Radiation Protection Criteria and Standards: Their Basis and Use, Hearings Before the Special Subcommittee on Radiation of ..., 86-2 ..., May 24, 25, 26, 31; June 1, 2, and 3, 1960

At the recent Congressional Hearings Selected Materials on Radiation Protection Criteria and Standards: Their Basis and Use by the Special Subcommittee on Radiation, we promised to submit interpretations of the data on shorter lived radionuclides from fallout found in the soils near Chicago, as they are related to radiological health. We are pleased to submit our statement at this time.

The data on radioactivity in soil referred to in the recent Hearings Selected Materials on Radiation Protection Criteria and Standards: Their Basis and Use, were developed by Dr. P. F. Gustafson of Argonne National Laboratory in a program initiated May 1957. These data together with their interpretation are contained in a paper presented by Dr. Gustafson in December 1959 at the annual meeting of the Radiological Society of North America. By permission of the author this paper is attached. Also attached are graphs of these data prepared by us, which will appear in the July issue of Radiological Health Data, a monthly publication of the Department of Health, Education, and Welfare, together with data sheets for March and April 1960 which were received after the preparation of the graphs.

It will be noted that the total (natural sources and fallout) gamma radiation levels reported in the attached paper are direct measurements made at 3 feet above the ground using an ionizing chamber. To estimate the contribution to this total level from the radionuclides in fallout, studies were performed by Dr. Gustafson to determine (a) the distribution of these materials in depth within the soil, (b) shielding effect of the soil, and (c) the effect of multiple scattering of gamma rays within the soil. Based on these data the gamma dose rates at 3 feet above the ground were calculated, As a second approach a similar calculation of the above

*See page 300.

788

Mr. James T. Ramey

RADIATION PROTECTION

rate was made for the natural occurring radionuclides in soil to which
value was added an estimate of the contribution
of cosmic rays, fol-

lowed by the mathematical subtraction of this calculated dose rate
(from natural sources) from the total measured radiation level in order
to estimate the contribution of fallout. In addition to these necessary
estimations (in contrast with direct measurements) must be added the
usual uncertainties of sample collections and measurements. Within the
relatively large range of uncertainties necessarily inherent in such
studies we are in agreement with the data and interpretations contained
in the attached paper by Dr. Gustafson.

In addition to the interpretations given in the attached paper we have calculated the theoretical out-of-doors exposures based on the same data and methods of calculations as used by Dr. Gustafson, These calculations are summarized as follows:

(c)

140

Dose rate in April 1960

140

(Barium -lanthanum were not measured but it would be expected
that these would contribute some additional gamma exposure.
Although their half-lives are relatively short (12.8 days and 40
hours) they are present initially in relatively great abundance
in fission products.)

As one point of reference in interpreting the calculations it will be recalled that the external gamma radiation exposure from natural background is usually considered to be 100 millirads per year. Thus, the calculated out-of-doors doses from fallout (based on these soil data) are about 1/3 of natural background for the period May 1957 to April 1959 and about 1/2 for the peak year (1959). The calculated out-of-doors dose rate in April 1960 was about 1/5 that of natural background. In the absence of further atmospheric nuclear weapons testing, it would be expected that the downward trend of dose rates would continue since the radiological decay of the radionuclides more than compensate for the small additional deposition from the atmosphere. Eventually, of course, the gamma dose rate will level off to that from cesium-137 (half-life of 27 years).

RADIATION PROTECTION

789

Mr. James T. Ramey

The struc

The shielding effect of normal structures will reduce the in-door radiation exposures from those calculated out-of-doors. Using an ionization chamber Dr. Gustafson found a reduction factor of approximately five for in-door dose rates compared to those out-of-doors. tures used for this test, however, were either relatively large frame buildings or were of the cinder-block construction. It would be expected that a more usual small frame home would have somewhat less shielding effects possibly a factor of 2 or 3. The actual radiation exposure that persons might receive would depend, of course, not only on the type of structure but also on the length of time it was occupied.

If we can be of further assistance, please do not hesitate to let me know.