It has been experimentally verified that the Ultra-Low-Level Counting System for the Gallex solar neutrino experiment is capable of measuring the expected solar up silon-flux to plus or minus 12% during two years of operation

Hampel, W.

Heusser, G.

Huebner, M.

Kiko, J.

Kirsten, T.

Schlotz, R.

Schneider, K.

English

NASA Technical Reports Server

422 SH 8.1-2
RESULTSOF ULTRA-LOWLEVEL 71Ge COUNTING
FORAPPLICATION IN THE "GALLEX"-SOLARNEUTRINOEXPERIMENT
AT THE GRANSASSOUNDERGROUNDPHYSICSLABORATORY
Hampel, W., Heusser, G., HUbner, M., Kiko, J.
Kirsten, T., Schneider, K. and Schlotz, R.
Max-Planck-lnstitut fur Kernphysik, Postfach 103980
D-6900 Heidelberg, Federal Republic of Germany
ABSTRACT
It has been experimentally verified that the
Ultra-Low-Level Counting System for the "Gallex"-
solar neutrino experiment is capable to measure
the expected solar v-flux to +12% during two
years of operation.
I. Introduction. More than a decade after Davis implemented his
Chlorine detector, experimental solar neutrino spectroscopy is still in
a state of infancy and lacks behind existing technical possibilities.
The only available experimental result, a deficit of solar 8B-neutrinos
as nmasured with the Cl-detector, has caused serious doubts on the ap-
propriateness of our understanding of stellar structure and evolution.
Urgently required is the detection of the pp-neutrinos from the princi-
pal fusion reaction p+p . d+e++_e.Their flux is largely model indepen-
dent and directly related to the solar luminosity. The extremely low
energy of pp-neutrinos (maximum 420 keV) limits the choice of potential
radiochemical detector materials. The Gallium detector, based on
71Ga(ve,e-)71Ge is by far the most promising pp-neutrino detector. The
newly for_ed "Gallex-collaboration" (I) intends to perform a solar neu-
trino experiment based on this reaction at the Gran Sasso Underground
Physics Laboratory (Italy) (2), The major goals of this experiment are:
- to provide the first experimental proof that the sun is producing
its energy by nuclear fusion.
- to limit or identify neutrino mass differences through eventual _e-
disappearance between sun and detector via neutrino oscillations.
The parameter range accessible in this way is many orders of magni-
https://ntrs.nasa.gov/search.jsp?R=19850026795 2020-03-20T16:49:55+00:00Z
tude below t h a t  f o r  o ther  v-osci 1 l a t i o n  experiments. 
- t o  i d e n t i f y  the cause of the "so la r  neu t r ino  puzzle" posed by the 
Ch lo r ine  so la r  neu t r ino  experiment (3) .  
2. Out l ine  o f  experiment components 
( 1 ) Underground labora to ry  
Cosmic ray-induced (p  ,n) react ions produce 7 1 ~ e  from 7 1 ~ a  j u s t  
as so la r  neu t r ino  capture, The overburden requ i red  t o  suppress 
- 
. the cosmic ray-induced 7 1 ~ e  product ion r a t e  t o  <2% o f  the  expect- 
,' 
- 
-' ed neu t r i no  induced product ion r a t e  i s  3100 m.w.e. (correspond- 
- 2  -1 i n g  t o  a muon f l u x  o f  Ip=21 m d ). The generous space a v a i l a b l e  
i n  the Gran Sasso underground labora to ry  i s  sh ie lded by 3200- 
3500 m.w.e., hence s u f f i c i e n t  f o r  the experiment. Background re -  
69 ac t i ons  w i l l  a l s o  be monitored by 6 9 ~ a  (p,n) Ge. This r eac t i on  
7 1 has an even l a r g e r  y i e l d  than 71~a(p ,n )  Ge, whereas pp-neutrinos 
d o h o t  $duce 6 9 ~ e .  
( 2 )  Target 
The h igh  costs  o f  Gal 1 ium fo r ce  us t o  choosg t he  small e s t  t a r g e t  
s i z e  which i s  s t i l l  s u f f i c i e n t  t o  produce a - +lo% r e s u l t  w i t h i n  
4 years o f  measurements ( t h i s ,  assumes the standard so la r  model 
neu t r ino  f l u x ) .  Given the achieved condi t ions (see below), t h i s  
corresponds t o  30 t o f  Ga (converted i n t o  8m GaC1 so lu t ion) .  
The requ i red  radiochemical p u r i t y  o f  t h i s  s o l u t i o n  has been de- 
monstrated t o  be achievable even i n  the m u l t i t o n  scale. Ga ac- 
qu i  s i  t i o n  i's pending. 
(3 )  Ge e x t r a c t i o n  system 
The q u a n t i t a t i v e  and reproducib le  ex t rac t i on  o f  Ge (as v o l a t i  1 e 
GeC14) from GaClg s o l u t i o n  and i t s  conversion i n t o  GeH4 (Germane) 
t o  be used as count ing gas has been developed and tes ted  i n  t he  
e a r l i e r  BNL MPI 1.3 t Ga p i l o t  experiment (4).  
(4)  Counting system 
" ~ e  1.43 d) decays by K (88%) L (10.3%), and M(1.7%) 
e l ec t ron  capture. Auger-electrons and stopped 'x-rays a re  counted 
i n  GeH4/Xe gas mix tures i n  m in ia tu r i zed  p ropor t iona l  counters 
made o f  u l t r apu re  mater ia ls .  Up t o  8 counters can be accommodat- 
ed i n  the  we l l  o f  a we l l  type NaI p a i r  spectrometer. Apar t  from 
i t s  ant ico inc idence mode, i t  serves i n  the  coincidence mode t o  
4 2 4 SH 8.1-2
identify 69Ge decays (TI/2=I.6 d; EC; EC+y; and e+ _" 2_). The Nal
crystal is surrounded by plastic scintillator anticoincidence
shields and passive Fe/Pb shields. The data from all active compo-
nents (proportional counters, Nal halves, plastic scintillators)are
fed into the PDP/LSI computer both for on line evaluation and for
storage on magnetic tape ("Mulia" system) (4).
3. Results of Ultra-Low Level Background Reduction
In the proposed configuration of the Gallex experiment one expects per
run an average of 7 71Ge decays during the first month afteY extraction
according to the standard solar model. From this it follows that back- 1
ground rates of order _ I c/I0 days must be achieved. Apart from the meas-
ures described above, this goal is in particular approached by a complete
pulse shape analysis using a transient digitizer to reject background,
mainly from Compton electrons and from electronic noise.
The energy deposition from Auger electrons and X-rays emitted in the
71Ge electron caoture decay results in a spectrum with 2 peaks: the_
peak at _I.2 keV and the K peak at _I0.4 keV, Counting efficiencies c are
determined using active 71Ge in prototype counters. Thus, the _cceptance
fields of 95% are defined in an energy-pulse form parameter plot for K-
and L peak (Fig.1).lThe pulse form is most efficiently parameterized as
I _PS(t)dt/S(tma x) where S(t) is the pulse heightG.l= S(T)/S(tmax). T
at time t after pulse °v onset, tmax is the time at which S(t) is maximal,
and T is an empirically selected parameter, here T=160 nanoseconds. In
this way, background pulses simulated by Compton electrons from a 60Co
source are rejected with an efficiency of 96.3% (L peak) and 99.9% (K
peak).
The values previously achieved with many counters (CL=25%, b=O.2 cpd;
CK=38%, b=O.08 cpd) would allow to determine a solar neutrino rate of
90 SNU with a standard error of 14% in a two year experiment (50 extrac-
tions). 90 SNU is the rate compatible with the Cl detector result inter-
preted in terms of astrophysical causes ("consistent" model). Meanwhile,
improvements have been made and background rates measured with some coun-
ters are about a factor of two lower than the values listed above. Fig.2
displays the results of a 51.5 day background measurement obtained with
one of the best counters. Here bK=O.04 cpd, bL=O.08 cpd. This would allow
to measure 90 SNU with a 30 t Ga detector during two years of operation
425 SH 8.1-2
to _ 12%, as compared to +I0% if the background were zero. Additional ad-
vances have been recently made with an improved counter design. Its dead
volume is reduced to 5% by directly evaporating the Al-cathode onto the
quartz tube and directly sealing the tungsten anode wire into the quartz
ends. With this counter type we obtain an overall efficiency of 74%
(_K=46%' _L=28%)"
E.ergy [keV] El_crgy [kcV]
0 2 l 6 8 R) 12 I.t 0 2 /- 6 8 R) 12 ! I.
,s)_.._. '.... ,w)........_unt_ t39 BacksroundI v- _or calibration "" _unt_ _3
751 '_l Count_ ¢t43 75 "_-'-I $1. S da_ds
I U
._()i " . . . • .- .. ., .. . . • ,. .• _ :m ,.;. ¢ , .._. ,:,.",
• .l •
0 |(NI _(N) _Nt ICN) ."dN) ( ItW) 2IN| ;$4N) |iX) FWN)
(:ha.:Icl - N.ml|lN.r (:ha|_ncl - Numl_r
/
Fig.l: 71Ge spectrum and 95% accept- Fig.2: Result of 51.5 days of
ance fields for genuine 71Ge background counting of coun-
yz< decays, The pulse shape para- ter #43 applying pulse
meter G.I is defined in the shape analysis with the
j t_xt. MULIA system. Acceptance
J windows for K and L peak
are indicated.
References
(I) Members of the collaboration: T. Kirsten (spokesman), W. Hampel,
G. Eymann, G. Heusser, J. Kiko, E, Pernicka, B. Povh, M. Schneller,
K. Schneider, H. V_Ik (HEIDELBERG) - K. Ebert, E. Henrich, R.Schlotz
" (KARLSRUHE)- R.L. M_Bbauer (MONCHEN) I. Dostrovsky (REHOVOT)-
M. Cribier, G. Dupont, B. Pichard, J. Rich, M. Spiro, D. Vignaud
(SACLAY) -G. Berthomieu, E. Schatzman (NICE) - E. Fiorini,E.Bellotti,
- O. Cremonesci, C. Liguori, S. Ragazzi, L. Zanotti (MILANO) -
L. Paoluzzi, S. D'Angelo, R. Bernabei, R. Santonico (ROMA).
(2) A. Zichichi, AIP Conf.Proc. 96, 52 (1983).
(3) J. Rowley, B. Cleveland, R. Davis, AIP Conf.Proc. 126, I (1985).
(4) W. Hampel, AIP Conf.Proc. 126, 162 (1985).


Results of ultra-low level 71ge counting for application in the Gallex-solar neutrino experiment at the Gran Sasso Underground Physics Laboratory

Results of ultra-low level 71ge counting for application in the Gallex-solar neutrino experiment at the Gran Sasso Underground Physics Laboratory

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