in air and in phantom, and photon calibrations 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 tumor doses for patients. This paper presents some of the results of the intercomparisons and discusses the implications of the findings.

Thermal quenching of luminescence in six thermoluminescent dosimetry phosphors—II: Quenching of Thermoluminescence

Abstract Since the results of the measurement of the X-ray-excited radioluminescence as a function of temperature (Part I of this paper) of TL dosimetry materials were difficult to interpret in terms of quenching of thermoluminescence, the direct measurement was made of the TL light sum and the glow-peak height as a function of heating rate. In order to overcome the problem of thermal lag between the heating planchet and the phosphor, a method was developed to achieve a high degree of thermal coupling by mixing powdered phosphor and gold and pressing the mixture to the surface of a gold disk. The glow curves from these gold plaques were recorded with a reader that produced heating programs which were linear to a few percent from rates of 4°C/min up to 640°C/min. By this technique glow-peak heights and light sums were measured for CaF2:Mn, natural fluorite, LiF(TLD-100), CaSO4:Mn, Li2B4O7:Mn, and terbium-activated lithium aluminosilicate glass over the full range of heating rates. Considerable variability was found in the thermal quenching characteristics of the various phosphors. All but the fluorite and the Li2B4O7:Mn showed some decrease in light output at high heating rates. LiF exhibited a decrease for low heating rates as well, with a broad maximum in the light output at about 100°C/min.

Neutron beam dosimetry at the NRL cyclotron

A 35 MeV deuteron beam impinging upon a thick Be target is being employed to generate a neutron beam for radiobiological experiments of relevance to later possible fast neutron therapy trials. The primary calibration of the beam has been based upon tissue-equivalent plastic ionization chambers, calibrated in turn with 60Co gamma -rays. CaF2:Mn and 7LiF (TLD-700) thermoluminescent dosemeters have been employed to determine the gamma -ray dose component in the neutron beam, by a method depending upon the ratio of fast neutron sensitivities of the two phosphors.

Anomalous fading of CaF2:Mn thermoluminescent dosimeters☆

Abstract A surprising effect encountered in the application of the thermoluminescence of CaF 2 :Mn to dosimetry is the initially high fading rate. This fading is much more rapid than would be expected considering the high-temperature, apparently uncomplicated glow curve of CaF 2 :Mn. This effect is present when rapid heating rates (∼20°C/min) are used to obtain the glow curve, but when slow heating rates (∼20°C/min) are employed no fading is observed. It was determined that the glow curve is a composite of several closely spaced components of varying stability. With rapid heating rates all the glow curve components are shifted upward in temperature, causing preferential thermal quenching of the stabler (higher temperature) components. The composite glow curve is thus distorted and the unstable components, being attenuated to a lesser degree, give a greater contribution to the thermoluminescence output.

Displacement correction factor for fast‐neutron dosimetry in a tissue‐equivalent phantom

The displacement correction factor to be used for analysis of fast-neutron dosimetric measurements using air-filled EG and G tissue-equivalent ion chambers in a tissue-equivalent phantom has been investigated using the MANTA neutron radiotherapy beam generated by 35-MeV deuterons on a thick Be target. The displacement correction factor inferred from these measurements is 0.970 for the EG and G IC-17 (1.0-cm3) ion chamber, and is 0.989 for the EG and G IC-18 (0.1-cm3 ion chamber. This multiplicative displacement correction factor has no significant dependence on depth in the phantom or on neutron beam size.

Thermal quenching of luminescence in six thermoluminescent dosimetry phosphors—I: Quenching of X-ray-excited radioluminescence

Abstract The role played by the thermal quenching of luminiscence in thermoluminescent (TL) dosimeters has not been investigated previously. If thermal quenching is operative in the temperature region of the glow-peak, changes in heating rate which cause the temperature position of the peak to change would result in non-constant light sums. The direct measure of the TL light sum or the glow-peak height as a function of heating rate is technically difficult. The main problem is that the phosphor temperature tends to lag behind the heating-planchet temperature by an amount which is a function of the heating rate and the degree of thermal coupling. Thus a thermocouple attached to the planchet provides an erroneous measure of the phosphor temperature. Initially (Part I of this paper) this problem was side-stepped by studying the thermal quenching of the X-ray-excited radioluminescence under static thermal conditions. Because of the observed similarity of these emission spectra to the corresponding thermoluminescence spectra, the emitting centers involved in both processes are probably the same for each phosphor studied. The resulting curves of brightness vs. temperature were found generally to exhibit large discontinuities in the vicinity of the TL glow-peak temperatures, and were therefore difficult to interpret in terms of quenching of thermoluminescence. For this reason, the measurements of Part II of this paper were undertaken.