Milliroentgen Dosimetry with Thermoluminescence

DAVID A. PATTERSON AND HERBERT FRIEDMAN U. S. Naval Research Laboratory, Washington 25, D. C.

(Received August 2, 1957)

The primary purpose of this letter is to point out that thermoluminescence can be used for the measurement of extremely small radiation exposures in the range of ionization chambers and film badges.

THE release of energy absorbed by a luminescent material can occur in a number of different ways. Thermolumines­

cence (hereafter abbreviated as T.L.) is one of these and, as its name implies, it is distinguished by the fact that the emission of light occurs when the material is heated after irradiation. Each T.L. phosphor has a characteristic "glow curve" of light emission vs temperature. These glow curves and the insight they afford into fundamental luminescence processes have received consid­erable attention from many authors.1

A number of practical applications of the energy-storing capabilities of T.L. materials have been made. Tousey, Watanabe, and Purcell2 have used this property in CaSO4:Mn to study the ultraviolet emission of the sun; Kossel et al.3 have applied the

FIG. 1. Thermoluminescence emission "light sum" vs x-ray dose in milliroentgens.

same phosphor to medical x-ray dose determinations; Daniels and Rieman4 have extensively studied the T.L. dosimetry pro­perties of LiF and A12O3; and recently Ginther and Kirk5 have developed a sensitive, stable T.L. dosimeter system using CaF2: Mn. In these practical applications, the phosphor is exposed at room temperature to the ionizing radiation and then heated. The total T.L. emission (or light sum) is measured and is generally proportional to the radiation dose received.

During an experimental study of a number of T.L. materials at this laboratory it was found that the well-known CaSO4:Mn phosphor was capable of detecting considerably smaller radiation doses than most previously reported personnel dosimetry devices.

The maximum sensitivity measurements were made with an RCA 5819 photomultiplier at one end of a polished cylindrical tube and the phosphor and heater at the other end. The cylinder and photomultiplier were water-cooled and a thin quartz filter in the cylinder provided further thermal protection for the multi­plier photosurface.

The CaSO4:Mn preparation technique was that described in reference 2. A thin layer of the resulting powder was deposited from an ethyl alcohol suspension on to aluminum foil which also served as a reflector. After irradiation the sample was covered with a thin transparent quartz disk and placed, quartz down, in the top of the polished cylinder. A small electric heater placed flat against the foil was used to heat the sample nonlinearly to more than 350°C in about three minutes. The exposed sample area was about 1.9 square inches. X-ray exposures were made with a Keleket Industrial Thermax machine (70 kv, Mo target) and measured with a Victoreen roentgen ratemeter.

The minimum dose measured with CaSO4:Mn was 10 milli­roentgens. This is to be compared with lower limits for most personnel dosimeters in the region of 1-100 roentgens. For example, Ginthers' CaF2:Mn5 has a lower limit of about 1 r, Daniels4 has reported 6 r measurements with LiF, and Schulman's6

Ag-activated phosphate glass has a practical minimum of 10 r in routine use. Self-indicating systems such as color center growth in tenebrescent materials are less sensitive. Actual measurements of lower doses were not carried out. However, since the light sums are quite linear with dose (Fig. 1) and no appreciable "pre-dose" T.L. was found, extrapolation to lower doses seems justified. This extrapolation indicates that x-ray doses as low as 0.2 mr are detectable with this phosphor. The dependence of sensitivity on the energy of the incident radiation was not in­vestigated in detail. However, even if one assumes an energy dependence of around 10:1 for response to 70 kvp x-rays vs 1.2-Mev 7 radiation a computation shows that detection of at least 2 mr is possible for the harder radiation. The response obtained from actual exposures to a few roentgens of Co60 γ rays supports this expectation.

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FIG. 2. Glow curve for CaSO4:Mn taken with constant heating rate 6°C/min. The peak is at 85°C.

A practical limitation on the system was noted, however, in the decay of the light sum after exposure. There is a fairly rapid initial loss of some 40% in the first 8-10 hours. In three days about 70% of the T.L. is gone and after eight days the loss is around 85%. Although these figures are approximate they serve to indicate that strictly accurate dose determinations would require knowledge of both the time lapse between exposure and reading and of the T.L. decay curve.

The glow curve for CaSO4:Mn (Fig. 2) shows that the bulk of the T.L. emission is released at relatively low temperature. I t is this which accounts for the gradual decrease in the light sum after exposure.

The attractive features of the use of T.L. materials as dosimeters have been pointed out by other authors. They include re-usability, simplicity and economy of manufacture, ruggedness, and in­definite shelf life.

The CaSO4:Mn phosphor is reported as an example of the low-dose measurement capabilities of an old and well-known phosphor. Its principal attributes are extreme sensitivity and associated small size. Its prime limitation is the decay of light sum with time after exposure which at the end of an 8 hour day could result in an error of a factor of 2 in dose. An associated problem might be the dependence of the light sum on exposure temperature but Watanabe et al.2 found little effect for tempera­tures up to 70 °C. I t is possible that an analytical study of this phosphor (e.g., effects of Mn concentration and coactivators) could shift the glow peak to higher temperatures and reduce or eliminate the decay. The results reported here indicate a fruitful direction in which very sensitive dosimetry devices may be sought.

1 G. F . J. Garlick, Luminescent Materials (Oxford University Press, New York, 1949); J. T. Randall and M. H. F. Wilkins, Proc. Roy. Soc. (London) A184, 347 (1945).

2Tousey, Watanabe, and Purcell, Phys. Rev. 83, 792 (1951); K. Watanabe, Phys. Rev. 83, 785 (1951).

3 Kossel, Mayer, and Wolf, Naturwissenschaften 41, 209 (1954); Burger, Lehmann, and Mayer, ibid. 41, 209 (1954).

4 F. Daniels and W. P. Rieman, Final Rept. Chem. Proc. Agency Proj. No. 4-12-80-001 (1954).

5 R. J. Ginther and R. D. Kirk, Report of NRL Progress, September, 1956; J. Electrochem. Soc. 104, 365 (1957).

6 Schulman, Shurcliff, Ginther, and Attix, Nucleonics 11, 52 (1953).

December 1957 B O O K R E V I E W S 1137