FLUCTUATIONS OF Ca.Fm(Eu) SCINTILLATOR BACKGROUND

Z. DVORAK, K. STUKHEIL

ItTstitute of Radiation Dosimetry of the Czechoslovak Academy of Scieneies, Na Truhla~ee 64, 180 86 PraKue

(Received 8 M a y , 1989)

ABSTRACT: B a c k g r o u n d signal of the scintillation detector routinely used for atmospheric Kr-85 counting follows fluctuations,and their connection with cosmic ray variations is searched for. A strong correlation between the 27-day moving average of sunspot numbers and the background is obtained from the 1988 data.

I. INTRODUCTION

Revising the background of our counting chamber with CaF2(Eu) scintillation crystal, we noticed its long-term fluctuations.This chamber is routinely used for the counting of Kr-85 samples from the air (atmospheric concentration is about 1Bq/m a, half-live 18.76 y). Details of this system were published (Wilhelmova, TomA~ek, Dvorak, 1988).

2. DATA

The time series of this background during the year 1988 is shown on the top of Figure 1 and 2. Daily means (UT) are plotted against the time in Julian days in these figures. Counting of Kr-a5 sample a s well as regular calibrations interrupted the background measurements and caused gaps in our d a t a .

3 . DISCUSSION

Our chamber operates under a I0 cm lead shielding and thus the changes of terrestrial radiation (namely due to radon and its daugthers in the air of laboratory) cannot significantly influence the background signal.Therefore, the connection with cosmic ray v a r i a t i o n s is searched for. Our data are compared with the cosmic rays indices (neutron monitor data) from Czechoslovakia: Lomnicky stlt (Kudela, 1989) and other sites (Solar-Geoph}~ioal Data, 1988, 1989) in Figure i. Correlations between measurements from selected sites are shown rightside and the correlation coefficients are listed in Table I,

A surprising result was found when cosmic ray indices and our data were compared with the Sun activity. For this

Solar Physics 123: 189-192, 1989. �9 1989 Kluwer Academic Publishers, Printed in Belgium.

190 Z. DVOI~K AND K. STUKHEIL

purpose we used the sunspot numbers from URSIGRAMs (Solar- Geophysical Data, 1988, 1989). The comparison of time course of sunspot numbers and our data reveals an agreement in details as well as in the general trend - of. Figure 2. The correlation coefficients between the 27-day moving average of sunspot numbers and cosmic ray indices shows (Table II) that the correlation with the background of our counting chamber is the strongest one.

This behaviour of our detector will be the subject of a thorough examination. In order to verify this result we introduce a new additional system for the regular background record (independent of our Kr-85 programme).

ACKNOWLEDGEMENTS

The authors gratefully acknowledge dr. Josip Kleezek for his help, discussions, and comments; and E. Baldrianova for her

technical assistance.

REFERENCES :

KUDELA, K.: 1989, personal communication, Inst.Exp.Phys., Slov.Acad.Sci., Ko~iee, CSSR.

Solar-Geophysical Data: 1988,1989, Part I, No.522-532, U.S. De~grtment of Commerce, Boulder.

WILHELMOVA, L., TOMASEK, M, DVORAK, Z.: 1986b, Monitoring of the Atmospheric Activity of Kr 85 in Prague, Nucl,Inst. Methods B ! ? : 5 6 8 .

TABLE i: Correlation eoeffitiens between the daily mean of cosmic ray indices from the pairs of different sites or between the daily mean of cosmic r a y indices from one of these sites and the background of CaFm<Eu) scintillator (Praha) during 1988.

site r d e g r e e

of freedom

Lomnicky ~tlt - Praha Kiel - Lomnieky ~tlt

Kiel - Praha Kiel - Climax Kiel - Thule

0 , 3 6 3 226 0 . 6 6 3 303 0 . 7 0 9 189 0 . 9 2 0 267 0 , 9 6 3 303

FLUCTUATIONS OF CaF2 (Eu) SCINTILLATOR BACKGROUND

600I /1 PRAHA l

550 o . . . . 160 . . . . . ~o ' ~6o

1750

[am.~,

1450 100 200 300

6200

~64

5700 100 200 300

42001 . . . . CLII~',AX

o ~oo 200 300

i co I .

0 100 200 300 doy

191

80o[ . " : ' ,

�9 % " - i" "N}

f " . .,%[,,2 b , :~.. ~,:~

550 1450 lore, JL 1750

5700 5200 kle~.

bS00 l I ' " ' ~ {' " ' ";" }"6":

�9 ' . , , A " ' " 550L 5700 6200

kiet

4200[ . . . . . . . .

3700L . . . . . 5700 6200

kiet

'5~176 " .i}

4000 t " ] 5700 6200 kle~.

FiE.l: Cosmic ray indices and the background of the CaY2(Eu) scintillator during 1988.

day - Julian day b - daily mean of the CaF2 (Eu) scintillator background

[counts per I000 s] lom.s - daily mean of the cosmic ray indices (K. Kudela,

198g) [counts per I ps] kiel, thule, climax - daily mean of the cosmic ray indices

(Solar-Geophysical Data, 1988-9), [counts per" 36 s]

192 Z. D'~O~K AND K. STUKHEIL

TABLE II: Correlation coefficients between the 27-day moving average of the negative sunspot numbers and daily mean of the CaF2(Eu) scintillator background or the daily mean of cosmic ray indices during 1988.

s i t e r d . o . f

Lomnicky ~tlt 0 . 3 3 1 307 Thule O. 580 288 Climax O. 614 254 Kiel 0.672 290 P r a h a [CaF2(Eu) ] 0.810 190

750

b

650

5 5 0 26o 0

50

-R

-100

-250 0

, / Yl,

300

i

" ' 36o do),

800 '.% :

b .t. a "~.~, .

- ~ : ~

500 , , , -200

4500)

thu[e

4000 0

." % ~,-, .g|.. ,~ # , "~ ; ,% ,%~ . . . .

w t :

i t i -200

Fig.2: Solar activity and the background of CaF2(Eu> scintillator during 1988.

day, b, thule - cf. Figur e 1 -R - the negative value of sunspot numbers from URSIGRAMs

(Solar-Geophysical Data, 1988-9) and their 27-day moving average (used also for correlation plots at the bottom)