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(Si) for the GALLEX solar neutrino experiment
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A329 (1993) 541-550 North-Holland
Section A
The miniaturized proportional counter HD-2(Fe) /(Si) for the GALLER solar neutrino experiment R. Wink, P. Anselmann, D. D6rflinger, W. Hampel, G. Heusser, T. Kirsten, P. M6gel, E. Pernicka, R. Plaga and C. Schlosser Max Planck Institut für Kernphysik, P.O. Box 10 39 80, W-6900 Heidelberg, Germany
Received 14 December 1992
The miniaturized proportional counters used for the detection of 7'Ge in the solar neutrino project GALLEX are characterized . We also report on the construction techniques applied to build these counters and to achieve the described performance . A very low counting background is achieved by, among other things, careful selection of the materials used for construction . The total 7'Ge detection efficiency after applying cuts to reduce the background is about 66% .
1. Introduction The solar neutrino experiment GALLEX [1-3] is taking data relevant to the Sun since June 1991 at the Gran Sasso underground laboratory [4]. It monitors the low energy neutrino-flux of the sun. In a tank containing 30 .3 t of natural gallium in the form of 101 t GaC1 3 hydrochloric aqueous solution, solar neutrinos produce a measured rate of (0.8 ± 0.2) atoms of "Ge per day. With a "Ge half life of 11 .43 days, about 1071 Ge-atoms are present after 3 weeks of exposure . These atoms (and 1 mg of inactive germanium carrier) are extracted and chemically converted to the gas germane (GeH 4) [5,6]. At the end the 7' Ge is contained in about 0.3 ml (STP) of germane. In order to count the 7' Ge-atoms the electron-capture back reaction to 7'Ga has to be observed: 7'Ge(e- , ve) '1 Ga . This reaction results in X-rays and/or Auger electrons which can be detected in gas proportional counters . The energies of the Auger electrons and X-rays are given in table 1 . Necessary requirements for the detection of such a small number of "Ge-atoms are: - a high detection efficiency ; - a very low and time stable background to reliably distinguish signal from background; - a low threshold (< 0.5 keV); - a technique to transfer the few 7'Ge-atoms quickly from the target tank to the detector, so as to minimize the time between the end of bombardment
and the start of counting, because of the relatively short half life of nGe . A miniaturized proportional counter can meet these criteria. However, the minimal size of the proportional counter is limited by the volume needed for the GALLEX counting gas (800 Torr Xe (70 vol.%)-GeH 4 (30 vol.%)), which is about 1 ml . The choice of the gas mixture is the result of a complicated optimalization [7-9]. Miniaturization of the proportional counter helps to reduce background because: - less material is needed for the construction of the counter. Any material, despite of careful selection, is still a source of radioactive background, scaling with mass (self contamination ; see section 3); - the miniaturized counter is a smaller target for external background sources (like 'Y'S, muons, - - - ). A "Ge energy spectrum acquired with such a proportional counter is shown in fig. 1. The energy resolution of the L-peak (1 .2 keV) is typically M 45% Table 1 Energies of the of Auger electrons and X-rays of the 7'Ge decay [27] Probability 1%]
Captured electron
41 .4 5 .2 41 .1
K K K
10 .3
2 .0
L M
0168-9002/93/$06 .00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
Sum energy of Auger electrons [keV] 10.37 0.11 1 .l2, 1 .15 1 .3 0.16
Energy of X-ray [keV]
10.26 9.25, 9 .22 -
54 2
R. Wink et al. / Miniaturized proportional counters for GALLEX
700 N
71 Ge energy spectrum
c ô
L - Peak
500 300
z
K- Peak
100
0
5
10
15 Energy ( keV ) Fig. 1 . 7IGe energy spectrum .
(FWHM), therefore a low threshold of only 0.5 keV is needed . The K-peak (10.4 keV) has a typical FWHM of - 28%. The "Ge decay is a "point like event" inside the proportional counter, hence the signal of a charge sensitive preamplifier has a fast rise . On the other hand a background event caused for instance by an ionization track inside the proportional counter (e .g ., from a Compton electron) usually is an extented event. It leads to a slowly rising signal . Then, the pulse shape of the signal can be used to distinguish "Ge from background events (see fig. 2) . The remaining counter background and its origin are discussed in section 4. The chlorine solar neutrino experiment by Davis and coworkers [101 involves a similar counting problem and miniaturized counters were developed to detect the electron capture of "At . The aim of this work was to improve these "Davis-type" counters and then apply them to 7'Ge-detection . The construction of extremly low level background counters is of general interest for many potential other uses . Success depends critically on many details. This motivates us to pass on our
700 600 500 E u L 400 LM
x m 300 d N
D_
200
2. Counter construction Starting from the experience with Davis-type proportional counters, many new counter types were developed at Heidelberg during the last 10 years. [8,13191. This row is labeled "HD" for "Heidelberg" . The first type was "HD-l", "HD-2" comes in three variations: one with an evaporated aluminum cathode ("HD-2"), and two with solid cathode made from iron ("HD-2(Fe)") or silicon ("HD-2601. Experience was increasing in that order. First we describe those items which are common to most counter types. 2.1 . Cleaning of quartz parts After the glassblower has fixed all quartz parts together, they are first cleaned over night in aqua regia, then put for 2 h into n-hexane and at the end for few seconds in hydrofluoric acid . Between each cleaning step they are rinsed with quartz distilled water and dried at 120°C for 2 h. After the last cleaning with HF it is crucial to rinse the two capillaries very well . For this, distilled water is filled into the counter through the open top end and subsequently pressed out with nitrogen/ hydrogen (4%) gas through the capillaries. This rinsing of the capillaries is one important step for the successful sealing of the anode and cathode wires (see below) . Otherwise, remaining impurities react during the sealing procedure with the wire, resulting in breaks . 2.2. Anode and cathode wire of the HD counter types
100 0
experience by a scrutinous description of the procedures and recipies . In section 2 we describe the counter construction and the techniques to mechanically treat, clean and assemble the required parts. Various measures led to an increase of counting efficiency compared to Davis-type counters . The many years of counter development have always been accompanied by radioimpurity measurements and basic studies of background components [11,121. All individual construction parts of the proportional counters have been selected with the help of non-destructive low-level Ge-spectroscopy and partly also of radiochemical methods. The sensitivity level for the Ge-detection was as low as mBq/kg once large samples (> 0.5 kg) were available (section 3).
r
0
__1
_i
40
i
i
80
i
i
120
i
160
200
Time [nsecl Fig. 2. Pulse shape of a 71 Ge-pulse compared to a background pulse.
As standard wire for the anode and for making the first part of the electrical connection from the cathode to the outside we use since 1987 a 13 wm diameter tungsten wire from Good Fellows Inc. We tried before also 7.5 wm and 50 wm tungsten wires as cathode
R. Wink et al. / Miniaturized proportional counters for GALLEX
wires, and 7.5 p,m tungsten wires as anode wire . From the technical point of view, there is no difference in sealing in (see below) the 7.5 p,m or 13 pm tungsten wire, but because of the aging effect (see section 4) and mechanical stability we decided to use the 13 pm tungsten wire as standard wire . The most critical technical part of the construction of all HD counter types is to vacuum seal the cathode
543
and anode wires directly into the "Suprasil" quartz capillaries, because of the different expansion coefficients of tungsten and quartz (tungsten: 5 X 10 -6 K-t , "Suprasil" quartz: 5 X 10 -7 K-1 ). Our technique is to make the capillaries about 10 cm longer than needed (and shown in fig. 3) . Next we press nitrogen/ hydrogen (4%) gas through the counter for >_ 15 min to remove the air completely . The lower end of the capillaries are
Low-Level Proportional Counter (a) Davis-type
cathode
anode wire
side window
spring
(b) HD-1-type aluminum cathode (evaporated -1 pm) plug
Cr /Ni spring 125 pm
Cr/Ni ring 125 pm
anode 7.5 pm W wire
stopcock
cathode wire 50 pm W or 13 pm W
gas inlet
(c) HD - 2 - type anode wire 13 pm W Cr/Ni spring 125pm
plug end window
cathode wire 13pm W
gas inlet
stopcock
l
I
HD-2 : evaporated aluminum HD-2 (Fe) : iron tube HD-2 (Si) : silicon tube
0
1
2
Fig. 3. Schematical drawings of the different proportional counter types.
3
4
5 cm
544
R. Wink et al. / Miniaturised proportional counters for GALLEX
put into water to avoid back diffusion of air into the capillary once the gas flow is stopped. The gas flow is stopped at the very moment when the glassblower starts to heat the seal-in point with a flame of 2000°C. With these provisions, there are almost no failures . In earlier attempts while using noble gases or pure nitrogen, the wire often broke in the sealing process . The technique described works only with tungsten wire diameters _< 50 wm . For technical reasons it is necessary to seal in the anode wire after the cathode wire . Therefore one has to protect the cathode wire against heat during the operation for the anode wire, otherwise the cathode wire is no longer vacuum tight and/or oxidized . After both wires are sealed in, the capillaries are cut to optimum length . Afterwards the tungsten wires are melted off with the torch to the same length as the capillaries . Direct electrical contacts to the outside with thin tungsten wires are not possible . Instead, one has to connect the thin tungsten wires to stronger (125 wm copper) wires . We tried four different ways : - filling the capillaries with electrical contactive silver ; - filling the capillaries with electrical contactive carbon ; - soldering the tungsten wire with copper by putting special solder into the capillaries together with the copper wire and heating it together under vacuum up to 600°C ; - filling the capillaries with pure liquid gallium . We chose the last method for making electrical contacts for two reasons : - the capillaries are homogeneously filled with a metal, so disturbing discharges of the high voltage against the quartz during the counting are avoided ; - pure gallium is liquid at high room temperatur, so it is possible to change the copper wire, if necessary, e .g ., when changing the preamplifier. The soldering technique makes the "best" electrical contact, but lacks the above mentioned advantages . We now describe details of the individual counter type variants.
2.3. Davis-type counter This counter type is similar to the proportional counters which are used by Ray Davis et al . i n the chlorine experiment . They are made from hyperpure "Suprasil" quartz and a massive iron tube cathode (see fig . 3) . The anode wire is tungsten with diameters varying from 5 Wm to 13 Wm . The anode and cathode wires are guided through quartz tubes of 10-15 cm length . They are sealed in at the end of the quartz tubes by the graded seal technique, where by a sequence of transition glasses the expansion coefficient
of tungsten is better matched by the final glass than by quartz . The long tubes are necessary to keep the 4° K containing glass far enough away from the active volume . The quartz tubes are also used for the counting gas inlet . They are filled with mercury to avoid additional dead volume . The mercury increases the capacity from 0 .3 pF to 4-5 pF . The volume efficiency is defined as the ratio of the active volume to the total volume filled with counting gas . The active volume is the volume inside the cathode, in which ' 1 Ge-decays can be detected . The average volume efficiency is 78% . The 7 ' Ge detection efficiency is nearly proportional to the volume efficiency . The main part of the dead volume is between the iron cathode and the quartz wall . One major point of the development of the HD-1 counter type (see below) was to decrease this dead volume . For calibration the Davis-type counters have a side window (see section 4b) .
2.4. HD-1 counter type The HD-1 counter type (see fig . 3) depicts the following three main changes compared to the Davistype counter: - The cathode is evaporated directly onto the inside of the quartz tube . This avoids any dead volume between the quartz and the cathode . The volume efficiency is therefore increased from ^- 78% to - 90% . The cathode is made from aluminum with a thickness of - 1 Wm . In two cases copper was used as material for the cathode . To achieve low background an evaporation apparatus was built and a thorium free wire was used for the evaporation procedure. - The anode and cathode wires are directly sealed into the quartz by a new technique (see above) . This new development avoids the necessity of the 10-15 cm long tubes for the transition from quartz to glass, which have to be filled with mercury to avoid additional dead volume . The absence of the long tubes filled with mercury reduces the capacity of the counter from 5 pF to < 1 pF . - The position of the stopcock is near the active volume . The dead volume between the active volume and the stopcock is now negligible, it is no longer necessary to fill the volume between the active volume and the stopcock with mercury, which is normally done during the counter filling procedure . So, other counter filling procedures are possible (like, e .g ., freezing the counting gas into the counter). Since counters have to be baked to 200°C before filling, we could not benefit from the latter improvement since we failed to find a vacuum grease which can be heated to 200°C and which at the same time is free of radioactive impurities . The radius and the length of the cathode are vari-
545
R. Wink et al. / Miniaturized proportional counters for GALLEX
able, because they are hand made by the glassblower, therefore the determination of the size of the active volume has a typical error of 4% . Since the volume efficiency is nearly proportional to the 71 Ge detection efficiency, this variation will increase the error of the determination of the 't Ge detection efficiency. So one major point for the development of the HD-2 counter type was to increase the precision of the volume efficiency .
the counter filling, one risks the destruction of the counter by pushing mercury inside the active volume, because the mercury will alloy with the aluminum of the cathode. Therefore we tried to use iron as material of the cathode, but the evaporation causes problems . The amount of mercury used to fill the tube is small compared to the amount in the Davis-type counters .
2 .5 . HD-2 counter type
The two differences between the HD-2 and HD2(Fe) counter type are: - The HD-2(Fe)-counters have a solid iron cylinder as cathode. We are able to fit the iron cathode very well into the quartz, such that the additional dead volume between the quartz and the solid cathode is in the order of 3% only . - For the calibration the HD-2(Fe)-counters have an end window instead of a side window (see section 4b).
The HD-2 counter type (see fig. 3) has two main differences compared to the HD-1 counter type . These are: - The dimensions (length and radius) of the quartz tube are standardized . The quartz parts are machined with tolerances as small as typical for machining metals. The uncertainty on the volume efficiency is in principle < 1% . Due to the standardization the volume efficiency is increased to 93% . - Between the stopcock and the active volume there is a quartz tube, to be filled with mercury to avoid the problems during the baking of the counter before counter filling as described above. Having again a tube between the stopcock and the active volume to be filled with mercury has one (minor) disadvantage . In the case of a handling error during
2.6. HD-2(Fe) counter type
2.7. HD-2(Si) counter type The only difference of this counter type compared to the HD-2(Fe) counter type is the cylinder used as cathode to be made from silicon rather than from iron . The wall thickness of the silicon cathode is chosen to be about 0.4 mm. Smaller wall thicknesses lead to
Table 2 Summary of properties for the different counter types Cathode Material Thickness [wm] dimensions of active volume Length [mm] Radius [mm] Volume [ml] Veff [%]
Position of stopcock near to active volume Mercury between stopcock and active volume Side window End window Cathode wire [wm] Anode wire [wm] Wires sealed directly into quartz 10-15 cm long quartz-glass transition tubes Capacity [pF] Contact beween tungsten and copper wires
Davis-type
HD-1
HD-2
HD-2(Fe)
HD-2(Si)
no
yes
no
no
no
yes yes (no) 13 5-13
no yes (no) 7.5-50 7.503)
yes yes (no) 13 13
yes no yes 13 13
yes no yes 13 13
no
yes
yes
yes
yes
yes 4-5
no <1 contactive silver or carbon
no <1
no <1
no <1
soldered
gallium
gallium
massive tube iron 300-500 individual 25 2.5 0.5 78
massive soldered
evaporated aluminum 1 individual 25-34 2.5-3.4 0.5-1 .2 90
evaporated aluminum 1 standardized 32 .5 3 .2 1.0 93
massive tube iron 170 standardized 32 .5 3.0 0.9 90
massive tube silicon 400 standardized 32.5 2.8 0.8 86
R. Wink et al. /Miniaturized proportional counters for GALLEX
54 6
Table 3 Impurity concentrations of various quartz batches. The Suprasil 1 was used for the counter construction. Note that concentrations for ordinary quartz glass are given in ng/g whereas they are given in [pg/g] for Suprasil samples Element
K
Concentration Suprasil 1 [pg/g]
U
Th Na Sc
Cr Fe
Co
Zn As
Br Sb La Ce Nd Sm Eu Tb
Yb Hf W
Au
1260 < 100 <3 3800 1 .3 39 3500 250 230 <2 29
Suprasil AN [pg/g]
3 .7 < 20
< 800 < 100 <2 390 1 .2 61 3900 120 710 2 7 6 .2 8 .1 32
< 0 .2
3 .1
1.1
-
-
-
<3 <8 5 .3
-
-
<4 7 61
Suprasil 300
[pg/g]
20500 < 100 <3 6500 2 .7 57 4300 120 170 <1 <8 4 .3 3 .7 < 30 300 < 0 .8 1 6 4 <4 <5 5.7
Ordinary quartz glass [ng/g] 86 40 1330 0 .19 3 .4 630 1 .2 37 0 .3 0 .2 0.9 33 101 67 12 1 .1 16 16 49 1 0 .012
extremely brittle cathodes . The goal of using silicon instead of iron as cathode material was to achieve an even better counter background, because silicon can be produced very pure by zone refinement and crystallization. Some counters of this type have a shaped cathode to correct the end effect of the electric field, which results in a larger detection efficiency. A summary of the most important details of the different counter types is given in table 2.
3. Selection of construction materials The body of the counter consists of quartz which, with respect to purity is available in various qualities. To find the optimal material for our purpose we have analyzed several samples of each quality (ordinary quartz glass, Suprasil 1, Suprasil AN and Suprasil 300; all from Heraeus, Hanau, Germany) by means of instrumental neutron activation. The results for K, Th, U and various other elements are given in table 3. As expected, ordinary quartz glass is not pure enough for our purpose. Among Suprasil's, the least marketed quality, Suprasil 1, still contains measurable amounts of potassium . For uranium and thorium concentrations only upper limits could be determined, which are quite satisfactory . The respective contributions to the proportional counter background are discussed in section 4.1 . Suprasil AN would be a slightly better choice concerning radioactive impurities but it was not available in the dimensions needed for the counter constructions. Therefore Suprasil 1 was used for all counters . Interestingly, the supposedly highest quality of the quartz glass, Suprasil 300, contained 16 times more potassium than Suprasil 1 . To determine the background contribution from primordial radionuclides in the iron cathode of the HD-2(Fe) counter type a sample of 17 .3 g iron from the batch used for the fabrication of the cathodes was analyzed by instrumental neutron activation . The results are summarized in table 4. It was especially difficult to find iron with sufficiently low "Co impurity for the cathode of the HD2(Fe) counter type . For some materials compromises had to be made since no contamination free samples with the right performance could be found; e.g ., the sealing glue used for the capillary tubes contains about 0.9 Bq 4° K/kg and the contact glue for the silicon cathode and its tungsten wire is contaminated with 0.8 Bq 4°K/kg and 0.02 Bq 108mAg/kg . Here only the minimal necessary amounts are applied in order to keep their background contributions at an uncritical level.
Table 4 Concentrations of primordial radionuclides in iron used for cathodes of the HD-2(Fe) counter type Element
Material Research FE Matz WE 81) Concentration [ng/g]
Material Research FE Matz WE 88)
Johnson Matthey IRON PURATRONIC
K
93 -
= 100 <1 0 .05
2 < 0.03 < 0.07
U
Th
-
54 7
R. Wink et al. / Minlatunzed proportional counters for GALLEX Table 5 Concentrations of primordial radionuclides in silicon used for cathodes of the HD-2(Si) counter type [28] Concentration
Isotope 238 U
< 2 ppt < 0.1 ppt < 0.1 ppb
232 Th
40 K
Among all commercially available substances silicon has the highest chemical purity achievable by zone-refining methods . Table 5 shows the limits on primordial radionuclides in the silicon used in the present work for the cathode of the HD-2(Si) counter type (FZ-silicon). It is not possible to use pure silicon for the cathodes because the high resistance of very pure silicon leads to a strong dependence of the counting rate on the gas gain . The material used in this work was doped with boron to a concentration of 2-3 x 10 18 atoms/cm' which corresponds to specific resistance of 6 x 10 -2 fl cm (about 1-2 ft from one end of the counter to the other). The company producing the silicon dopes the material before the zone-refinement cleaning procedure. Due to the high difference in the segregation coefficient between boron and the most dangerous contaminants (uranium, thorium, potassium) no radioactive impurity is introduced by doping silicon . Silicon can contain the radioactive isotope 32 Si (which has a not well determined half life of about 100 yr). The decay of this isotope can lead to non negligible background in the counters if the raw silicate used for silicon production was not carefully selected [20] . The silicate rock used for the production of silicon in the present work originated most probably from fistsized silicate rock pieces [21] in which the 32 Si content is expected to be low, because the slow authigenic growth
of quartz contaminated with atmospherically produced 32Si can only happen on the surface of the silicate pieces . However the producing company cannot absolutely guarantee that no sand was processed in part of the raw material for the silicon used in the present work, so there is the possibility of a sizable 32 Si contamination. This is also true for the Suprasil quartz used, for which the origin of the raw silicate is even more unclear. Especially for the quartz it will be very difficult to select suitable raw materials because the commercial producers use huge batches of SiC1 4 of mixed origin in the Suprasil production . Part of the ongoing program to improve the counters will therefore be to check the 32 Si purity of the materials used by "milking" the daughter 32 P from large samples of the raw material used for the production of silicon and quartz .
4. Counter performance 4.1 . Relevant counter background of the counter types HD-2(Fe)I(Si) used for the GALLEX experiment at Gran Sasso A summary of the counter background measurements done at the passive side of the GALLEX counting system "GFL" at Gran Sasso [22,23] is given in table 6 . Within statistics all counters have a similar background . We observed no striking difference in the background between HD-2(Fe) and HD-2(Si) counters . Only the counter #99 has a slightly higher background . A typical number for the integral counter background above 0.4 keV is 0 .6 counts per day (cpd) and in the region of interest in GALLEX 0.04 cpd in the L- and 0 .02 cpd in the K-window. These values are low com-
Table 6 Summary of background measurements Counter No .
103 113 106 110 114 99 107 102 112 Sum Sum [cpd]
Type
HD-2(Fe) 1-113-260 HD-2(Si) HD-2(Si) HD-2(Si) HD-2(Fe) HD-2(Fe) HD-260 HD-2(Fe)
Counting time [days]
Number of events
74.3 246 .0 20 .5 435 .3 53 .3 195 .0 100 .9 34 .8 43 .0 1203 .1
37 114 11 223 41 184 73 16 34 733 0.61
E>0.4keV
L peak
K peak
all
fast
all
fast
6 34 3 47 7 32 8 3 6 146 0.12
3 14 1 14 1 7 1 2 2 45 0 .04
5 10 0 19 3 20 6 3 0 66 0.05
1 6 0 6 2 7 2 0 0 24 0 .02
R. Wink et al. /Miniaturized proportional counters for GALLEX
548
Table 7 Background sources for the HD-2(Fe) proportional counters in the passive shield Source External sources
Muons Neutrons Gamma rays Rn +progenies K, Th, U in copper of the shielding material
Internal sources
K in quartz Th in quartz U 1n quartz 6° Co in iron cathode K in iron cathode 226Ra in iron cathode Th in iron cathode U in iron cathode Tritium in counting gas 85 Kr in counting gas Sum
Activity or flux at the position of the proportional counter 3 x 10 -8 cm -2 s-1 < 10 -6 cm -2 s -1 < 10 -6 cm -2 s -1 < 0 .5 Bq m-3 < 2, 1, 1 mBq/kg
TU= tritium unit=[ 3H]/[H]=10-18.
0 .005
< 0 .001 < 0.02 < 0.006 < 0.02
0 .04 mBq/kg
0.0001 < 0.0002 < 0.03 < 0.02 0.001 < 0.2 < 0.017 < 0.03 0.023 < 0.01 < 0.39
< 0.01 mBq/kg < 1 .2 mBq/kg < 7 mBq/kg 0.06 mBq/kg < 3 mBq/kg < 0.3 mBq/kg < 0.4 mBq/kg 6 TU <0.12Bgm -3
pared with the measured "Ge signal of 0.3 cpd (L- and K-window together) at the beginning of counting . For the comparison of these background rates with former values see section 4c). Table 7 summarizes the most relevant background sources for the latest set of HD-2(Fe) counters at the relevant counting position . Some source strengths can be given only with large errors . This applies, e.g ., to the neutron- and -y-fluxes at the position of the counter. They depend also on their individual shielding geometry. In these cases conservative upper limits are given. The neutron flux at Gran Sasso underground laboratory has been measured [24] and the -y-flux was assumed to be similar to that determined at the MPI Low Level laboratory at Heidelberg . For the radioimpurities see section 3. The corresponding count rates given in the last column of table 7 were either experimentally determined or, in case of Fe, calculated using the Monte Carlo method [8]. Cosmogenic radionuclides produced in the material when stored above ground have also been considered but could not be analyzed, except for short-lived "Co (T1 /2 = 70 .9 d) in the copper of the shield (about 0.3 mBq/kg). Cosmogenic 54 Mn in the iron cannot be larger than the sea level production rate of 5 mBq/kg [12] . 85Kr, always present in modern xenon, is minimized using xenon produced before 1961 when the 85 Kr concentration in the atmosphere (from nuclear fuel reprocessing) was still low. In order to avoid tritium in the counting, tritium free water (47 mTU #1) and low tritium conä1
Count rate > 0.5 keV [cpd]
taining chemicals (7 .3 f 1.4 TU) [6,25] are used in the conversion step of GeCl 4 to GeH4. Background events, caused by the background sources listed in table 7, occur mainly in the region labeled "slow" in fig. 4, only tritium events appear dominantly in the region labeled "fast" .
0
4
2
Energy [keV] 6 8 10 12
14
16
18 20
100 °
stow
V 80 N
û t`
t
8
60
o
d g
20
°
o
â
°
ôo~~
É 40
0
o
°
-b
° °
fast
°
° E
K-window L -window C-nlmg time : 274.6 days integral : 141 events
0
500
1000 1500 Energy [channels]
e
2000
Fig. 4 . Typical background spectrum of counter #110 . Plotted is risetime versus energy. The plot is divided into different regions for the characterization of the background sources (see table 8) . The regions of interest in GALLEX are the L and K-windows. The risetime for overflow events is not defined and therefore meaningless .
54 9
R. link et al. / Miniaturized proportional counters for GALLEX
Table 8 Identified background sources in the different regions of fig. 4 Region
Typical count rate [cpd]
Fast
0.14
L-window plus K-window Overflow
0.06
Slow
Identified sources
0.38
background sources listed in table 7 (excluded: tritium) < 0.35 cpd tritium: 0.023 cpd 222Rn plus daughters: 0.005 cpd tritium: 0.007 cpd 222Rn plus daughters: 0.005 cpd 222Rn plus daughters: 0.009 cpd
0.09
Fig. 4 shows a risetime versus energy plot of a typical background measurement. The regions of interest in GALLEX are labeled L-window and K-window. For discussion of the nature of the background pulses, the plot is divided into three additional parts, labeled "fast", "slow", and "overflow" . The L- and K-windows are subregions of the region labeled "fast" . The identified background sources in the different regions are listed in table 8. 4.2. Calibration window For the counter calibration we use a 1s3Gd source, which excites the characteristic X-rays of cerium . The K . energy of cerium is slightly below the L-absorption edge of xenon (which is part of the counting gas) and the Kß energy is slightly above. Therefore the counting gas is completely transparent for K. but not for Kp photons . They will be absorbed by electrons of the xenon L-shell. The L-shell will be refilled by electrons
of the outer shells of the xenon atom . As a result of all possible combinations, one gets four energy peaks at the following energies : 1.0 keV, 5 .1 keV, 9.8 keV and 35 keV [7]. This 153 Gd-cerium source is our standard calibration source . It is well known from literature [see, e.g ., 26] that in proportional counters there is an aging effect of the anode wire, caused by depositions of radicals from the quenching gas on the anode wire . This produces irregularities in the electric field at the place of deposition . The amount of the deposits on the anode wire is (among other things) proportional to the number of electrons which reach the anode wire . If a counter is calibrated only through a side window, only a small part of the anode wire is illuminated and therefore subject to deposits . Therefore this small part of the anode wire is no longer representative of the total anode wire . We have measured the critical dose for forming deposits in our counters to be 5 X 10 12 electrons per millimeter anode wire . We have learned from our comparisons of external calibrations through
Table 9 Background rates and counting efficiencies are listed for different background measurements. In the line second to last we quote the statistical error which the GALLEX experiment would yield after four years of running at an assumed signal of 90 SNU (1 SNU =1 solar neutrino unit =1 neutrino capture per 10 36 target atoms per second). In the last line the amount of gallium is listed which would be needed to reach an error of 7.9%, the value which we aim for at present. References for the counting systems used : [17,22,23,271 Counter type
Davis
Davis
Davis
Davis
HD-1
HD-2(Fe)
Number of counters Total counting time [days] Counting gas Pressure [Tory] Counting system
8 241 GeH4 940-1000 MPI ADP 32 28 0.52 0.095
1 22 GeH4 500 MPI ADP 34 23 0.22 0.05
1 54 Xe-GeH 4 1200 MPI ADP 25 37 0.44 0.15
7 277 Xe-GeH 4 1500 MPI Mulia 24 38 0.22 0.11
3 147 Xe-GeH 4 1520 MPI Mulia 26 41 0.25 0.20
3 70 GeH4 850-920 FFL ADP 35 30 0.144 0.014
HD-2(Fe) HD-2(Si) 9 1203 Xe-GeH 4 800 GFL Camac 30 36 0.04 0.02
L-efficiency [%] K-efficiency [%] L-background [cpd] K-background [cpd] Stat . error of GALLEX after 4 years of measurement [%] Amount of Gallium required to obtain an error of 7.9% [tons]
12.7
11 .1
12.3
10 .8
11 .5
8.6
7.9
56 .4
48.2
53 .6
46 .4
48 .8
35 .5
30 .0
550
R. Wink et al. / Miniaturized proportional counters for GALLEX
a side window and internal calibrations with "GeH 4 that one can never be sure that a calibration through a side window is representative for the total response of the counter, even if the energy calibration spectrum looks normal . The reason is that a deposit develops only at the position of the side window which leads to a variation in the gas gain over the length of the counter. We have never seen a disagreement between an external calibration through an end window and an internal calibration . The calibration through an end window can be made very similar to an internal calibration, if the mean free path in the counting gas of the applied X-rays is of the order of the length of the cathode or even longer . In our case the free mean paths of the cerium Kß X-rays is 100 mm, which is large compared to the size of the counter. 4.3. Comparison One way to compare the different counter types is to compare the statistical error to be expected in the GALLEX experiment after a running time of four years with different counter types [27]. The main inputs for this calculation are the counting efficiencies and the counter backgrounds (see table 9). Furthermore it is assumed, that the systematic effects with all used counter types are negligible, which may not be true for counters calibrated through a side window (see section 4.2). During ten years of development we have decreased the reachable statistical error for the GALLEX experiment for four years of running from 12 .7% to 7.9%, which is close to the theoretical limit for no background at all, which is at 7% . 5. Summary The precision in the measurement of the solar neutrino flux achievable in GALLEX is limited by the size of the (very expensive) target, so any improvement in counting efficiency and background is equivalent to a larger target. In this sense the improvements described in this paper are equivalent to an amount of 26 tons of gallium . Moreover, the results of this development allow to better understand the systematics of the counter background pulses, due to a low counter capacity (better pulse shape recording) . A superior calibration technique available for these counters assures one that the external calibration of the counter properly reflects the total response of the counter. This was experimentally verified in all cases.
Acknowledgement This work has been supported by the German Federal Minister for Research and Technology under contract number 06HD5541 . References [1] T. Kirsten, in : Inside the Sun, Proc . IAU Coll., Versailles, eds. G. Berthomieu and M. Cribier (Kluwer, Dordrecht, 1990) p. 187. [2] P. Anselmann et al., Phys . Lett. B285 (1992) 376. [3] P. Anselmann et al ., Phys . Lett. B285 (1992) 390. [4] E. Bellotti, Nucl. Instr. and Meth. A264 (1988) 1. [5] E. Henrich and K. Ebert, Angewandte Chemie 31 (1992) 1283 . [6] C. Schlosser, PhD Thesis #2, Universität Heidelberg, 1992 . A. Urban, PhD Thesis, Technische Universität München, 1989 . [8] R. Plaga, PhD Thesis #2, Universität Heidelberg, 1989 . [9] R.Plaga, in : Proc. PASCOS 91, Boston, eds. P. Nath and S. Reucroft (World Scientific, Singapore, 1992) p. 107. [10] R. Davis, in : Inside the Sun, Proc. TAU Coll ., Versailles, eds. G. Berthomieu and M. Cribier (Kluwer, Dordrecht, 1990) p. 171. [11] G. Heusser, Nucl . Instr. and Meth . B17 (1986) 418. [12] G. Heusser, Nucl . Instr. and Meth . B58 (1991) 79 . [13] G. Heusser and O. Schaeffer, Earth and Planetary Sci. Lett . 33 (1977) 420. [14] K. Schneider, Thesis #2, Universität Heidelberg, 1984 . [15] R. Wink, Thesis #2, Universität Heidelberg, 1986 . [16] G. Eymann and T. Kirsten, Nucl . Instr. and Meth . A258 (1987) 266. [17] R. Wink, PhD Thesis #2, Universität Heidelberg, 1988 . [18] P. Anselmann, Thesis #2, Universität Heidelberg, 1990 . [19] R. Plaga and T. Kirsten, Nucl . Instr. and Meth . A309 (1991) 560. [20] R. Plaga, Nucl. Instr. and Meth. A309 (1991) 58 . [21] Dr . Hutzler, Wacker Chemietronic, priv . comm ., Burghausen, Germany, 1989 . [22] G. Heusser, Proc . 2nd. Conf. Trends in Astroparticle Phys ., Aachen, ed . P.C. Bosetti (Teubner, Stuttgart, Leibzig, 1993) to appear. [23] R. Wink, in : Particles and Cosmology, Proc . Int. School, Baksan Valley, eds. V.A . Marveev, E.N . Alexeev, V.A . Rubakov and I.I . Tkachev (World Scientific, 1992) p. 66 . [24] P. Belli et al ., Nuovo Cimento 101A (1989) 959. [25] I. Carmi and I. Dostrovsky, priv . comm ., Weizmann Inst ., Rehovot, Israel. [26] J. Va'vra, Nucl . Instr. and Meth . A252 (1986) 547. [27] W. Hampel, Habilitationsschrift #2, Universität Heidelberg, 1987 . [28] Wacker Chemietronic, analyse+material, priv. comm ., Burghausen, Germany, 1989. #2 Available on request also from the library of MPI f.
Kernphysik, Heidelberg, Germany.
Nuclear Instruments and Methods in Physics Research A329 (1993) 541-550 North-Holland
Section A
The miniaturized proportional counter HD-2(Fe) /(Si) for the GALLER solar neutrino experiment R. Wink, P. Anselmann, D. D6rflinger, W. Hampel, G. Heusser, T. Kirsten, P. M6gel, E. Pernicka, R. Plaga and C. Schlosser Max Planck Institut für Kernphysik, P.O. Box 10 39 80, W-6900 Heidelberg, Germany
Received 14 December 1992
The miniaturized proportional counters used for the detection of 7'Ge in the solar neutrino project GALLEX are characterized . We also report on the construction techniques applied to build these counters and to achieve the described performance . A very low counting background is achieved by, among other things, careful selection of the materials used for construction . The total 7'Ge detection efficiency after applying cuts to reduce the background is about 66% .
1. Introduction The solar neutrino experiment GALLEX [1-3] is taking data relevant to the Sun since June 1991 at the Gran Sasso underground laboratory [4]. It monitors the low energy neutrino-flux of the sun. In a tank containing 30 .3 t of natural gallium in the form of 101 t GaC1 3 hydrochloric aqueous solution, solar neutrinos produce a measured rate of (0.8 ± 0.2) atoms of "Ge per day. With a "Ge half life of 11 .43 days, about 1071 Ge-atoms are present after 3 weeks of exposure . These atoms (and 1 mg of inactive germanium carrier) are extracted and chemically converted to the gas germane (GeH 4) [5,6]. At the end the 7' Ge is contained in about 0.3 ml (STP) of germane. In order to count the 7' Ge-atoms the electron-capture back reaction to 7'Ga has to be observed: 7'Ge(e- , ve) '1 Ga . This reaction results in X-rays and/or Auger electrons which can be detected in gas proportional counters . The energies of the Auger electrons and X-rays are given in table 1 . Necessary requirements for the detection of such a small number of "Ge-atoms are: - a high detection efficiency ; - a very low and time stable background to reliably distinguish signal from background; - a low threshold (< 0.5 keV); - a technique to transfer the few 7'Ge-atoms quickly from the target tank to the detector, so as to minimize the time between the end of bombardment
and the start of counting, because of the relatively short half life of nGe . A miniaturized proportional counter can meet these criteria. However, the minimal size of the proportional counter is limited by the volume needed for the GALLEX counting gas (800 Torr Xe (70 vol.%)-GeH 4 (30 vol.%)), which is about 1 ml . The choice of the gas mixture is the result of a complicated optimalization [7-9]. Miniaturization of the proportional counter helps to reduce background because: - less material is needed for the construction of the counter. Any material, despite of careful selection, is still a source of radioactive background, scaling with mass (self contamination ; see section 3); - the miniaturized counter is a smaller target for external background sources (like 'Y'S, muons, - - - ). A "Ge energy spectrum acquired with such a proportional counter is shown in fig. 1. The energy resolution of the L-peak (1 .2 keV) is typically M 45% Table 1 Energies of the of Auger electrons and X-rays of the 7'Ge decay [27] Probability 1%]
Captured electron
41 .4 5 .2 41 .1
K K K
10 .3
2 .0
L M
0168-9002/93/$06 .00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
Sum energy of Auger electrons [keV] 10.37 0.11 1 .l2, 1 .15 1 .3 0.16
Energy of X-ray [keV]
10.26 9.25, 9 .22 -
54 2
R. Wink et al. / Miniaturized proportional counters for GALLEX
700 N
71 Ge energy spectrum
c ô
L - Peak
500 300
z
K- Peak
100
0
5
10
15 Energy ( keV ) Fig. 1 . 7IGe energy spectrum .
(FWHM), therefore a low threshold of only 0.5 keV is needed . The K-peak (10.4 keV) has a typical FWHM of - 28%. The "Ge decay is a "point like event" inside the proportional counter, hence the signal of a charge sensitive preamplifier has a fast rise . On the other hand a background event caused for instance by an ionization track inside the proportional counter (e .g ., from a Compton electron) usually is an extented event. It leads to a slowly rising signal . Then, the pulse shape of the signal can be used to distinguish "Ge from background events (see fig. 2) . The remaining counter background and its origin are discussed in section 4. The chlorine solar neutrino experiment by Davis and coworkers [101 involves a similar counting problem and miniaturized counters were developed to detect the electron capture of "At . The aim of this work was to improve these "Davis-type" counters and then apply them to 7'Ge-detection . The construction of extremly low level background counters is of general interest for many potential other uses . Success depends critically on many details. This motivates us to pass on our
700 600 500 E u L 400 LM
x m 300 d N
D_
200
2. Counter construction Starting from the experience with Davis-type proportional counters, many new counter types were developed at Heidelberg during the last 10 years. [8,13191. This row is labeled "HD" for "Heidelberg" . The first type was "HD-l", "HD-2" comes in three variations: one with an evaporated aluminum cathode ("HD-2"), and two with solid cathode made from iron ("HD-2(Fe)") or silicon ("HD-2601. Experience was increasing in that order. First we describe those items which are common to most counter types. 2.1 . Cleaning of quartz parts After the glassblower has fixed all quartz parts together, they are first cleaned over night in aqua regia, then put for 2 h into n-hexane and at the end for few seconds in hydrofluoric acid . Between each cleaning step they are rinsed with quartz distilled water and dried at 120°C for 2 h. After the last cleaning with HF it is crucial to rinse the two capillaries very well . For this, distilled water is filled into the counter through the open top end and subsequently pressed out with nitrogen/ hydrogen (4%) gas through the capillaries. This rinsing of the capillaries is one important step for the successful sealing of the anode and cathode wires (see below) . Otherwise, remaining impurities react during the sealing procedure with the wire, resulting in breaks . 2.2. Anode and cathode wire of the HD counter types
100 0
experience by a scrutinous description of the procedures and recipies . In section 2 we describe the counter construction and the techniques to mechanically treat, clean and assemble the required parts. Various measures led to an increase of counting efficiency compared to Davis-type counters . The many years of counter development have always been accompanied by radioimpurity measurements and basic studies of background components [11,121. All individual construction parts of the proportional counters have been selected with the help of non-destructive low-level Ge-spectroscopy and partly also of radiochemical methods. The sensitivity level for the Ge-detection was as low as mBq/kg once large samples (> 0.5 kg) were available (section 3).
r
0
__1
_i
40
i
i
80
i
i
120
i
160
200
Time [nsecl Fig. 2. Pulse shape of a 71 Ge-pulse compared to a background pulse.
As standard wire for the anode and for making the first part of the electrical connection from the cathode to the outside we use since 1987 a 13 wm diameter tungsten wire from Good Fellows Inc. We tried before also 7.5 wm and 50 wm tungsten wires as cathode
R. Wink et al. / Miniaturized proportional counters for GALLEX
wires, and 7.5 p,m tungsten wires as anode wire . From the technical point of view, there is no difference in sealing in (see below) the 7.5 p,m or 13 pm tungsten wire, but because of the aging effect (see section 4) and mechanical stability we decided to use the 13 pm tungsten wire as standard wire . The most critical technical part of the construction of all HD counter types is to vacuum seal the cathode
543
and anode wires directly into the "Suprasil" quartz capillaries, because of the different expansion coefficients of tungsten and quartz (tungsten: 5 X 10 -6 K-t , "Suprasil" quartz: 5 X 10 -7 K-1 ). Our technique is to make the capillaries about 10 cm longer than needed (and shown in fig. 3) . Next we press nitrogen/ hydrogen (4%) gas through the counter for >_ 15 min to remove the air completely . The lower end of the capillaries are
Low-Level Proportional Counter (a) Davis-type
cathode
anode wire
side window
spring
(b) HD-1-type aluminum cathode (evaporated -1 pm) plug
Cr /Ni spring 125 pm
Cr/Ni ring 125 pm
anode 7.5 pm W wire
stopcock
cathode wire 50 pm W or 13 pm W
gas inlet
(c) HD - 2 - type anode wire 13 pm W Cr/Ni spring 125pm
plug end window
cathode wire 13pm W
gas inlet
stopcock
l
I
HD-2 : evaporated aluminum HD-2 (Fe) : iron tube HD-2 (Si) : silicon tube
0
1
2
Fig. 3. Schematical drawings of the different proportional counter types.
3
4
5 cm
544
R. Wink et al. / Miniaturised proportional counters for GALLEX
put into water to avoid back diffusion of air into the capillary once the gas flow is stopped. The gas flow is stopped at the very moment when the glassblower starts to heat the seal-in point with a flame of 2000°C. With these provisions, there are almost no failures . In earlier attempts while using noble gases or pure nitrogen, the wire often broke in the sealing process . The technique described works only with tungsten wire diameters _< 50 wm . For technical reasons it is necessary to seal in the anode wire after the cathode wire . Therefore one has to protect the cathode wire against heat during the operation for the anode wire, otherwise the cathode wire is no longer vacuum tight and/or oxidized . After both wires are sealed in, the capillaries are cut to optimum length . Afterwards the tungsten wires are melted off with the torch to the same length as the capillaries . Direct electrical contacts to the outside with thin tungsten wires are not possible . Instead, one has to connect the thin tungsten wires to stronger (125 wm copper) wires . We tried four different ways : - filling the capillaries with electrical contactive silver ; - filling the capillaries with electrical contactive carbon ; - soldering the tungsten wire with copper by putting special solder into the capillaries together with the copper wire and heating it together under vacuum up to 600°C ; - filling the capillaries with pure liquid gallium . We chose the last method for making electrical contacts for two reasons : - the capillaries are homogeneously filled with a metal, so disturbing discharges of the high voltage against the quartz during the counting are avoided ; - pure gallium is liquid at high room temperatur, so it is possible to change the copper wire, if necessary, e .g ., when changing the preamplifier. The soldering technique makes the "best" electrical contact, but lacks the above mentioned advantages . We now describe details of the individual counter type variants.
2.3. Davis-type counter This counter type is similar to the proportional counters which are used by Ray Davis et al . i n the chlorine experiment . They are made from hyperpure "Suprasil" quartz and a massive iron tube cathode (see fig . 3) . The anode wire is tungsten with diameters varying from 5 Wm to 13 Wm . The anode and cathode wires are guided through quartz tubes of 10-15 cm length . They are sealed in at the end of the quartz tubes by the graded seal technique, where by a sequence of transition glasses the expansion coefficient
of tungsten is better matched by the final glass than by quartz . The long tubes are necessary to keep the 4° K containing glass far enough away from the active volume . The quartz tubes are also used for the counting gas inlet . They are filled with mercury to avoid additional dead volume . The mercury increases the capacity from 0 .3 pF to 4-5 pF . The volume efficiency is defined as the ratio of the active volume to the total volume filled with counting gas . The active volume is the volume inside the cathode, in which ' 1 Ge-decays can be detected . The average volume efficiency is 78% . The 7 ' Ge detection efficiency is nearly proportional to the volume efficiency . The main part of the dead volume is between the iron cathode and the quartz wall . One major point of the development of the HD-1 counter type (see below) was to decrease this dead volume . For calibration the Davis-type counters have a side window (see section 4b) .
2.4. HD-1 counter type The HD-1 counter type (see fig . 3) depicts the following three main changes compared to the Davistype counter: - The cathode is evaporated directly onto the inside of the quartz tube . This avoids any dead volume between the quartz and the cathode . The volume efficiency is therefore increased from ^- 78% to - 90% . The cathode is made from aluminum with a thickness of - 1 Wm . In two cases copper was used as material for the cathode . To achieve low background an evaporation apparatus was built and a thorium free wire was used for the evaporation procedure. - The anode and cathode wires are directly sealed into the quartz by a new technique (see above) . This new development avoids the necessity of the 10-15 cm long tubes for the transition from quartz to glass, which have to be filled with mercury to avoid additional dead volume . The absence of the long tubes filled with mercury reduces the capacity of the counter from 5 pF to < 1 pF . - The position of the stopcock is near the active volume . The dead volume between the active volume and the stopcock is now negligible, it is no longer necessary to fill the volume between the active volume and the stopcock with mercury, which is normally done during the counter filling procedure . So, other counter filling procedures are possible (like, e .g ., freezing the counting gas into the counter). Since counters have to be baked to 200°C before filling, we could not benefit from the latter improvement since we failed to find a vacuum grease which can be heated to 200°C and which at the same time is free of radioactive impurities . The radius and the length of the cathode are vari-
545
R. Wink et al. / Miniaturized proportional counters for GALLEX
able, because they are hand made by the glassblower, therefore the determination of the size of the active volume has a typical error of 4% . Since the volume efficiency is nearly proportional to the 71 Ge detection efficiency, this variation will increase the error of the determination of the 't Ge detection efficiency. So one major point for the development of the HD-2 counter type was to increase the precision of the volume efficiency .
the counter filling, one risks the destruction of the counter by pushing mercury inside the active volume, because the mercury will alloy with the aluminum of the cathode. Therefore we tried to use iron as material of the cathode, but the evaporation causes problems . The amount of mercury used to fill the tube is small compared to the amount in the Davis-type counters .
2 .5 . HD-2 counter type
The two differences between the HD-2 and HD2(Fe) counter type are: - The HD-2(Fe)-counters have a solid iron cylinder as cathode. We are able to fit the iron cathode very well into the quartz, such that the additional dead volume between the quartz and the solid cathode is in the order of 3% only . - For the calibration the HD-2(Fe)-counters have an end window instead of a side window (see section 4b).
The HD-2 counter type (see fig. 3) has two main differences compared to the HD-1 counter type . These are: - The dimensions (length and radius) of the quartz tube are standardized . The quartz parts are machined with tolerances as small as typical for machining metals. The uncertainty on the volume efficiency is in principle < 1% . Due to the standardization the volume efficiency is increased to 93% . - Between the stopcock and the active volume there is a quartz tube, to be filled with mercury to avoid the problems during the baking of the counter before counter filling as described above. Having again a tube between the stopcock and the active volume to be filled with mercury has one (minor) disadvantage . In the case of a handling error during
2.6. HD-2(Fe) counter type
2.7. HD-2(Si) counter type The only difference of this counter type compared to the HD-2(Fe) counter type is the cylinder used as cathode to be made from silicon rather than from iron . The wall thickness of the silicon cathode is chosen to be about 0.4 mm. Smaller wall thicknesses lead to
Table 2 Summary of properties for the different counter types Cathode Material Thickness [wm] dimensions of active volume Length [mm] Radius [mm] Volume [ml] Veff [%]
Position of stopcock near to active volume Mercury between stopcock and active volume Side window End window Cathode wire [wm] Anode wire [wm] Wires sealed directly into quartz 10-15 cm long quartz-glass transition tubes Capacity [pF] Contact beween tungsten and copper wires
Davis-type
HD-1
HD-2
HD-2(Fe)
HD-2(Si)
no
yes
no
no
no
yes yes (no) 13 5-13
no yes (no) 7.5-50 7.503)
yes yes (no) 13 13
yes no yes 13 13
yes no yes 13 13
no
yes
yes
yes
yes
yes 4-5
no <1 contactive silver or carbon
no <1
no <1
no <1
soldered
gallium
gallium
massive tube iron 300-500 individual 25 2.5 0.5 78
massive soldered
evaporated aluminum 1 individual 25-34 2.5-3.4 0.5-1 .2 90
evaporated aluminum 1 standardized 32 .5 3 .2 1.0 93
massive tube iron 170 standardized 32 .5 3.0 0.9 90
massive tube silicon 400 standardized 32.5 2.8 0.8 86
R. Wink et al. /Miniaturized proportional counters for GALLEX
54 6
Table 3 Impurity concentrations of various quartz batches. The Suprasil 1 was used for the counter construction. Note that concentrations for ordinary quartz glass are given in ng/g whereas they are given in [pg/g] for Suprasil samples Element
K
Concentration Suprasil 1 [pg/g]
U
Th Na Sc
Cr Fe
Co
Zn As
Br Sb La Ce Nd Sm Eu Tb
Yb Hf W
Au
1260 < 100 <3 3800 1 .3 39 3500 250 230 <2 29
Suprasil AN [pg/g]
3 .7 < 20
< 800 < 100 <2 390 1 .2 61 3900 120 710 2 7 6 .2 8 .1 32
< 0 .2
3 .1
1.1
-
-
-
<3 <8 5 .3
-
-
<4 7 61
Suprasil 300
[pg/g]
20500 < 100 <3 6500 2 .7 57 4300 120 170 <1 <8 4 .3 3 .7 < 30 300 < 0 .8 1 6 4 <4 <5 5.7
Ordinary quartz glass [ng/g] 86 40 1330 0 .19 3 .4 630 1 .2 37 0 .3 0 .2 0.9 33 101 67 12 1 .1 16 16 49 1 0 .012
extremely brittle cathodes . The goal of using silicon instead of iron as cathode material was to achieve an even better counter background, because silicon can be produced very pure by zone refinement and crystallization. Some counters of this type have a shaped cathode to correct the end effect of the electric field, which results in a larger detection efficiency. A summary of the most important details of the different counter types is given in table 2.
3. Selection of construction materials The body of the counter consists of quartz which, with respect to purity is available in various qualities. To find the optimal material for our purpose we have analyzed several samples of each quality (ordinary quartz glass, Suprasil 1, Suprasil AN and Suprasil 300; all from Heraeus, Hanau, Germany) by means of instrumental neutron activation. The results for K, Th, U and various other elements are given in table 3. As expected, ordinary quartz glass is not pure enough for our purpose. Among Suprasil's, the least marketed quality, Suprasil 1, still contains measurable amounts of potassium . For uranium and thorium concentrations only upper limits could be determined, which are quite satisfactory . The respective contributions to the proportional counter background are discussed in section 4.1 . Suprasil AN would be a slightly better choice concerning radioactive impurities but it was not available in the dimensions needed for the counter constructions. Therefore Suprasil 1 was used for all counters . Interestingly, the supposedly highest quality of the quartz glass, Suprasil 300, contained 16 times more potassium than Suprasil 1 . To determine the background contribution from primordial radionuclides in the iron cathode of the HD-2(Fe) counter type a sample of 17 .3 g iron from the batch used for the fabrication of the cathodes was analyzed by instrumental neutron activation . The results are summarized in table 4. It was especially difficult to find iron with sufficiently low "Co impurity for the cathode of the HD2(Fe) counter type . For some materials compromises had to be made since no contamination free samples with the right performance could be found; e.g ., the sealing glue used for the capillary tubes contains about 0.9 Bq 4° K/kg and the contact glue for the silicon cathode and its tungsten wire is contaminated with 0.8 Bq 4°K/kg and 0.02 Bq 108mAg/kg . Here only the minimal necessary amounts are applied in order to keep their background contributions at an uncritical level.
Table 4 Concentrations of primordial radionuclides in iron used for cathodes of the HD-2(Fe) counter type Element
Material Research FE Matz WE 81) Concentration [ng/g]
Material Research FE Matz WE 88)
Johnson Matthey IRON PURATRONIC
K
93 -
= 100 <1 0 .05
2 < 0.03 < 0.07
U
Th
-
54 7
R. Wink et al. / Minlatunzed proportional counters for GALLEX Table 5 Concentrations of primordial radionuclides in silicon used for cathodes of the HD-2(Si) counter type [28] Concentration
Isotope 238 U
< 2 ppt < 0.1 ppt < 0.1 ppb
232 Th
40 K
Among all commercially available substances silicon has the highest chemical purity achievable by zone-refining methods . Table 5 shows the limits on primordial radionuclides in the silicon used in the present work for the cathode of the HD-2(Si) counter type (FZ-silicon). It is not possible to use pure silicon for the cathodes because the high resistance of very pure silicon leads to a strong dependence of the counting rate on the gas gain . The material used in this work was doped with boron to a concentration of 2-3 x 10 18 atoms/cm' which corresponds to specific resistance of 6 x 10 -2 fl cm (about 1-2 ft from one end of the counter to the other). The company producing the silicon dopes the material before the zone-refinement cleaning procedure. Due to the high difference in the segregation coefficient between boron and the most dangerous contaminants (uranium, thorium, potassium) no radioactive impurity is introduced by doping silicon . Silicon can contain the radioactive isotope 32 Si (which has a not well determined half life of about 100 yr). The decay of this isotope can lead to non negligible background in the counters if the raw silicate used for silicon production was not carefully selected [20] . The silicate rock used for the production of silicon in the present work originated most probably from fistsized silicate rock pieces [21] in which the 32 Si content is expected to be low, because the slow authigenic growth
of quartz contaminated with atmospherically produced 32Si can only happen on the surface of the silicate pieces . However the producing company cannot absolutely guarantee that no sand was processed in part of the raw material for the silicon used in the present work, so there is the possibility of a sizable 32 Si contamination. This is also true for the Suprasil quartz used, for which the origin of the raw silicate is even more unclear. Especially for the quartz it will be very difficult to select suitable raw materials because the commercial producers use huge batches of SiC1 4 of mixed origin in the Suprasil production . Part of the ongoing program to improve the counters will therefore be to check the 32 Si purity of the materials used by "milking" the daughter 32 P from large samples of the raw material used for the production of silicon and quartz .
4. Counter performance 4.1 . Relevant counter background of the counter types HD-2(Fe)I(Si) used for the GALLEX experiment at Gran Sasso A summary of the counter background measurements done at the passive side of the GALLEX counting system "GFL" at Gran Sasso [22,23] is given in table 6 . Within statistics all counters have a similar background . We observed no striking difference in the background between HD-2(Fe) and HD-2(Si) counters . Only the counter #99 has a slightly higher background . A typical number for the integral counter background above 0.4 keV is 0 .6 counts per day (cpd) and in the region of interest in GALLEX 0.04 cpd in the L- and 0 .02 cpd in the K-window. These values are low com-
Table 6 Summary of background measurements Counter No .
103 113 106 110 114 99 107 102 112 Sum Sum [cpd]
Type
HD-2(Fe) 1-113-260 HD-2(Si) HD-2(Si) HD-2(Si) HD-2(Fe) HD-2(Fe) HD-260 HD-2(Fe)
Counting time [days]
Number of events
74.3 246 .0 20 .5 435 .3 53 .3 195 .0 100 .9 34 .8 43 .0 1203 .1
37 114 11 223 41 184 73 16 34 733 0.61
E>0.4keV
L peak
K peak
all
fast
all
fast
6 34 3 47 7 32 8 3 6 146 0.12
3 14 1 14 1 7 1 2 2 45 0 .04
5 10 0 19 3 20 6 3 0 66 0.05
1 6 0 6 2 7 2 0 0 24 0 .02
R. Wink et al. /Miniaturized proportional counters for GALLEX
548
Table 7 Background sources for the HD-2(Fe) proportional counters in the passive shield Source External sources
Muons Neutrons Gamma rays Rn +progenies K, Th, U in copper of the shielding material
Internal sources
K in quartz Th in quartz U 1n quartz 6° Co in iron cathode K in iron cathode 226Ra in iron cathode Th in iron cathode U in iron cathode Tritium in counting gas 85 Kr in counting gas Sum
Activity or flux at the position of the proportional counter 3 x 10 -8 cm -2 s-1 < 10 -6 cm -2 s -1 < 10 -6 cm -2 s -1 < 0 .5 Bq m-3 < 2, 1, 1 mBq/kg
TU= tritium unit=[ 3H]/[H]=10-18.
0 .005
< 0 .001 < 0.02 < 0.006 < 0.02
0 .04 mBq/kg
0.0001 < 0.0002 < 0.03 < 0.02 0.001 < 0.2 < 0.017 < 0.03 0.023 < 0.01 < 0.39
< 0.01 mBq/kg < 1 .2 mBq/kg < 7 mBq/kg 0.06 mBq/kg < 3 mBq/kg < 0.3 mBq/kg < 0.4 mBq/kg 6 TU <0.12Bgm -3
pared with the measured "Ge signal of 0.3 cpd (L- and K-window together) at the beginning of counting . For the comparison of these background rates with former values see section 4c). Table 7 summarizes the most relevant background sources for the latest set of HD-2(Fe) counters at the relevant counting position . Some source strengths can be given only with large errors . This applies, e.g ., to the neutron- and -y-fluxes at the position of the counter. They depend also on their individual shielding geometry. In these cases conservative upper limits are given. The neutron flux at Gran Sasso underground laboratory has been measured [24] and the -y-flux was assumed to be similar to that determined at the MPI Low Level laboratory at Heidelberg . For the radioimpurities see section 3. The corresponding count rates given in the last column of table 7 were either experimentally determined or, in case of Fe, calculated using the Monte Carlo method [8]. Cosmogenic radionuclides produced in the material when stored above ground have also been considered but could not be analyzed, except for short-lived "Co (T1 /2 = 70 .9 d) in the copper of the shield (about 0.3 mBq/kg). Cosmogenic 54 Mn in the iron cannot be larger than the sea level production rate of 5 mBq/kg [12] . 85Kr, always present in modern xenon, is minimized using xenon produced before 1961 when the 85 Kr concentration in the atmosphere (from nuclear fuel reprocessing) was still low. In order to avoid tritium in the counting, tritium free water (47 mTU #1) and low tritium conä1
Count rate > 0.5 keV [cpd]
taining chemicals (7 .3 f 1.4 TU) [6,25] are used in the conversion step of GeCl 4 to GeH4. Background events, caused by the background sources listed in table 7, occur mainly in the region labeled "slow" in fig. 4, only tritium events appear dominantly in the region labeled "fast" .
0
4
2
Energy [keV] 6 8 10 12
14
16
18 20
100 °
stow
V 80 N
û t`
t
8
60
o
d g
20
°
o
â
°
ôo~~
É 40
0
o
°
-b
° °
fast
°
° E
K-window L -window C-nlmg time : 274.6 days integral : 141 events
0
500
1000 1500 Energy [channels]
e
2000
Fig. 4 . Typical background spectrum of counter #110 . Plotted is risetime versus energy. The plot is divided into different regions for the characterization of the background sources (see table 8) . The regions of interest in GALLEX are the L and K-windows. The risetime for overflow events is not defined and therefore meaningless .
54 9
R. link et al. / Miniaturized proportional counters for GALLEX
Table 8 Identified background sources in the different regions of fig. 4 Region
Typical count rate [cpd]
Fast
0.14
L-window plus K-window Overflow
0.06
Slow
Identified sources
0.38
background sources listed in table 7 (excluded: tritium) < 0.35 cpd tritium: 0.023 cpd 222Rn plus daughters: 0.005 cpd tritium: 0.007 cpd 222Rn plus daughters: 0.005 cpd 222Rn plus daughters: 0.009 cpd
0.09
Fig. 4 shows a risetime versus energy plot of a typical background measurement. The regions of interest in GALLEX are labeled L-window and K-window. For discussion of the nature of the background pulses, the plot is divided into three additional parts, labeled "fast", "slow", and "overflow" . The L- and K-windows are subregions of the region labeled "fast" . The identified background sources in the different regions are listed in table 8. 4.2. Calibration window For the counter calibration we use a 1s3Gd source, which excites the characteristic X-rays of cerium . The K . energy of cerium is slightly below the L-absorption edge of xenon (which is part of the counting gas) and the Kß energy is slightly above. Therefore the counting gas is completely transparent for K. but not for Kp photons . They will be absorbed by electrons of the xenon L-shell. The L-shell will be refilled by electrons
of the outer shells of the xenon atom . As a result of all possible combinations, one gets four energy peaks at the following energies : 1.0 keV, 5 .1 keV, 9.8 keV and 35 keV [7]. This 153 Gd-cerium source is our standard calibration source . It is well known from literature [see, e.g ., 26] that in proportional counters there is an aging effect of the anode wire, caused by depositions of radicals from the quenching gas on the anode wire . This produces irregularities in the electric field at the place of deposition . The amount of the deposits on the anode wire is (among other things) proportional to the number of electrons which reach the anode wire . If a counter is calibrated only through a side window, only a small part of the anode wire is illuminated and therefore subject to deposits . Therefore this small part of the anode wire is no longer representative of the total anode wire . We have measured the critical dose for forming deposits in our counters to be 5 X 10 12 electrons per millimeter anode wire . We have learned from our comparisons of external calibrations through
Table 9 Background rates and counting efficiencies are listed for different background measurements. In the line second to last we quote the statistical error which the GALLEX experiment would yield after four years of running at an assumed signal of 90 SNU (1 SNU =1 solar neutrino unit =1 neutrino capture per 10 36 target atoms per second). In the last line the amount of gallium is listed which would be needed to reach an error of 7.9%, the value which we aim for at present. References for the counting systems used : [17,22,23,271 Counter type
Davis
Davis
Davis
Davis
HD-1
HD-2(Fe)
Number of counters Total counting time [days] Counting gas Pressure [Tory] Counting system
8 241 GeH4 940-1000 MPI ADP 32 28 0.52 0.095
1 22 GeH4 500 MPI ADP 34 23 0.22 0.05
1 54 Xe-GeH 4 1200 MPI ADP 25 37 0.44 0.15
7 277 Xe-GeH 4 1500 MPI Mulia 24 38 0.22 0.11
3 147 Xe-GeH 4 1520 MPI Mulia 26 41 0.25 0.20
3 70 GeH4 850-920 FFL ADP 35 30 0.144 0.014
HD-2(Fe) HD-2(Si) 9 1203 Xe-GeH 4 800 GFL Camac 30 36 0.04 0.02
L-efficiency [%] K-efficiency [%] L-background [cpd] K-background [cpd] Stat . error of GALLEX after 4 years of measurement [%] Amount of Gallium required to obtain an error of 7.9% [tons]
12.7
11 .1
12.3
10 .8
11 .5
8.6
7.9
56 .4
48.2
53 .6
46 .4
48 .8
35 .5
30 .0
550
R. Wink et al. / Miniaturized proportional counters for GALLEX
a side window and internal calibrations with "GeH 4 that one can never be sure that a calibration through a side window is representative for the total response of the counter, even if the energy calibration spectrum looks normal . The reason is that a deposit develops only at the position of the side window which leads to a variation in the gas gain over the length of the counter. We have never seen a disagreement between an external calibration through an end window and an internal calibration . The calibration through an end window can be made very similar to an internal calibration, if the mean free path in the counting gas of the applied X-rays is of the order of the length of the cathode or even longer . In our case the free mean paths of the cerium Kß X-rays is 100 mm, which is large compared to the size of the counter. 4.3. Comparison One way to compare the different counter types is to compare the statistical error to be expected in the GALLEX experiment after a running time of four years with different counter types [27]. The main inputs for this calculation are the counting efficiencies and the counter backgrounds (see table 9). Furthermore it is assumed, that the systematic effects with all used counter types are negligible, which may not be true for counters calibrated through a side window (see section 4.2). During ten years of development we have decreased the reachable statistical error for the GALLEX experiment for four years of running from 12 .7% to 7.9%, which is close to the theoretical limit for no background at all, which is at 7% . 5. Summary The precision in the measurement of the solar neutrino flux achievable in GALLEX is limited by the size of the (very expensive) target, so any improvement in counting efficiency and background is equivalent to a larger target. In this sense the improvements described in this paper are equivalent to an amount of 26 tons of gallium . Moreover, the results of this development allow to better understand the systematics of the counter background pulses, due to a low counter capacity (better pulse shape recording) . A superior calibration technique available for these counters assures one that the external calibration of the counter properly reflects the total response of the counter. This was experimentally verified in all cases.
Acknowledgement This work has been supported by the German Federal Minister for Research and Technology under contract number 06HD5541 . References [1] T. Kirsten, in : Inside the Sun, Proc . IAU Coll., Versailles, eds. G. Berthomieu and M. Cribier (Kluwer, Dordrecht, 1990) p. 187. [2] P. Anselmann et al., Phys . Lett. B285 (1992) 376. [3] P. Anselmann et al ., Phys . Lett. B285 (1992) 390. [4] E. Bellotti, Nucl. Instr. and Meth. A264 (1988) 1. [5] E. Henrich and K. Ebert, Angewandte Chemie 31 (1992) 1283 . [6] C. Schlosser, PhD Thesis #2, Universität Heidelberg, 1992 . A. Urban, PhD Thesis, Technische Universität München, 1989 . [8] R. Plaga, PhD Thesis #2, Universität Heidelberg, 1989 . [9] R.Plaga, in : Proc. PASCOS 91, Boston, eds. P. Nath and S. Reucroft (World Scientific, Singapore, 1992) p. 107. [10] R. Davis, in : Inside the Sun, Proc. TAU Coll ., Versailles, eds. G. Berthomieu and M. Cribier (Kluwer, Dordrecht, 1990) p. 171. [11] G. Heusser, Nucl . Instr. and Meth . B17 (1986) 418. [12] G. Heusser, Nucl . Instr. and Meth . B58 (1991) 79 . [13] G. Heusser and O. Schaeffer, Earth and Planetary Sci. Lett . 33 (1977) 420. [14] K. Schneider, Thesis #2, Universität Heidelberg, 1984 . [15] R. Wink, Thesis #2, Universität Heidelberg, 1986 . [16] G. Eymann and T. Kirsten, Nucl . Instr. and Meth . A258 (1987) 266. [17] R. Wink, PhD Thesis #2, Universität Heidelberg, 1988 . [18] P. Anselmann, Thesis #2, Universität Heidelberg, 1990 . [19] R. Plaga and T. Kirsten, Nucl . Instr. and Meth . A309 (1991) 560. [20] R. Plaga, Nucl. Instr. and Meth. A309 (1991) 58 . [21] Dr . Hutzler, Wacker Chemietronic, priv . comm ., Burghausen, Germany, 1989 . [22] G. Heusser, Proc . 2nd. Conf. Trends in Astroparticle Phys ., Aachen, ed . P.C. Bosetti (Teubner, Stuttgart, Leibzig, 1993) to appear. [23] R. Wink, in : Particles and Cosmology, Proc . Int. School, Baksan Valley, eds. V.A . Marveev, E.N . Alexeev, V.A . Rubakov and I.I . Tkachev (World Scientific, 1992) p. 66 . [24] P. Belli et al ., Nuovo Cimento 101A (1989) 959. [25] I. Carmi and I. Dostrovsky, priv . comm ., Weizmann Inst ., Rehovot, Israel. [26] J. Va'vra, Nucl . Instr. and Meth . A252 (1986) 547. [27] W. Hampel, Habilitationsschrift #2, Universität Heidelberg, 1987 . [28] Wacker Chemietronic, analyse+material, priv. comm ., Burghausen, Germany, 1989. #2 Available on request also from the library of MPI f.
Kernphysik, Heidelberg, Germany.