Update on the measurement of the solar neutrino flux with the Homestake chlorine detector

! [gill I =I'_,I~'| ',l-"b"k,'~[l~"]l PROCEEDINGS SUPPLEMENTS

~.~ KI,~EVlEI~

Nuclear Physics B (Proc. Suppl.) 38 (1995) 47-53

UPDATE ON THE MEASUREMENT OF THE SOLAR NEUTRINO FLUX WITH THE HOMESTAKE CHLORINE DETECTOR B. T. Cleveland, T. Daily, R. Davis, Jr., J. Distel, K. Lande, C. K. Lee, P. Wildenhain 4. and J. Ullman b aDepartment of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104-6396 bphysics Department, Herbert Lehman College, New York, NY The Home,stake chlorine solar neutrino detector has been measuring the flux of v~ from the decay of ~Be and SB since 1970. The average measured flux is 2.55 + 0.25 SNU.

1. I N T R O D U C T I O N

AND HISTORY

At this conference we remember the many fundamental contributions that Bruno Pontecorvo made to neutrino physics. His interest and insight into physics served as a constant reminder to us of the joy of our field. One of the first of these contributions was Pontecorvo's suggestion in 1946 that the aTC1 - 37Ar method was ideally suited for the detection of neutrinos []]. The most interesting question that could be addressed by a ve detector was the experimental determination of the flux of lJc emitted by the Sun. Neutrinos were the only probe that could directly observe the nuclear fusion reactions in the core of the Sun that were assumed to be the source of the radiant energy emitted by the Sun. In 1965, almost 20 years after Pontecorvo first suggested the use of 37C1 as a neutrino detector, Raymond Davis began construction of a 37C1 based solar neutrino detector employing perchloroethylene at the Homestake Mine in Lead, South Dakota. Thus, from Pontecorvo's novel suggestion there evolved a solar neutrino detector which took advantage of: 1) the availability of an inexpensive, nonhydrogen containing chlorine compound, C2C14, that is a liquid at room temperature, 2) the ease with which the noble gas, argon, can be removed from the chlorine containing liquid, *This research is supported by the U.S.National ScienceFoundation grant AST91-15517 0920-5632/95/S09.50© 1995 Elsevier Science B.V. All rights reserved. SSDI 0920-5632(94)00732-2

3) the high efficiency with which a7Ar can be recognized by its decay back to 37C1 by K orbital electron capture, 4) the unique characteristics of the nuclear reactions in the A = 37 system that permit precise determination of the energy dependence of the cross section for neutrino interactions on aTc1.

2. D E T E C T O R RESPONSE NEUTRINOS

TO SOLAR

The Homestake perchloroethylene solar neutrino detector responds to neutrinos via the reaction Ue + 37C1 ~ a7Ar + e-

Since the threshold for this reaction is 814 keV, the detector is sensitive to neutrinos from all neutrino generating reactions in the Sun except for p-p fusion, namely: 7Be electron capture, the decay of a3C, 15N and 8B and P + e - + P . In addition to being produced by neutrinos, 3TAr atoms are also produced by (p,n) interactions with 37C1. The dominant source of such protons is from the photonuclear interaction of high energy cosmic ray muons. To a lesser extent, protons are also produced in the detector by neutrons from the surrounding rock that generate protons via (n,p) reactions within the detector. The number of 3TAr atoms in the detector at any given time is:

48

B. 77. Cleveland et al./Nuclear Physics B (Proc. SuppL) 38 (1995) 47-53

N((I)icri + BKGND) NoTo(1 - e -t/T°) where Oi is the electron neutrino flux from the i th neutrino generating reaction in the Sun, ai is the average cross section for these neutrinos, No = 2.18x10 a°, the number of target atoms of aTC1 in the detector, To = 50.5 days, the mean lifetime of afar, and t is the time since the last extraction. Our typical detector exposure time is 60 days, but that occasionally varies [2,3]. 3. D E T E C T O R

OPERATION

The detector consists of a cylindrical tank, 6.1 meters in diameter and 14.6 meters long. About 95% of the detector volume is filled with perehloroethylene and the remaining 5% with helium gas. Since diffusion time constants for argon are much longer than the time between extractions, the neutrino and background produced StAr remains dissolved in the liquid in close proximity to the production site. The argon gas is removed from the perchloroethylene by sweeping the liquid with a flow of helium gas via a set of eductors. All but two of these eductors maintain equilibrium between the argon dissolved in the detector liquid and that in helium gas above the liquid. The remaining two eductors maintain a flow of helium gas that carries argon gas out of the tank and through a cryogenically cooled charcoal trap. The argon atoms are adsorbed on the charcoal while the helium gas passes through the charcoal and returns to the tank. The eductors are driven by a liquid circulation pump that draws perchloroethylene out of the b o t t o m of the tank and then forces it back into the tank through the eductor system. 4. 3tAr E X T R A C T I O N In order to monitor the extraction process, a small amount of stable, isotopically labeled argon carrier gas is added to the detector. The carrier gas used alternates between a6Ar and aSAr. This carrier gas, about 0.1 cc at STP, is added at the end of the previous extraction. By following the carrier gas through the extraction process, we can

monitor the efficiency of the various steps in this process. The extraction of argon from the tank has been experimentally demonstrated to be exponential, with the fraction of argon gas remaining in the tank equal to e -~V where V is the total volume of gas that has passed through the charcoal trap and ol is a constant [4]. c~ is measured for each run. The observed values of ~ for each of the extractions from the Homestake tank are shown in Figure 1. The mean value of a is 7.645 4- 0.159 megaliters -1. There is no indication of any variation of the value of oi over the period of detector operation since 1970. We typically sweep the tank with 390 kiloliters of helium giving an extraction efficiency of 95%. After completion of the extraction cycle from the tank, the argon gas is removed from the charcoal trap and purified by gas chromatography and its volume measured. The purified argon gas is then put into a very small proportional counter. The decays of a f a r are followed for approximately one year. At the end of that time the gas is removed from the counter, the volume of argon gas is again measured and its isotopic composition determined by mass spectroscopy. The fraction of carrier gas that is recovered from the proportional counter, corrected for extraction fraction, is called the gas processing efficiency. In Figure 2, we show the gas processing efficiency for each of the extractions since 1970. Again, there is no apparent variation of efficiency with time. The average gas processing efficiency is 95.8 4- 0.7%. Two runs show low efficiency. In each case, a known problem arose during gas processing and was corrected for in determining the final result. 5. aTAr D E C A Y S aTAr decays back to 37C1 by orbital electron capture. 90% of these captures involve electrons from the K shell, releasing 2.82 keV. Of the K shell captures, 90% result in the emitted energy being released as Auger electrons, the remaining 10% result in the emission of soft x-rays. The argon gas, together with 7% methane, is put into a very small proportional counter, typically 6 mm diameter by 30 mm long. The

B. T Cleveland et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 47-53

10

.

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

. . . .

I

49

. . . .

I

. . . .

I

. . . .

i

'

I

,

*D •

•

•,lA

fD

0

(9 gl

.o *d

Mean Value Gas Extraction Coeff: 7.645 +/- 0.159

, , ,

0

I , , , ,

I

i

J

i

,

I

I

. . . .

. . . .

'

3o

20

4o

so

e,o

i

,

70

,

,

,

I

,

,

,

so

,

I

. . . .

9o

I

,

:oo

,

,

,

I

,

H•

,

,

,

~2o

~3o

R ~ Number

Figure 1 : Gas E x t r a c t i o n Coefficient vs. R u n N u m b e r 1.2

I

÷.

i

I

I

I

i

~

i

100

!10

120

130

1.0

0.8

+

J

0.6

0.4

Mean: 0.958 +/- 0.007 0.2

0.0

30

40

50

60

70

80

90

Run Number

Figure 2 : Gas Processing Efficiency vs. R u n Nllmber

50

B.T. Cleueland et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 47 53

37Ar decays that result in the emission of Auger electrons give rise to fast rising, monoenergetic pulses. Most background events involve throughgoing electrons. The signals from these background events are neither monoenergetic nor fast rising. The rate of background events that satisfy the energy-rise time criteria we impose is very small, typically one event per 100 days or less. There are two geometric and electric field effects that result in undetected 37Ar decays. The first is that there are regions of the gas volume of the proportional counter in which decays are not recorded - "dead regions". The main "dead region" is a space between the counter cathode (iron) and the counter envelope (quartz) that is left to allow for differential expansion during counter bakeout. There is also a small region of the counter filling tube in which gas can lurk. The second geometric reason for non-detected 3TAr decays is that at the ends of the proportional counter the accelerating electric fields are smaller than in the counter center, resulting in lower amplitude pulses. Recently, we have added guard rings to the cathodes to reduce the fringing field effect [5]. The effective geometric efficiency of each counter is measured by filling that counter with a known amount of 37Ar and determining the fraction of those decays that are observed and satisfy our selection criteria. Since the geometric parameters of each counter are very well known, we also calculate the geometric efficiency of each counter. There is very good agreement between the measured and calculated geometric efficiencies. During the counting period, each counter is periodically calibrated with an external 5"~Fesource. The emitted x-rays are 5.9 keV. We also see a Fe escape peak in the counter at 2.96 keV, very close to the 2.82 keV peak from 37Ar K orbital capture. From these calibrations we are able to determine the counter pulse amplitudes and rise time that corresponds to 3TAr and the counter resolution thus defining the event acceptance window. We fit the observed decay rate from a given extraction to

Ae -t/T° + B

where A is the amplitude of a7Ar decay rate and B is the rate of background events inside our acceptance window [6]. We have traditionally used one full width at half maximum as the acceptance window. We also test the stability of the result by varying the acceptance window and seeing how A varies. Of course, for each acceptance window we must use the appropriate counter geometric efficiency. In Figure 3 we show the combined time distribution of the events within the acceptance window for all extractions.

6. B A C K G R O U N D S As we mentioned above, the largest background is due to the remnant of cosmic ray muons that reach our depth. The background contribution from cosmic rays was determined by measuring the 37Ar production vs. depth in a set of small portable perchloroethylene tanks at shallower depths, up to 700 m.w.e., and then extrapolating to the depth of the solar detector. This extrapolation [7,8] is done by scaling the 3TAr production rate with the depth dependence of the total muon flux and the average muon interaction probability by E(lt) °7. In 1980, E. Fireman [9] suggested that we measure the extrapolation by use of a 39K target, in the form of a solution of KOH, where cosmic ray interactions convert 39K into 37Ar. The potassium detector was compared with the portable perchloroethylene detectors at shallow depths and then operated at the full depth adjacent to the main chlorine detector. These measurements give a cosmic ray background of 0.055 + 0.015 3TAr per day. The earlier analytic extrapolation procedures agree very well with this measurement [10]. The neutron flux out of the walls of the detector room was sampled at several locations around the room. Combining that measured neutron flux with the effective cross section for (n,p) followed by 37Cl(p,n)37Ar gives a neutron induced background of 0.025 i 0.025 3TAr per day. We plan to remeasure the neutron flux with the hope of reducing the uncertainty in this background.

B. T Cleveland et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 47-53

tq

51

104

t) ° w.t

6 U

$

e~

4 3

e$ .,a

2

e~ o

r~

1 0 -2-

0

50

100

150

200

250

300

Time After End of Bxtracti0n (Days) Figure 3 • T i m e Distribution of Events W i t h i n the A c c e p t a n c e W i n d o w o,8

'

0.7

runs

runs

runs

runs

18-51

52-89

91-124

18-124

'

r

.

.

.

.

i

.

.

.

.

~

.

.

.

.

I

.

.

.

.

4.0

tM "10

0.6

.q: O ¢3

3.0

0.5

wi

qO O e-t

0.4

2,0

O e-.D

0.3 II

O eZ

._m O ¢D

0.2

1.0

0.1 0.0

1

1970

i

b

J

L

~

1975

i

i

i

i

[

1 980

i

i

J

I

1985

J

i

i

i

J

*

J

r

k

1990

Year Figure 4 : T i m e - Averaged 3TAr P r o d u c t i o n Rate

0.0

52

B.T. Cleveland et al./Nuclear Physics B (Proc. Suppl.) 38 (1995) 4 ~ 5 3

7. E X P E R I M E N T A L

CHECKS

A number of experimental checks have been carried out to ensure that the detector functions properly and that the recovery of the neutrino produced aZAr is not impeded. The most crucial of these tests is to verify that aTAr is not bound to the remnants of C2C14 molecule after the neutrino interaction where the recoil energy of the argon atom or ion is about 20 - 30 eV. Although such a binding is highly unlikely, it was tested by making C2C14, with a6C1 atoms. 36C1 is unstable with a half life of 3x105 years and beta decays into a6Ar. Since the energy spectrum of the 36Ar in these decays is similar to that of a7Ar produced by solar neutrino interactions, full recovery of the 3eAr from the 36C1 labeled C2C14 would imply that argon atoms with small recoil energies are not trapped in the remnant molecule. After an appropriate time period the 36Ar was swept out of the labeled C2C14. The recovery of 36Ar was 1.02 4- 0.039 of that predicted from the 36C1 life time indicating that molecular trapping did not occur. 8. S Y S T E M A T I C

ERRORS

We have examined the various factors discussed above to determine the contribution each of them makes to the systematic error. These contributions are given in the following table, and combine in quadrature to a total of =t=7%. Extraction Efficiency

4- 1.5%

Counting Efficiency

4- 3.0%

Target Volume

i 0.2%

Background Effects

4- 6.0%

Other

-t- 2.0%

9. S O L A R

NEUTRINO

RATE

The number of 37Ar - like events measured by the proportional counters is now corrected for (1) counter efficiency and (2) extraction efficiency to determine the number of 3TAr atoms present in the detector at time of extraction. After background subtraction, we have:

E(~iai) = 2.55 4- 0.17 (statistical) -t- 0.18 (systematic) SNU. giving a combined result of: 2.55 ~ 0.25 SNU. We have also divided these data into three equal time intervals, 1970-77, 1978-85 and 198693, as shown in Figure 4. The average solar neutrino fluxes for these intervals are: Period

Average Neutrino Flux

1970 -- 77

2.52 4- 0.41 SNU

1977 - 85

2.27 4- 0.30 SNU

1986 -- 93

2.78 -4- 0.35 SNU

We have also evaluated these data with a larger energy window, two FWHM, and find the overall rate to be 2.52 4- 0.23 SNU, essentially the same as for one F W t t M . REFERENCES

1. B. Pontecorvo, Chalk River Report PD-205 (1946). 2. R. Davis, Prog. in Part. and Nuel. Phys., 32 (1994) 13. 3. R. Davis, Proc. of the Int. Workshop on Neutrino Telescopes, Venice, February 22-24, 1994, ed. by M. Baldo Ceolin. 4. R. Davis, D. S. Harmer, and K. C. Hoffman, Phys. Rev. Lett. 20 (1968) 1205. 5. R . T . Kouzes and D. Reynolds, I E E E Trans. on Nucl. Sci., 36, no. 1 (1989) B46. 6. B. T. Cleveland, Nucl. Instr. and Methods, 214 (1983) 451. 7. A. W. Wolfendale, E. C. M. Young, and R. Davis, Jr., Nature Phys. Sci., 238 (1972) 130. 8. G. L. Cassiday, Proc. 13th Int. Cosmic Ray Conf., August 17-30, 1973, Denver, vol. 3, pp. 1958, Colorado Associated Univ. Press, Boulder (1973).

B.T. Cleveland et aL /Nuclear Physics B (Proc. Suppl.) 38 (1995) 4 ~ 5 3

9. E.L. Fireman, B. T. Cleveland, R. Davis, Jr., and J. K. Rowley, Proc. Conf. on Solar Neutrinos and Neutrino Astronomy, Lead, S.D., Aug. 23-25, 1984, ed. by M. L. Cherry, K. Lande, and W. A. Fowler, Am. Inst. of Phys. Conf. Proc., 126 (1985) 22. 10. (3. T. Zatsepin, A. V. Kopylov, and E. K. Shirokova, Yad. Fiz., 33 (1981) 378.

53