INCREASED WITH

A

ACTIVATION

ANALYSIS

LOW-BACKGROUND

I. R. Barabanov, A. M. Kuryshev,

ACCURACY

DETECTOR UDC 543.53

V. N. Gavrin, and I. V. Orekhov

The minimum concentration of an impurity m e a s u r e d by activation analysis depends, other conditions being equal, on the degree of purification of the isotope produced by irradiation and on the efficiency and background of the detector with which its activity is measured. If a gaseous radioactive isotope is formed as the result of irradiation, the minimum m e a s u r e d concentration can be reduced by s e v e r a l o r d e r s of magnitude by high purification of the isotope produced and by the use of a detector with high efficiency and low background [1]. This paper d e s c r i b e s briefly a device for the m e a s u r e m e n t of calcium m i c r o i m p u r i t i e s through the activation method by separation and m e a s u r e ment of the activity of aTAr f o r m e d as a r e s u l t of the 4~ c~)aTAr reaction. The calcium impurity is important in a r a d i o c h e m i c a l experiment for the detection of solar neutrinos [2]. Such a device can be used without change in all c a s e s where a radioactive isotope of an inert gas is obtained as the result of irradiation. A simplified d i a g r a m of the device is shown in Fig. 1. The bubbler 1 containing the m a t e r i a l under investigation in solution is connected, a f t e r irradiation in a fast-neutron flux, to the device through the g r o u n d - g l a s s joints 2 and 3 and helium is bubbled through the irradiated liquid. The bubbling time d e pends on the dimensions of the bubbler. When a bubbler ~ 10 cm a in size is used, complete extraction of the argon formed is achieved in a few minutes of bubbling at a rate of - l c m 3 / s e e . The condenser 4 ( - 78~ and zeolite 5 are intended for the removal of vapor from the liquid. Isotopes of radon, xenon, and krypton a r e separated from the gas mixture in the carbon trap 6 (-78~ The extracted argon is adsorbed from the helium flow by the carbon trap 7 (-196~ After the completion of bubbling, the helium is pumped out of the trap 7 at a t e m p e r a t u r e of - 196~ and then the extracted gas together with a p r e l i m i n a r i l y deposited portion of c a r r i e r gas is pumped through the titanium p u r i f i e r (~ 1000~ by the Toeppler pump 8 [3] into the small proportional counter 9,0.5 cm 3 in volume. The counter is included in an anticoincidence s y s t e m for subtraction of c o s m i c - r a y background based on a plastic scintillator 5 cm in d i a m e t e r and 10 cm tong scanned by an F]~U-93 photomultiplier, the inherent radioactivity of which m a k e s a negligibly s m a l l cor_,tribution to the background. Counter and plastic scintillator a r e surrounded by a passive shield c o n s i s t ing of 1 . 5 c m of m e r c u r y , 10cm of electrolytic copper, and 4 0 c m of iron to reduce the counter background

2

Y

~-

Measurement of helium bubbling rate 5 6 7

8

To vacuum

system

Fig. !.

Simplified d i a g r a m of argon extraction device.

Translated from Atomnaya l~nergiya, Vol. 37, No. 6, p. 503, D e c e m b e r , 1974. submitted April 30, 1974.

Original letter

9 19 75 Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, mierofilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

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f r o m the ~/radiation oZ the r a d i o a c t i v e i m p u r i t i e s which a r e contained in the m a t e r i a l s of o r d i n a r y l a b o r a t o r y r o o m s . The background of such a s y s t e m is 5 c o u n t s / d a y o v e r the e n t i r e p u l s e - h e i g h t r a n g e and 0.33 c o u n t s / d a y in the region of e n e r g y deposition f r o m 3TAr decay, and can be m a d e even l o w e r if a p u l s e - s h a p e a n a l y s i s c i r c u i t is used and the counting equipment is located in an underground r o o m . The use of such counting equipment and the efficient s y s t e m f o r p u r i f i c a t i o n of the isotope produced m a k e s it p o s s i b l e to m e a s u r e with e a s e a c a l c i u m concentration of 10-6g C a / g with a f a s t - n e u t r o n flux of 104 n / c m 2- s e c , which is e a s i l y achievable by using P o - Be s o u r c e s of c o m p a r a t i v e l y low activity that a r e safe to handle and 10 g of s a m p l e m a t e r i a l If n e c e s s a r y , the m i n i m u m m e a s u r e d c a l c i u m concentration in a 10-g s a m p l e g o e s to a value of ~ !0 -t~ g C a / g with the use of neutron fluxes of 1013 n / c m 2- sec and long i r r a d i a t i o n t i m e s if t h e r e a r e no s e c o n d a r y chains f o r the f o r m a t i o n of 3TAr. A f u r t h e r i n c r e a s e in s e n s i t i v i t y is also p o s s i b l e by i n c r e a s ing the absolute size of the sample. LITERATURE I. 2.

3. 4.

CITED

G. E. Kocharov and V. O. Naidenov, in: Proceedings V All-Union School of Space Physics [in Russian], Apatity, I00 (1968). R. Davis, Phys. Rev. Lett., 20, 1205 (1966). L R. Barabanov, V. B. Veshnikov, and A. A. Pomanskii, Pribory V. N. Gavrin et al. , Kosmicheskie Luchi, No. 1O, 20 (1969).

i Tekh.

Eksperim.

, 2, 109 (1967).

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