Publication Abstract

Investigation of photon spectra and contributions to air kerma rates in the environment near nuclear facilities using portable germanium gamma spectrometry

Investigation of photon spectra and contributions to air kerma rates in the environment near nuclear facilities using portable germanium gamma spectrometry

G.J. Hunt

Portable spectrometers based on NaI or Ge detectors have for some decades been important additions to the tools available for monitoring and assessment of radioactivity and radiation dose rates in the environment due to both natural and anthropogenic sources (eg Beck et al., 1972). The contribution of these spectrometers is now well established (eg ICRU, 1994). The high resolution of the Ge detector is of particular use in rapidly identifying and assessing radionuclides of significance. Portable spectrometry is of importance in assessing deposited activity and contributions to gamma air kerma rates, and the rapid availability of such results would be significant in the event of a nuclear emergency, in addition to the more usual application of measuring concentrations of radionuclides in environmental samples. '

Environmental gamma ray spectra may be contributed by a number of photon sources of different energies. First, there is the contribution due to natural background: photons derived from cosmic rays present a continuum of energies but the. primordial radionuclides and their decay chains (mainly 232Th, 238U and 40K) produce a number of peaks at characteristic energies. 137Cs is widespread in the environment due to the effects of weapons-test fallout and, particularly in Europe, the effects of the Chernobyl accident. In the UK marine environment there is also a widespread source of 137Cs due to liquid radioactive waste discharges from Sellafield. The resulting effect on photon spectra is from the 137mBa daughter of 137Cs which produces a characteristic spectral peak at 662 keV. Near nuclear sites, additional effects may be observed, either due to discharges (atmospheric or liquid) or to direct radiation. Atmospheric discharges may give rise to deposited radionuclides or to radiation from the plume: where there is irradiation of air due to cooling of a reactor biological shield a particular component may be due to 41Ar at 1294 keV. Where the primary circuit of a gas-cooled reactor may be exposed there may be contributions due to 16N at 6.13 MeV and 7.12 MeV, as well as to any 60Co in the reactor steel at 1173 keV and 1332 keV. From the photon spectra may be derived the around concentrations of contributing nuclides (eg ICRU, 1994) and resulting gamma air kerma rates. These can be important in control of radiation exposure to workers and public in both normal and emergency situations.

In the course of developing a portable gamma spectrometry system, primarily for use following an emergency, spectra were measured in July-August 1996 at a number of locations in the environment near nuclear establishments in the UK. Examples of these spectra are presented in this paper to illustrate the range of observations and the utility of the portable Ge system. Spectra are analysed in terms of radionuclide contributions and gamma air kerma rates. Particular features are highlighted.

Reference:

G.J. Hunt* (2000) Investigation of photon spectra and contributions to air kerma rates in the environment near nuclear facilities using portable germanium gamma spectrometry. Proceedings of the 10th International Congress of the International Radiation Protection Association, Hiroshima, Japan, 15-19 May 2000

G.J. Hunt*

Proceedings of the 10th International Congress of the International Radiation Protection Association, Hiroshima, Japan, 15-19 May 2000