Etched track detectors in solar neutrino experiments

~

Pergamon

Nu¢i T m ~ lt.,dj_~. Meas.,

Vol.22, Nm 1--4,pp. 591-598, 1993 ElsevierSctenceLid Primedm OguatBritain 09694107W94$6.00+.00

ETCHED TRACK DETECTORS IN SOLAR NEUTRINO EXPERIMENTS R ILlt~,* T. ~trr~,* J. SKVAR~,*M. KR~IAR,tA. L n ~ t

S. K A u ~ and M. l~Jn~

• "J Stefan"Institute,Umversttyof L)ubljana,POB 100, 61111 Ljubljana,Slovenm;t'R. Boikovi6"Imutute,POB 1016, 41001 Zagreb,Cmatm,~Facuityof Engineering,AomoriUmvm'mty,2-3-1 Kobata,Aomon030, Japan

ABSTRACT Within the framework of LOREX experiment the mineral Lorandite (TIAsS2) of the old (5 x I0 e years) arsenic mine Allchar (Kavadarci, Macedonia) will be used to estimate average solar neutrino flux from the amounts of 2°sPb isotope (Tx/s = 1.5 x l0 T years) induced by 2°ST1 (v,, e-) 2°sPb reaction. In order to evaluate the amount of the 2°sPb isotope induced by 2°iPb (n, -f) z°sPb reaction the thermal neutron flux inside and above the mine was measured by our high sensitive etched track neutron dmemeters (CR-39/BE, CR-39/BN-1). The thermal neutron flux was found to be (2.7 ± 0.4) x I0 -s cm -2 s -I. In addition to cosmic muon interactions and spontaneous fissions the neutrons can be produced through various (a, n) reactions due to the high alpha particle flux from the decay chains of uranium and thorium. Therefore radon concentration in the mine was measured by our diffusion chamber type dosemeter utilizing CR-39 detector and was found to be 1.5 x 104 Bq m -s. The pcm/bilities to extend the application of etched track detectors to another solar neutrino experiments are discussed.

KEYWORDS Boron radiator; CR-39; lorandite; LOREX; neutron monitoring; radiation detectors; radon; solar neutrino detectors; thallium; etched track detectors.

INTRODUCTION A primary interest of the study of solar neutrinos arises from the fact that they can reveal the inner structure of the Sun. Detection of solar neutrinos at the Earth may also reveal yet undetected intrinsic properties of neutrinos. Theory indicates that the solar neutrino flux on the Earth is about 7 x I0 x° cm -2 s -I (Be&call and Ulrich, 1988; Person, 1990). The neutrino has very low (~ 10- ~ cm 2) cross section for interactions with matter so its measurement is exceedingly difficult. To 1986 the world's only solar neutrino detector was used in the experiment conducted in the Homastake Gold Mine at Lead, South Dacota, U.S.A., using STCl (~,, e-) STAr reaction. Now there are three additional man made solar neutrino detectors in action and fourth is planned to start in 1995. The so called Soviet American Gallium Experiment (SAGE) at the Baksan Nentrino Observatory in the Northern Caucasus, Russia and Gallium Experiment (GALLEX) under the Gran SMso in the Abruzzi Appennines, Italy, are based on the utilization of riGa (~,, e-) n G e reaction. The Kamiokande, located $00 km west of Tokyo, Japan in a zinc mine, uses ~, + e- --* + e- reaction. The most massive and ambitious detector now under construction, the Sudbury 591

592

R. ILI(~ et al.

Neutrino Obmrvatory (SNO), near Toronto, Canada is baasd mainly on us + d --* p + p + e - reaction. Various mineral deposits in the Earth that have been unattended for millic~ of years were proposod for a time averaged neutrino flux measurements. At Los Alamos National Laboratory a geochemical molybdenum mineral experiment is at the remmrch and development stage. Here the solar neutrino flux will be estimated from the me~urement of 9STc induced by ~Mo (us, e-) SmTc reaction in molybdenite ore from a very deep deposit in Colorado (Schwarzchild, 1990). Similar experiment, named LOREX (Lorandite Experiment), where the mineral iorandite (TIA~=) is used as solar neutrino detector via 2°STI (v,, e-) =°sPb is in problem (Proc., 1990). Thallium mineralization was found to be in arsenium mine Allchar, Kavadarci, Macedonia. Recently another geochemical experiment was proposed (Haxton, 1990). It was suggested to measure small amounts of lSexe in deep telluride ores which can be induced by l=eTe (vs, e-) l=eI reaction. Here the l=eI is transformed to 12SXe by beta decay. Table 1. Proton-proton chains of nuclear reaction in the sun (Bahcall, 1990; Wolfenstein and Beier, 1989) Chmn I

Chain 1I

Chain m

Chain IV

p+p ---,d+.-+~,. (~.TS~) or p+e--I-p -.* d-I-~, (0.25~) d + p --* SHe + "7 aHe+aHe ~ 4He+p..t-p (8S~)

aHe+aHe ..-., 7Be+ "I (16%)

8He+p --., *He..I-e++v,

(2xi0-6~)

7Be+e- -* TLi+ v, (99.9~)

?Be+p --* 8B+'y (0.1~)

7Li+p --* 4He+4He

SB -* 8Be+e++ u, SBe ~ '6He + "He

The current solar neutrino experiments measure lower neutrino fluxes than the expected value from standard solar model (SSM) as described by Bahcall and Ulrich (1988). To explain this discrepancy the new experiments should be designed and/or new neutrino physics becomes necemary. Particular attention should be paid to the recalibration of detectors and characterization of the background radiation. For instance, more than 30 nuclear reactions should be conmdered to analyze the background problems of the LOREX experiment (Freedman et a/., 1979). To the best of our knowledge Solid State Nuclear Track Detectors (SSNTDs) have not been yet used in solar neutrino experiments. The aim of the present work is twofold: (i) to analyze the potential application of SSNTDs in solar neutrino experiments for the assessment of background radiation and (ii) to present our first results obtained within the framework of LOREX project. Our primary concern is with neutrons and alpha particles for which the corresponding intensities in the Allchar mine have not yet been estimated.

SOLAR NEUTRINO EXPERIMENTS According to the SSM energy is generated in the interior of the Sun by fusion. The chains of nuclear reactions in the Sun are given in Table 1. The resulting neutrino spectra is shown in Fig.1. Experiments carried out so far have provided only limited information about energies and number of neutrinos emitted by the Sun. The basic characteristics of the current and future experiments are given in Table 2. The primary detector in Homestake experiment is a 380 000 ] tank of perchlorethylene (C=Oi4) located 1478 m under ground in the mine. In this passive radiochemical experiment the detector is expmed to solar neutrino for one or two halflives (31 days) of the STAr neutrino induced reaction products. The detector is then purged with helium

SOLAR NEUTRINO DETECTION

593

gas to collect the sTAr isotope.. The energy threshol&~or the STCl(v,, e-)STAr reaction is 0.81 MeV.

~

t0 ~2

~ 10lo

zBe

P4

'E

I0s

P :P

i

i00 10 4 10 2

0.1

I NEUTRINO ENERGY (MeV)

10

Fig.1. Solar neutrino spectrum calculated by standard solar model (Bahcall, 1990) The Kamiokande H is an electron scattering experiment. This water Cherenkov detector is sensitive to the 8B solar neutrino. A solar neutrino signal must be extracted from background of various sources, indicating applicability of nuclear detectors with high sensitivity and selectivity such as SSNTDs. The SAGE experiment has been running with the initial 30 tons of its planned 60 ton liquid metallic gallium. The GALLEX experiment uses 30 tons of natural gallium in an aqueous gallium chloride solution. The transmutation threshold for gallium is only 0.23 MeV. Thus much of the pp neutrino spectrum can be monitored with gallium detector. In these experiments some neutrinos have been detected, but the number appears to be significantly lower than what was predicted by SSM. The Homestake and Kamiokande experiments give about 25 and 45% of the neutrino flux predicted by SSM (Hirata eta/., 1992; Shi et a/., 1992). The preliminary observed capture rates in the SAGE and GALLEX experiments were 20 + 15/20 (stat) 4- 32 (w/st) and 83 4- 19 (stiLt) 4- 8 (w/st) SNU respectively, compared to 132 SNU (Abasov eta/., 1991; Hirata et a/., 1990; Shiet al., 1992). 1 SNU (Solar Neutrino Unit) is 10-a° captures/atom second. The SNO detector will have 1000 tons of very pure DzO in a transparent vessel surrounded by 5000 tons of H~O which serves as the radioactive shield (Balantekin and Loreti, 1992; Ewan, 1992). The Cherenkov light generated by electrons produced in neutrino interactions will be detected by an array of photon detectors. The characterization of the background radiation field in the cavity (20 m in dis, 30 m high), located 2072 m under ground in the nickel mine, by SSNTDs is recommended. The SNO detector will be sensitive only to SB neutrinos. In the 9SMo detector thousands of tons of molybdenite ore will be processed to get a few million atoms of 9STc. An ultra sensitive mass spectrometer will enable separation of 9STc atoms from other technetium isotopes. This experiment will provide a measure of the solar neutrino flux averaged over the last several million years. Some of the problems associated with background from muons and other sources may su~_m~f_ully be solved by SSNTDs. The proposal to use telluride ore as a solar neutrino detector which might tell us on the SB neutrino flux variation on a very long time scale (l~Xe is stable so the record is permanent) is considered as an exceedingly dimcult experiment (Haxton, 1990). The use of 7Li, lIB, SlBr,/OAr and SlSln as solar neutrino detectors have been considered (Bshcall and Ulrich, 1988). More details on the LOREX experiment are given in the next section.

R. ILIC et ~ .

594

Table 2. Bmic characterktics of current and future solar neutrino experiments Primary Detector

Reaction

Energy Treshold (MeV)

Capture Rate (SNU) predicted

Name of Experiment

S?Cl(~,, e-)STAr

0.814

7.9 -4- 2.6

2.1 4- 0.2

Homestake

H=O

s~ + e- --* v + e-

7.5"

0.46 r

Ksmiolamde 11

GaCls

r i g a (v,, e - ) n G e

0.233

132 4- ~

83 ± 21

GALLEX

Ga

71Ga (sp,, e-)71Ge

0.233

132 ± ~

20 -'- 38

SAGE

DzO

~e -~- d --* p ~ - p - ~ -

5.0

6.01 -4- 2.28

Mo mineral

9SMo (is,, e-)~l~c

1.68

17.4 -4- ls.s 11

TIAsSI

S°STl (~,, e-)S°sPb

0.054

268

12eTe(~,,, e-)lSe]

2.15

12.2

C2C[4

Te mineral

SNO

LOREX

a - effective treshold determined by electron observation r - relative to the predicted value

ETCHED TRACK DETECTORS IN LOREX EXPERIMENT Lorex p r o j e c t The Lorex is a complex mnlt/dieciplinary project proposed by Freedman et ~. (1976). It is based on the determination of =°sPb isotope induced by solar neutrino in thallium ore. The only suitable thallium deposit is in Allchar mine. Due to the long (1.5 x 107 years) halflife of =°sPb the solar neutrino flux averaged over the age of mineralisation, which was estimated to be 5 x I06 years (3akupi et eL, 1982), can be measured. The status of the project was surveyed in Int. Syrup. on Solar Neutrino Detection with Tl-205 (Proc., 1990). The current research is oriented to the: (i) development of suitable procedure for the separation of lorandite from the ore, (fi) determination of trace element in lorandite, (fii) investigation of erosion rate, (iv) extraction of Pb from ]orandite, (v) determination of log f, value for S°STl, (vi) improvement of techniques for selective measurement of S°sPb isotope and (vii) determination of background radiation. Detsik of the detectors are given in literature (Proc., 1990). 2°sPb can be produced by various ruction chains induced either by muons from coamic rays or by radioactive contaminants in the lorandite and other minerals. An analysis carried out by Preedman (1979) showed that 34 diffm~mt nuclear reactions induced by protons, neutrons, alphas, muous and pious directly or indirectly contribute to the production of 2°sPb. The main reactions are summarized in Table 8. From the Table $ it is evident that Z°sPb can be induced by thermal neutrons via =°4Pb (n, ~) =°sPb reaction. Neutrons can be produced by: (i) cosmic muons as a result of electronmsnetic interact/on with nuclei in the rock, intoraction of photons from cascades induced by 6 electrons, e+ - e- Im/rs and bremmtrshlung photons as well as muon capture by the nuclei in the rock, (ii) spontaneous bsion of the uranium and thorium isotopes presented as the impurities in the lorandite and other minerals and (iii) alpha capture reactions where the alpha

SOLAR NEUTRINO DETECTION

595

particlm are emitted in U and Th decay chains. In order to intimate the amount of 2°sPb induced by the 2°4pb (n, ~) reaction the thermal neutron flux in the mine should be known. For this purpose a high sensitive neutron doasmeter utilizing CR-89 detector was used (ni~ et eL, 1998; gutej and N~j|er, 1998). To eetimate the contribution of (e,, n) reaction to the production of neutrons the radon concentration in the mine wee also measured by our diffusion type radon dosemeter (Humar et ~., 1998; gutej eta/., 1988). In this connection it should be mentioned that neutron induced fiseionography with Makrofol KG was suc~m~ul]_y used for the measurement of uranium and thorium concentrations in Allchar ore (~ubi~i~ et eL, 1988). In addition fission track dating of minerals from the Allchar deposit carried out by Jakupi et aL (1982) emphasizes the role of etching technique in this and similar geochemical neutrino experiments. Table 8. Po~ible background reactions which can produce 2°sPb ewaporation and spallaUon of ~Bi,mTh, " U , ~Pb ~ ~ Pb ( ~ ' n) ~Pb T m Tl (T,~') mPb '

P n

"~

~,..,~---. ~,-,v~;xn+yp)

~---..., np

~Pb (n,7) mPb

DBi (~-,v~4n) ~Pb

U,Th

~

impurities ~ .

~

~Pb (n,2n) ~Pb

, a + ROCK ~

spontaneous fission

, mTl(p,n) ~ P b

n ,

n\

n + ROCK

, p - -

A program of the measurements of neutron and/or radon in Allchar mine started in 1987 ( K r ~ a r et eL, 1990). Since then four long time exposures have been performed. In the first run variety of CR-39 detectors foils (MA-ND/a, MA-ND/p, TASTRAK) alone and/or with boron (Li2B40?, BN-I) and polyethylene neutron converters were exposed at di~erent places of the mine for 829 days (from 21.10.1987 to 15.9.1988). From this preliminary experiment we learned that neutron flux in the mine is < 2 x 10-s cm -2 s - l , while the radon concentration was eetimated to be of the order of 104 Bq m -s. During continuoul improvement of our technique three additional rune were performed. The exposure time were 815 (7.6.1989 - 18.4.1990), 72 (18.4.1990 - 29.6.1990) and 658 (1.11.1990 - 22.8.1999) days respectively. The quantitative results obtained in the last three experiments are reported in the next subsection. The measuring stations are given in Fig. 2. ~gutrma measure~mat The thermal neutron dosematers used consisted of MA-ND/a (MOM, Hungary) detector and BN-1 or BE 10 (Kodak Pathe, France) neutron converter screens placed in AI radon tight foil. After expoeuro the detector foils were etched in 6.25 M NaOH at 70°C for 5 hours. The dceeme~ were calibrated using our neutron facility of TRIGA Mark H reactor and gold actiwstion foils (~utej and N~/er, 1998). The rseponas of the doeemeters was found to be 7.7 x 10-s and about 2.0 x 10-s traclm/n for BN-1 and BE-10 converters. From the data presented in Table 4 it is evident that the thermal neutron flux in the mine (poeition 4) k about 2.7 x 10-s neutrons/cm s s. The neutron flux at the entrance of the mine (poeition 1) and at the surface of soll above the

596

R. ILI{~ tn a/.

TI mineralization (position 6) was found to be about 60% higher and is comparable with value obtained at the two reference positions (Ljub]jana and Zalpmb) located some I000 km from the mine Unfortunately detectors at the another position were damaged and/or lost.

6 ENTRANCE HOR 823 m

DOLOMITE ~ANDEZITE COMPACT TI,- ore

0

50m

t

T [ - MINERALIZATION

I

Fig. 2. Profile of the Allchar mine with marked measuring stations

Table 4. Measured value of thermal neutron flux and radon concentratimz Pm/tion

Thermal neutron h x [cm-'a -z ]

(7.6.89- 18.4.90)" AlIchar 1 2 3 4 6

(1.11.90- 22.8.92) b

(4.4 -4- 0.6) x I0 -s

Radon Concentration [Bq m -s]

(7.6.89- 18.4.90)

(18.4.90- 29.6.g0)

(2.20 ± 0.30) x 10 z

(2.9.4- 0.7) x 10 z (1.56 ± 0.30) x 10 ~ (1.4s ± o.ss) x 1o~ (0.99 ± o.sz) x zo'

(1.4 ± o.xs) x lO' (1.61 ± 0.20) x 104 (z.s9 ± o.ze) x zo'

(2.7 ± 0.4) x 10-s (3.0 ± 1.2) x 10 -s

0.8)

Ljublj.a. b,~

(1.0 ±

Za~p.ebb,~

(1.7 d: 0.7) x 10-s

" - measured with BN-I convertor - measured with BE-10 convertor c . roof apartment of one floor house - roof apartment at the lOth floor

x 10 -a

SOLAR NEUTRINO DETECTION

]llatl~

597

~lnmaa.~mnna~tt

The radon dose_mAterused in this experiment is a diffusion chamber equipped with MA-ND/a detector. The dmemeter is a plastic cup (0.25 dms) closed by a membrane made of fiberglass filter. It is described in details elsewhere (Hums: eta/., 1993; ~utej et a/., 1988). After the exposure the detector foils were etched and evaluated according to the procedure described in previous section. The response of the dosemeter is 6.2 tracks cm-:/kBq m -s h. From the result presented in Table 4 it can be seen that the radon concentration in the mine was about 1.5 x 104 Bq m -s. In order to determine the production of :°5Pb isotope by neutrons and to analyze its yield from different sources further measurements by our techniques are recommended. In particular the depth profile of the thermal neutrons may offer useful information for the LOREX experiment.

Acknowledgements: The help and many useful suggestions of Dr. B.A. Logan (University of Ottawa), Mr. T. Tustoni~ and Dr. I. ~.limen (both R. Boikovi~ Institute) is acknowledged. We would like to express our thanks to Mr. T. Leov and Mr. D. Muksetov (Rudkop Enterprise) for their kind hospitality and technical assistance.

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