Counting tritiated water

Letters to the editors

632

International Journal of Applied Radiation and Isotopes, 1971, Vol. 22, pp. 632--633. Pergamon Press. Printed in Northern Ireland

between the mean values of the ]'4C activity shown in Table 2 using two tracers 9nNb and e°Co where the fl end point energy differs by a factor of 2.

Counting Tritlated Water

I. W. GOODIER

(Received 10 April 1971)

A. WILLIAMS

.Division of Radiation Science National Physical Laboratory Teddington Middlesex, England References

A COMPAV~XIW.assay has been made office commonly used scintiUating media for counting tritiated water in aqueous solutions, i.e. Bray solution, toluenetriton, Ne 240R and Ne 25(P (Nuclear Enterprises, Ltd) and InstagelR (Packard Instrument Company, Inc.). Solutions were counted in plastic vials, at 4°C, with a 3-channel Tricarb 3314 Liquid Scintillation Spectrometer equipped with two bi-alkali EMI 9635 Q B phototubes; efficiency with 3H Toluene standards was 52 per cent.

1. CAMPION P. J., TAYLOR J . G. V. a n d MERRITT J .

S. Int. J. appl. Radial. Isotopes 8, 8 (1960). 2. WILLIAMS A. and GOODmR I. W. Standardisation of Radionudides. Proceedings of a Symposium, Vienna 1966, IAEA Vienna, p. 153 (1967).

107

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FIG. 1. Results obtained with three different figures of merit (see text)• Left and middle scales are logarithmic. Each line connects the representative points for the combination under study. The last number on the right of the figure refers to the volume of tritiated water solution added to the medium.

Letters to the editors aH as tritiated water (C,E.A. Saday) from calibrated sources, appropriately diluted in bidistilled water, was added to various concentrations (1-4 ml, five samples per experiment) to each of the five following scintillators: Bray solution (PPO 7g, Popop 50 mg, naphthalene 50 g, p-dioxane I000 ml), toluene-Triton solution (PPO 4 g, Popop 100 mg, Triton X-100 330ml, toluene up to 1000ml), Ne 240a, Ne 250R and IustagelR. All reagents were high grade. After addition of Instagel, the mixture was put in a water bath at 37°(11 for 10 min. The samples were kept for 6 hr in the Tricarb refrigerator before starting counting. The comparison of counting rates was made using the three following figures of merit:

633

(Received 30 January 1971)

Those devices are more simple than scintillation cameras but their poor detection efficiency restricts their field to the one of low-energy radioisotopes (below 100keV). Using liquid dielectrics might give a solution to this problem. (For example, a 4-ram thick layer of liquid xenon has an efficiency of more than 50 per cent for 150-keV X-rays.) A direct charge collection argon-filled chamber allowed us to study conduction processes in liquefied noble gases. We were able to show: The existence of high mobility carriers (electrons) (for applied electric fields of 20 kV/cm, their mobility is about 30 cm s V -1 s"-1) which is required for electron avalanche. The strong dependence of the amount of collected charges with applied electric field and liquid purity, is) Such localization detectors might have interesting applications in high energy physics instrumentation for the energy loss of a minimum ionizing particle is adequate to create a detectable quantity of charges. That does not occur for the detection of X-rays or y-rays of the energy range used in nuclear medicine (a 100 keV y-ray creates a charge of about 6.10-16 coulombs). Their detection can only be done with charge multiplication through electron avalanche. Two options are available; to originate the avalanche in the liquid itself, or to have the two main steps of the detection done in different media: ionization in the liquid phase and avalanche in the gaseous phase of a same dielectric. Electronic avalanche processes in cryogenic liquids are not very well known yet. The triggering of the avalanche is not reliable, which is shown by efficiency changes. The intensity of applied electric fields (about 5.105 V/cm) causes secondary phenomenons (influence of the nature and surface of the electrodes).t 5, 6) Two phases systems already led to encouraging results. ~7~ It has been proved that electrons can escape the liquid phase and drift to the gaseous phase. Because of the high density of the dielectric, such noble liquid-filled devices might have interesting developments in the field of high efficiency and fairly good resolution localization detectors. R. ALLEMAND M. LAVAL J. F. PRUNIER L.E. T.L Centre d'gtudes nucl~aires de Grenoble C~dex 85-Grenoble-Gare, France

EXPERIMENTALresults in the field of gas-filled thermal neutron localization detectors {t) have been applied to the study of the visualization of X-ray and y-ray emitters for organ dynamic study or mappingJ t)

1. ALLEMANDR., BOURDELJ., CONVERT P., JACOB'-, J. and ROUDAUTE. Nouveaux d/tecteurs de loealisation de neutrons pour les dtudes de diffraction et de

signalz (cpm) . 10_6 figure A = background (cpm) figure B = signal (cpm) . volume of sample (ml) . 10_6 background (cpm) figure C = water content (sample as percentage total volume of vial), efficiency (cpm/dpm, per cent). Results are shown on Fig. 1. With the figure of merit A, the values range from 6 to 374, with figure B from 25 to 5981, best results being given by InstagelR, Ne 240R and Ne 250a. With figure C, on a linear scale, the spread is less large, from 193 to I010, but the pre-eminence of Instagel is acknowledged. As medical users of tritiated water, we feel that 10 ml Instagel plus 4 ml aqueous sample is a combination to be strongly recommended on the basis of efficiency, volume of aqueous sample incorporated and saving of scintillating solution. B. FaANqOXS

M. LIMANDAS Physics Laboratory Nephrological Clinic Htpital de l'AntiquaiUe 69 Lyon 5, France

International Journal of Applied Radiation and Isotopes, 1971, Vol.

22, pp. 633-634. Pergamon Press. Printed in Northern Ireland

The Use of Liquefied Noble Gases: A New Prospect for X-Ray and v-Ray Localization Detectors

References