MEASUREMENTS OF Cs'87_ IN HUMAN BEINGS IN THE UNITED KINGDOM" J. Rundo U.K.A.E.A. Research Group, Atomic Energy Research Establishment, Harwell ABSTRACT A method of determining the radio-caesium content of the human body using thallium-activated sodium iodide crystals and gamma-ray spectrometry is described, and results are presented for subjects measured between June 1956, and July 1957. The potassium content was determined simultaneously and the mean content of 16 adult males was 0.21 per cent of body weight. The mean caesium content of these subjects was 34.0 ue per gram of potassium with a standard OIE deviation of 7.6 uuc per gram. 1 INTRODUCTION The presence of Cs!97 in the human body was first reported by Miller and Marinelli! for American subjects in the second half of 1955 and similar observations for British subjects were first made in the spring of 1956.7 A method of calibration of the apparatus at Harwell has been devised and this memorandum describes the method and results obtained to date (mid-1957). We have not made a systematic study of the levels as a function of time or geo- graphical location. 2 TECHNIQUE The apparatus used consisted of a substantial lead shield with four crystals of NalI(TI), 1.75 in. diameter by 2 in. thick, placed over the subject at a height of 14 in. above the stretcher. The pulses from the photomultipliers were passed to a linear amplifier (AERE type 1049C) and the spectrum was determined with a 30-channel pulse amplitude analyser (AERE type 1091A). A description of the apparatus in its original form has been given by Owen.’ Some subjects were measured using a single crystal 4.25 in. diameter by 2 in. thick, placed centrally over the body at the same height as the small crystals. To minimise the risk of observing external contamination on skin or clothing, all subjects showered and changed into previously monitored pyjamas. Each measurement lasted 50 min. *This paper was received from the Atomic Energy Researcn Establishment as Report A.E.R.E. HP/M. 126, dated January 1958. 249 tat