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Liquid Scintillation Cou LARGE t Iau ID SCINTILLATION COUNTERS offer a unique combination 1 of high sensitivity and moderate energy resolution and hence are particularly useful for the rapid measurement of body radioactivity. I t is necessary that this radionctivity involve the emission of gamma rays or hard beta rays (and hence bremsstrahlung). In this category are not only a large number of common radioactive contaminants but also body radioactivity inducrd by neutron exposure (mainly W h e n the identity of the radionuclide is known, the large liquid scintdator will measure the amount in minimum time. When the identity of the radionuclide is not known, this instrument will screen a large number of people and pick out those who count above normal (these may be few and far between) for more detailed study in a Sa1 (Tl) human spectrometer. Negative values on potentially exposed personnel are of considerable value for exposure records. This system has been in operation at this laboratory for about 2 gears and has proved most satisfactory. Three large liquid scintillators for counting people have been developed and built a t this laboratory. Some of their characteristics am summarized in table 1. The first detector (Humco I) was put into operation in 1955 and has been the workhorse. About 4,032 From L a Alamos Scientific Lahratory, University of Californh, Lor Almos, Sew Mexico. Presented on 9 Y a y 1959 in a symposium, “The Acute Radiation Syndrome.” at the US. S a d Medicnl School, Bethesda. Maryland. This study is hased on work performed under the auspices of the L.S. Atomic Energy Corn-
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itoring With Large XTEBB offer a unique combination e energy resolution and hence are measurement of body radioactivity. vity involve the emission of gamma ! bremirstrahlung). In this category common radioactive contaminants >nuclide is known, the large liquid unt in minimum time. Wben the known, this instrument will screen those who count above normal .I for more detailed study in a Negative values on potentially nble value for exposure records. at this laboratory for about 2 years iry. Some of their characteristics een the workhorse. About 4,032 in a symposium. "The Acute Radiation :chool, Bethmda, Maryland. Thin study ispica of the U.S. Atomic Energy Compeople and 4,435 milk samples have been measured in this instrument.'-l The detector unit removed from the lead shield is shown Fburc 1. Humco I detector unit.
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-lilt for and operated a t the 1951) ice.' The photocathode coverage e noise threshold to about 90 kev. Ibenmne in which are dissolved er). The subject stands (fig.2) in t i and 1%feet in diameter. The .ahell design so that no door is reTANK WOES PLATPORY c a~ 1958 Geneva Atom. for Pew of the Geneva Conference, 4,255 the machine operating at about se data will be discussed more being completed for use a t this he design trend of the Geneva :mt it is a 4 r , horimntal system. the noise level below 90 kev. Fig- 3. Humco It. improved mc,dcl 114' nrkinnl L o o Alamw humma counter GENEVA MEA SURERI ENTS At the 1958 Geneva Atoms for Peace Conference, the ability of IL lsrge liquid scintillator to measure rapidly a large number of people was dearly demonstrated. Those measured merely removed their shoes and stood in the counter for 40 seconds. Pulses from a chmnel centered on 1.2 mev (the energy of the Compton edge of the K* gamma rays) were recorded. Statistical precision was 7 percent on an averuge Kco determination. In alniost a11 cases E"'was the dominant radioisotope. There were, however, about 20 insstances of definitely elevated counts. These turned out to be occupational workers in the nucleu science field and were referred to the N a I (Tl) spectrometer at the French exhibit. Cobalt' and radium contamination were found to be most common, often on clothing. There were 2 amusing incidents: A boy counted very high because he had a piece of uranium ore in his pocket; and a man who counted high insisted that he W B ~not wearing a wrist watch. but it turned out that he carried IL radiumdid The great majority of persons measured at Geneva, however, showed mainly Ku. The accumulated measurements were processed daily by an IBM-650 computer, and the results were printed out 89 a frequency distribution (fig. 4). A summary of the results is given in tables 2 and 3. It is clear that the observed standard deviation ia greater than that observed under laboratory conditions. A large part
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SCREEWING AND IDENTIFICATION In cases of accidents, one u u d y has a large number of people, some of whom map be contaminated with unknown radiocrctivity. Our procedure Is to run all through Humco I quickly, and study those who count above normal in our NaI (Tl)human spectrometer: ROW the superior energy resolution of the crystal (figs. 5 and 6) becomes important. This situation occurred at this labontay, in April 1959,
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.C.) from I d at hpra 11 exiDwtr*in 4 1 1 one of these ;R..\t , is shown in figure 7. From the .he uniounls present could be +stlmated by an Pa' calibration and iecay scheme data. The effectiveness of the liquid scintillator-crystal spectrometer :earn w s well demonstrated again a t the meeting of the Medial .idvisor! Board of the .%comicEnergy Commission a t h Alamos on 1.5 March 1958. The members were counted in Humco I; dl were normal except one (H.B.G.).whose excess activity wag identified as Coo b_v the crystal spectrometer. It was then recalled that this illember had participated as a volunteer in an experiment involving Cou-l~beledVitamin BIZ about a yew previously. The main advantage of increased photocathode coverage is ability to measure lower energy radiation. Of considerable interest in this connection is Srm:Ym, which emirs hard beta rays (&,,=2.2 mev)
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riscc r i q x d \ at vtie low energies The drop below 80 kev ia due to nbsc~prtoriin Lht- copper .-an In which the crystal is mounted and in rmirters by means of their bremsstrahlung must be done in the lowe n a r q regon (between about 50 to 300 kev) This is why it is important that the newer large liquid scintillators are able to look down 11) !IO kev and, perhaps, below. Measurements with o SP,!Y' source in a 7bpound phantom on the L'r Geneva counter gave a counting efficiency of 0.3 percent and factor of 30 below the mariroum permissible level for occupational workers ( 1 pc) but above present levels in the general population. Of course, there ~9 very little that one can do to identify the beta emitter, since the only difference between bremsstrahlung spectra is their poorly defined end points 'CYLn estimated lower limit of detection of about 0.03 N. After 8 person is exposed to fast neutrons, a major radioactivity induced in t h e body ia 14.97-hour NaN. This nuclide emits hard gamma ra-y (one 1.37 mev and one 2.75 rnev photon per disintegration) and thus can easily be detected in vivo. Experience a t this laboratory shows that 3 rads of feist neutrons produce about 1 pc Na". Thus very small neutron exposures (in the millirad region) can be detected. Actually, a real problem is that medium exposures may produce enough Na2' to jam the counter. 3. AIDERSON, E. c.: Radioactivity of people and milk: 1957. Science 128: The h Alamoa human counter. Radioactivity of peopk .nd Coo&. Science 125: 1273-1278, June 1957.