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The background components of germanium low-level spectrometers
418
Nuclear
THE BACKGROUND
COMPONENTS
Instruments
OF GERMANIUM
and Methods
in Physics Research B17 (1986) 418-422 North-Holland, Amsterdam
LOW-LEVEL
SPECTROMETERS
C&d HEUSSER Max-Planck-Institut ftir Kernphysik, POB 103980 O-6900 Heidelberg
FRG
been studied. The cosmic-ray-induced Background components of a Ge(Li) spectrometer operated below 15 m w.e. have component, including inelastic neutron scattering, is dominating. Most neutrons are produced within the shield itself by p-caputure and fast muon interaction. The activation products of these reactions can only be avoided by going deeper underground. This would also suppress the neutron-induced component more effectively than any anticoincidence device or neutron shield.
In modern physics the detection of very rare events is gaining steadily increasing importance. For these experiments (e.g. those detecting double beta decay or solar neutrino capture), extremely low background levels are essential. The detection system must therefore be constructed of materials which are very carefully selected with respect to low radioactive contamination. Nondestructive low-level gamma-ray spectroscopy is the most straightforward method. Many measurements of this kind have been carried out with germanium and NaI(T1) spectrometers to select construction materials for the detection system of a gallium solar neutrino experiment
PI. In order to further improve their sensitivity and also in the interest of low-level counting in general, a systematic study of the different background components
Fig. 1. Shield of Ge(Li) low-level spectrometer
0148-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
was performed. Germanium detectors are the most adequate instruments for such studies since they are relatively easy to construct with low internal contamination and provide valuable spectral information due to their high energy resolution.
2. Spectrometer The main instrument used in this study was a Ge(Li) detector of 26% relative efficiency. Its design and shielding are very similar to that of a smaller well-type Ge(Li) detector described earlier 121, which was used to select the construction material for this detector. By adding iron and lead around the elbow dipstick mounting above the dewar, its shield (fig. 1) was improved compared to ref. [2] and now consists of, in this order, 24 mm mercury in a lucite container, 72 mm old ships iron,
with the front opened
for sample exchange.
419
G. HemseT / Ba~kgro~#d co~~o~e~~s of Ge low-feuelspectrometers
I
Channe II
200
400 If
400
SO0 4
1000 1
Table 1 Background nudides
Number 1200 1400 "'I'
1600
1800 2001
Isotove “* Pb (Th) ‘I4 Pb (U) 208T1 (Th) ‘14Bi (U) %
count
rates of the main lines of primordial
Enerav lkeV1
radio-
Count rate hml
239
-z 0.006 (0.074)
352 583 609 1461
0.0059 & 0.0017 -= 0.003 (0.025) 0.0024 +_0.0012 0.009 i 0.0009
3. BackgrounrI components Energy Fig. 2. Background
spectrum
[ keV]
The residual background shown in fig. 2 as well as that of low level gamma spectrometers in general can be attributed to the following components: A - incompleteiy shielded external gamma rays, B - intrinsic contamination of detector and shield materials, C - =‘Bn and its daug hters in cavities of the shield, D - direct events of muons which are not vetoed (detection probability of the anticoincidence counter less than lOO%), E - cosmic-ray-induced radiation in detector and shield. Effects of the components A and D are demonstrated in fig. 3. Shown are spectra of an intrinsic Ge detector (34% relative efficiency) under various shielding conditions: unshielded, shielded by 5 cm iron and 10 cm lead, and an additional anticoincidence shield of proportional counters. The strongest lines of K and Th are not completely suppressed by the passive shield (component
spectrometerre-
of the Ge(Li)
corded during19 d.
an annular multiwire proportional counter made of copper (veto detector together with a pillbox-shaped proportional counter in front (fig. 1) and one with a well at the rear), and 100 mm low activity lead (< 5 dpm 208Tl/kg, -CIO dpm 2’4Bi/kg). The detector is operated in a laboratory covered by about 15 m w.e.
(meter water equivalent) like all our low-level spectrometers [2,3]. A background spectrum recorded during 19 d is shown in fig. 2. Its total count rate (80-2060 keV) corresponds to 4.5 cpm and the rates of individual lines of primordial radionuclides are given in table 1. For the lines of the thorium series, 3a upper limits and the count rates in the corresponding energy range (in parentheses} are given since no peaks are observed above the continuous background.
I Number
Channe 0
I 1
2/-
CI
-
500
/
I
i
t
10cmPb .
m
+ 5cmFe
Ant ice incidence
c
I
2mo
1500
fOo0
‘a l-
(Proport
ional
counter)
Q-
;
a 0
--1
-
8
d
a L
+-J
-2-
c
3
-3-
cn
Energy
[ keV]
Fig. 3. Background spectra of an intrinsic Ge detector under various shielding conditions: (a) without + 10 cm low acitivity lea& (c) like (b) but with proportional counter veto shield around
shield, (b) 5 cm old ships iron the lead shield.
If. UNDERGROUND
LABORATORIES
4. 214Pb and ‘14Bi with peaks at 3.52 and 609 keV present in background spectra of both detectors are at least partly originating from 222R.n enclosed in cavities of the shield (component C) since flushing nitrogen through the shield interior reduced their count rate noticeably. Large openings in the shield do not allow a complete removal of radon by this method and therefore a weak radium contamination of the detector or inner shield cannot be excluded (component B). It escapes direct detection since its only gamma line at 186 keV is of low (3.5%) abundance and has to be discriminated against the m~mum of the background spectra (figs. 2 and 3). The &K peak in fig. 2 definitely stems from component B. It is probably located somewhere in the detector cryostat. Pair production of high energy gamma rays (mainly cosmic-ray-induced) is responsible for the 511 keV peak (0.07 cpm). Even when the detector is operated in the well of a large (10 in. diameterx 9 in.) NaI(Tl) veto detector, the count rate of this peak is still 0.021 cpm. In this configuration, the total background count rate is slightly reduced (3.7 cpm), but the peaks of the primordial radionuclide lines are higher by a factor of 3 or more. The latter is due to conta~nations of the photomultiplier glass of the NaI(T1) detector 131.Besides higher background for U, Th, and K, this arrangement has in addition strongly reduced counting efficiencies for radionuclides with cascading gamma ray emission and is therefore only of advantage for special applications. Muons escaping registration by the anticoincidence device produce the same continuous background through direct ionization (component D) as seen in fig. 3 by the difference of the two lower spectra. The reduced total background with the NaI(Tl) anticoincidence counter is mainly caused by stronger suppression of these muonic events.
The cosmic-ray-induced radiation {component E) shows up in weak peaks at 596 keV in background spectra of both detectors; at 368 keV (Ge(Li) detector) and at 847 keV (intrinsic Ge spectrometer). They are identified as lines of inelastic neutron excitation on 74Ge of the detector, on ‘OOHg,and 56Fe of the inner shield. Their cross sections reach up to 1.2 b [4]. Fig. 4 displays the spectrum section of fig. 2, showing the 74Ge(n n’) line and the 609 keV iine of ‘14Bi which is compared with that of a similar detector operated at sea level (W. Kolb, PTB Braun~hweig, private ~~uni~ation). This detector is shielded by 8 mm of copper and 100 mm of low activity lead plus 100 mm of the same lead forming the walls of the counting room. Its background spectrum exhibits many other neutron-induced lines by either inelastic scattering or radiative capture. The higher intensity of the ‘14Bi line probably reflects
4a
440
460
483
520
Coo
CL04 0.08
-
0.07
-
J
O.C%-
I
z ‘0
o.c6-
”
0.04-
2 a
am-
T a
&a?-
_I
2
0.012r
_
f
0.011.
2
3
O.OlO-
c 8
o.owo.om0.007
-
0.006
-
0.005
-
0.004 I
I
0
540
I
560
583
6KI
Energy
f
6XJ
I
1
640
660
I
6fKJ
700
[ kav]
Fig. 4. Comparison of the 74Ge(n, d) excitation line in Ge detectors operated at sea level and at 15 m w.e.
more cavities in that shield which hold Rn. Since the detectors have similar efficiencies, the neutron flux (a 600 keV) in our laboratory is reduced by the same factor of 10 against sea level as the 74Ge (n, n’) peak. Just by intensity arguments it can be shown that the main neutron sources in our laboratory are negative muon capture and fast muon interaction. According to the mean attenuation length (about 150 g/cm2) of the nucleonic component, which delivers most neutrons at sea level, their flux should be reduced through 15 m w.e. by at least 4 order of magnitude. The cosmic ray secondary neutron flux is dependent on both altitude and latitude. It is also influenced by the solar cycle. At sea level and northern latitudes of 40° to 60° total fluxes of (6.5-7.2) X lop3 cmm2 s-’ are reported in the literature [5,6} and (1.1-3.2) x 10-3cm-2 s-r for thermal neutrons [6,7]. Neutron production due to 238U spontaneous fission and (a, n) reactions in our laboratory with 90 cm thick concrete walls containing about 0.9 ppm U and 1.4 ppm Th yield a flux of the order of lo-$ cm-* s-r when appyling production rates and attenuation lengths given by refs. [7-lo]. Depending on the type of rock, most of these neutrons seem to have thermal and epithermal energy [7]. Table 2 summarizes the neutron fluxes in our laboratory inside the shield of
G. Heusser / Background components of Ge low-level spectrometers
Table 2 Neutron fluxes of different origin in our laboratory shielded by 15 m w.e. Flux [cm-’ s-‘)
Source Nucleonic component (a, n) + fission
7 x10-4 6 x1O-4
the Ge(Li) detector estimated from different information. These numbers are, however, more qualitative than quantitative since large uncertainties up to 50% and more are involved in the calculations as well as in experimental results. In contrast to the nucleonic component, 15 m w.e. reduce the muon flux by only a factor of 2 to 3 [ll]. In our laboratory it is about 8 X 10U3 cmm2 s-t. The stopping rate of negative muons decreases from about 0.3 to 0.07 kg-’ min-’ [l&12]. The neutron production rate P,, of p--capture is given by the stopping rate Z,_, the capture probability into the nucleus W, and the neutron multiplicity m through
(1) WC is a function
of Z and can be taken, for example from ref. [12]. The neutron multiplicity for different elements has been measured by refs. [13,14]. Due to the higher multiplicity of lead and mercury and their larger mass in the shield their average production rate of 0.11 was used to calculate the flux neutron kg-’ min-’ given in table 2. For simplification, differences in the stopping power for negative muons were not taken into account. The neutron flux was obtained by multiplying the production rate P,, with the mean attenuation lenght (mean free path) of neutrons L, = A/(rL, with A being the atomic weight, CTthe total cross section and L the
Table 3 Radioisotope production rates by p--capture
421
Loschmidt number. For more qualitative estimates we have used the total cross sections for lead, iron, and mercury in the energy range l-2 MeV, mainly responsible for the excitation of the 596 keV 76Ge line [4]. L ranges from about 35 to 70 g/cm2. The flux from p--capture in table 2 is based on a weighted mean L of 60 g/cm’. To verify the flux obtained by the comparison of the peak count rates of the 74Ge(n, n’) line, we exposed our detector to a 24’Am-Be neutron source with approximately known source strength. In order to minimize the activation of the detector, the front of the shield (as in fig. 1) was opened and the source was mounted at one meter distance from the crystal for such a short time that the peak just appeared. The flux calibrated in this way (table 2) has large uncertainties due to low statistics, different neutron spectra of the “Am-Be source (average energy 4-6 MeV) and p--capture (evaporation spectrum [15]) and also because of nonidentica1 exposure conditions. Nevertheless, the agreement between this number and that from the two detectors at different altitudes is good. The missing neutrons, measured flux minus p--induced flux, may be attributed to the production by photonuclear reactions of fast muons though the production ratio of fast muons/p%apture seems to be high [ll], but the quality of the data is insufficient to draw a more definite conclusion. Activation is a further type of cosmic-ray-induced background. Here mainly p--capture and fast muon reactions have to be considered. Table 3 gives, as an example, the estimated pr~uction rates for the main radioisotopes of the targets Ge, Fe, and Pb by p--capture. They are obtained by replacing M in the neutron production eq. (1) by the yield y if the target element is monoisotopic. Details can be found, for example, in refs. [11,14]. So far, none of these isotopes could be identified, but it is clear that they, as an ensemble, contribute noticeably to the background. They are mostly formed in an excited state and therefore also contribute together with the stable p--capture products through their de-excitation gamma rays. Production rates via fast muons are probably of similar magnitude WI.
at 15 m w.e.
Target
Radionuelide
Production rate (atoms kg-’ min -‘I
5. Discussion
Fe
54Mll 56Mn
0.013 0.013
Ge
72Ga “Ga 74Ga
0.012 0.013 0.0065
Pb
*04Tl t0q.l m7T1
0.0085 0.018 0.017
In our laboratory more neutrons are produced in the shield itself than outside because of the higher density and higher Z (larger WC and m). As a consequence, the shielding thickness of a detector should be chosen not thicker than necessary to absorb external gamma rays. Any additional high-Z material adds to the neutron producing target. Since only a few percent of the neutrons are not of cosmogenie origin, a neutron shield on the outside would not be effective. Placing the neutron II. UNDERGROUND
LABORATORIES
422
G. Hewer
/ Background components of Ge low-level spectrometers
shield around the detector would strongly increase the whole shield (target) because of its necessarily bulky size. A better solution would be an anticoincidence counter covering the outside of the lead shield which effectively eliminates incoming muons. The veto time has to be chosen long enough (a few ten ps) to also cancel delayed excitation radiation. It can be better met when using gas proportional or Geiger-Mtiller counters than scintillation detectors which, because of their larger gamma sensitivity, cause a longer dead time. The lower spectrum in fig. 4 was recorded with a provisional veto shield of proportional counters around the lead shield. It reduced the neutron-induced 847 keV 56Fe peak by a factor of 4 + 1.5. The activation component can only be avoided by placing the detector deep underground. Beyond a depth of about 1000 m w.e. the cosmic-ray-produced neutron flux would be less than 1% of that from (QL,n) reactions and fission in our laboratory. Here, an outside neutron shield would be helpful if the neutroninduced background component would still be disturbing.
References (11 A gallium solar neutrino experiment will be performed by the international GALLEX Collaboration: MPI Kernphysik Heidelberg, KFK Karlsruhe, TU Munich, CEN Saclay, University of Nice, University of Milano, Univer-
sity of Rome, and WIS Rehovot. See for example, W. Hampel, AIP Conf. Proc. no. 26, Homestake (1984) p. 162. PI G. Heusser, W. Hampel and M. Hiibner, Proc. 2nd Int. Conf. on Low Level Counting, High Tatras 1980, Phys. Appl. 8 (1982) 127. [31 G. Heusser, these Proceedings (Low-level Counting) Nucl. Instr. and Meth. B17 (1986) 423. B.A. Magurno and R. [41 J.R. Stehn, M.D. Goldberg, Wiener-Chasman, BNL 325, 2nd ed., suppl. 2, ~01s. l-3 (1964). [51 NCRP Report no. 45 (1975). L.D. Stephens and H.W. Patterson, J. [61 M. Yamashita, Geophys. Res. 71 (1966) 3817. 171 Strippa Project SKB Techn. Report no. 85-06. PI Y. Feige, B.G. Oltman and J. Kastner, J. Geophys. Res. 73 (1968) 3135. 191 M.W. Kuhn, S.N. Davis and H.W. Bentley, Geophys. Res. Lett. 11 (1984) 607. WI R. Zito, D.J. Donahue, S.N. Davis, H.W. Bentley and P. Fritz, Geophys. Res. Lett. 7 (1980) 235. 1111 T. Kirsten and W. Hampel, Proc. 1st Int. Conf. on Low Level Counting, High Tatras 1975, Bratislava (1977) p. 427. Nucl. Phys. Al66 (1971) 145. WI S. Charalambus, S. Diaz, S. Kaplan and R. Tyle, Phys. P31 B. MacDonald, Rev. 139 (1965) B1253. u41 G. Heusser and T. Kirsten, Nucl. Phys. Al95 (1972) 369. U. Jahnke, K.H. Lindenberger, G. 1151 W.U. Schroder, Roschert, R. Engfer and H.K. Walter, Z. Phys. 268 (1974) 57.
Nuclear
THE BACKGROUND
COMPONENTS
Instruments
OF GERMANIUM
and Methods
in Physics Research B17 (1986) 418-422 North-Holland, Amsterdam
LOW-LEVEL
SPECTROMETERS
C&d HEUSSER Max-Planck-Institut ftir Kernphysik, POB 103980 O-6900 Heidelberg
FRG
been studied. The cosmic-ray-induced Background components of a Ge(Li) spectrometer operated below 15 m w.e. have component, including inelastic neutron scattering, is dominating. Most neutrons are produced within the shield itself by p-caputure and fast muon interaction. The activation products of these reactions can only be avoided by going deeper underground. This would also suppress the neutron-induced component more effectively than any anticoincidence device or neutron shield.
In modern physics the detection of very rare events is gaining steadily increasing importance. For these experiments (e.g. those detecting double beta decay or solar neutrino capture), extremely low background levels are essential. The detection system must therefore be constructed of materials which are very carefully selected with respect to low radioactive contamination. Nondestructive low-level gamma-ray spectroscopy is the most straightforward method. Many measurements of this kind have been carried out with germanium and NaI(T1) spectrometers to select construction materials for the detection system of a gallium solar neutrino experiment
PI. In order to further improve their sensitivity and also in the interest of low-level counting in general, a systematic study of the different background components
Fig. 1. Shield of Ge(Li) low-level spectrometer
0148-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
was performed. Germanium detectors are the most adequate instruments for such studies since they are relatively easy to construct with low internal contamination and provide valuable spectral information due to their high energy resolution.
2. Spectrometer The main instrument used in this study was a Ge(Li) detector of 26% relative efficiency. Its design and shielding are very similar to that of a smaller well-type Ge(Li) detector described earlier 121, which was used to select the construction material for this detector. By adding iron and lead around the elbow dipstick mounting above the dewar, its shield (fig. 1) was improved compared to ref. [2] and now consists of, in this order, 24 mm mercury in a lucite container, 72 mm old ships iron,
with the front opened
for sample exchange.
419
G. HemseT / Ba~kgro~#d co~~o~e~~s of Ge low-feuelspectrometers
I
Channe II
200
400 If
400
SO0 4
1000 1
Table 1 Background nudides
Number 1200 1400 "'I'
1600
1800 2001
Isotove “* Pb (Th) ‘I4 Pb (U) 208T1 (Th) ‘14Bi (U) %
count
rates of the main lines of primordial
Enerav lkeV1
radio-
Count rate hml
239
-z 0.006 (0.074)
352 583 609 1461
0.0059 & 0.0017 -= 0.003 (0.025) 0.0024 +_0.0012 0.009 i 0.0009
3. BackgrounrI components Energy Fig. 2. Background
spectrum
[ keV]
The residual background shown in fig. 2 as well as that of low level gamma spectrometers in general can be attributed to the following components: A - incompleteiy shielded external gamma rays, B - intrinsic contamination of detector and shield materials, C - =‘Bn and its daug hters in cavities of the shield, D - direct events of muons which are not vetoed (detection probability of the anticoincidence counter less than lOO%), E - cosmic-ray-induced radiation in detector and shield. Effects of the components A and D are demonstrated in fig. 3. Shown are spectra of an intrinsic Ge detector (34% relative efficiency) under various shielding conditions: unshielded, shielded by 5 cm iron and 10 cm lead, and an additional anticoincidence shield of proportional counters. The strongest lines of K and Th are not completely suppressed by the passive shield (component
spectrometerre-
of the Ge(Li)
corded during19 d.
an annular multiwire proportional counter made of copper (veto detector together with a pillbox-shaped proportional counter in front (fig. 1) and one with a well at the rear), and 100 mm low activity lead (< 5 dpm 208Tl/kg, -CIO dpm 2’4Bi/kg). The detector is operated in a laboratory covered by about 15 m w.e.
(meter water equivalent) like all our low-level spectrometers [2,3]. A background spectrum recorded during 19 d is shown in fig. 2. Its total count rate (80-2060 keV) corresponds to 4.5 cpm and the rates of individual lines of primordial radionuclides are given in table 1. For the lines of the thorium series, 3a upper limits and the count rates in the corresponding energy range (in parentheses} are given since no peaks are observed above the continuous background.
I Number
Channe 0
I 1
2/-
CI
-
500
/
I
i
t
10cmPb .
m
+ 5cmFe
Ant ice incidence
c
I
2mo
1500
fOo0
‘a l-
(Proport
ional
counter)
Q-
;
a 0
--1
-
8
d
a L
+-J
-2-
c
3
-3-
cn
Energy
[ keV]
Fig. 3. Background spectra of an intrinsic Ge detector under various shielding conditions: (a) without + 10 cm low acitivity lea& (c) like (b) but with proportional counter veto shield around
shield, (b) 5 cm old ships iron the lead shield.
If. UNDERGROUND
LABORATORIES
4. 214Pb and ‘14Bi with peaks at 3.52 and 609 keV present in background spectra of both detectors are at least partly originating from 222R.n enclosed in cavities of the shield (component C) since flushing nitrogen through the shield interior reduced their count rate noticeably. Large openings in the shield do not allow a complete removal of radon by this method and therefore a weak radium contamination of the detector or inner shield cannot be excluded (component B). It escapes direct detection since its only gamma line at 186 keV is of low (3.5%) abundance and has to be discriminated against the m~mum of the background spectra (figs. 2 and 3). The &K peak in fig. 2 definitely stems from component B. It is probably located somewhere in the detector cryostat. Pair production of high energy gamma rays (mainly cosmic-ray-induced) is responsible for the 511 keV peak (0.07 cpm). Even when the detector is operated in the well of a large (10 in. diameterx 9 in.) NaI(Tl) veto detector, the count rate of this peak is still 0.021 cpm. In this configuration, the total background count rate is slightly reduced (3.7 cpm), but the peaks of the primordial radionuclide lines are higher by a factor of 3 or more. The latter is due to conta~nations of the photomultiplier glass of the NaI(T1) detector 131.Besides higher background for U, Th, and K, this arrangement has in addition strongly reduced counting efficiencies for radionuclides with cascading gamma ray emission and is therefore only of advantage for special applications. Muons escaping registration by the anticoincidence device produce the same continuous background through direct ionization (component D) as seen in fig. 3 by the difference of the two lower spectra. The reduced total background with the NaI(Tl) anticoincidence counter is mainly caused by stronger suppression of these muonic events.
The cosmic-ray-induced radiation {component E) shows up in weak peaks at 596 keV in background spectra of both detectors; at 368 keV (Ge(Li) detector) and at 847 keV (intrinsic Ge spectrometer). They are identified as lines of inelastic neutron excitation on 74Ge of the detector, on ‘OOHg,and 56Fe of the inner shield. Their cross sections reach up to 1.2 b [4]. Fig. 4 displays the spectrum section of fig. 2, showing the 74Ge(n n’) line and the 609 keV iine of ‘14Bi which is compared with that of a similar detector operated at sea level (W. Kolb, PTB Braun~hweig, private ~~uni~ation). This detector is shielded by 8 mm of copper and 100 mm of low activity lead plus 100 mm of the same lead forming the walls of the counting room. Its background spectrum exhibits many other neutron-induced lines by either inelastic scattering or radiative capture. The higher intensity of the ‘14Bi line probably reflects
4a
440
460
483
520
Coo
CL04 0.08
-
0.07
-
J
O.C%-
I
z ‘0
o.c6-
”
0.04-
2 a
am-
T a
&a?-
_I
2
0.012r
_
f
0.011.
2
3
O.OlO-
c 8
o.owo.om0.007
-
0.006
-
0.005
-
0.004 I
I
0
540
I
560
583
6KI
Energy
f
6XJ
I
1
640
660
I
6fKJ
700
[ kav]
Fig. 4. Comparison of the 74Ge(n, d) excitation line in Ge detectors operated at sea level and at 15 m w.e.
more cavities in that shield which hold Rn. Since the detectors have similar efficiencies, the neutron flux (a 600 keV) in our laboratory is reduced by the same factor of 10 against sea level as the 74Ge (n, n’) peak. Just by intensity arguments it can be shown that the main neutron sources in our laboratory are negative muon capture and fast muon interaction. According to the mean attenuation length (about 150 g/cm2) of the nucleonic component, which delivers most neutrons at sea level, their flux should be reduced through 15 m w.e. by at least 4 order of magnitude. The cosmic ray secondary neutron flux is dependent on both altitude and latitude. It is also influenced by the solar cycle. At sea level and northern latitudes of 40° to 60° total fluxes of (6.5-7.2) X lop3 cmm2 s-’ are reported in the literature [5,6} and (1.1-3.2) x 10-3cm-2 s-r for thermal neutrons [6,7]. Neutron production due to 238U spontaneous fission and (a, n) reactions in our laboratory with 90 cm thick concrete walls containing about 0.9 ppm U and 1.4 ppm Th yield a flux of the order of lo-$ cm-* s-r when appyling production rates and attenuation lengths given by refs. [7-lo]. Depending on the type of rock, most of these neutrons seem to have thermal and epithermal energy [7]. Table 2 summarizes the neutron fluxes in our laboratory inside the shield of
G. Heusser / Background components of Ge low-level spectrometers
Table 2 Neutron fluxes of different origin in our laboratory shielded by 15 m w.e. Flux [cm-’ s-‘)
Source Nucleonic component (a, n) + fission
7 x10-4 6 x1O-4
the Ge(Li) detector estimated from different information. These numbers are, however, more qualitative than quantitative since large uncertainties up to 50% and more are involved in the calculations as well as in experimental results. In contrast to the nucleonic component, 15 m w.e. reduce the muon flux by only a factor of 2 to 3 [ll]. In our laboratory it is about 8 X 10U3 cmm2 s-t. The stopping rate of negative muons decreases from about 0.3 to 0.07 kg-’ min-’ [l&12]. The neutron production rate P,, of p--capture is given by the stopping rate Z,_, the capture probability into the nucleus W, and the neutron multiplicity m through
(1) WC is a function
of Z and can be taken, for example from ref. [12]. The neutron multiplicity for different elements has been measured by refs. [13,14]. Due to the higher multiplicity of lead and mercury and their larger mass in the shield their average production rate of 0.11 was used to calculate the flux neutron kg-’ min-’ given in table 2. For simplification, differences in the stopping power for negative muons were not taken into account. The neutron flux was obtained by multiplying the production rate P,, with the mean attenuation lenght (mean free path) of neutrons L, = A/(rL, with A being the atomic weight, CTthe total cross section and L the
Table 3 Radioisotope production rates by p--capture
421
Loschmidt number. For more qualitative estimates we have used the total cross sections for lead, iron, and mercury in the energy range l-2 MeV, mainly responsible for the excitation of the 596 keV 76Ge line [4]. L ranges from about 35 to 70 g/cm2. The flux from p--capture in table 2 is based on a weighted mean L of 60 g/cm’. To verify the flux obtained by the comparison of the peak count rates of the 74Ge(n, n’) line, we exposed our detector to a 24’Am-Be neutron source with approximately known source strength. In order to minimize the activation of the detector, the front of the shield (as in fig. 1) was opened and the source was mounted at one meter distance from the crystal for such a short time that the peak just appeared. The flux calibrated in this way (table 2) has large uncertainties due to low statistics, different neutron spectra of the “Am-Be source (average energy 4-6 MeV) and p--capture (evaporation spectrum [15]) and also because of nonidentica1 exposure conditions. Nevertheless, the agreement between this number and that from the two detectors at different altitudes is good. The missing neutrons, measured flux minus p--induced flux, may be attributed to the production by photonuclear reactions of fast muons though the production ratio of fast muons/p%apture seems to be high [ll], but the quality of the data is insufficient to draw a more definite conclusion. Activation is a further type of cosmic-ray-induced background. Here mainly p--capture and fast muon reactions have to be considered. Table 3 gives, as an example, the estimated pr~uction rates for the main radioisotopes of the targets Ge, Fe, and Pb by p--capture. They are obtained by replacing M in the neutron production eq. (1) by the yield y if the target element is monoisotopic. Details can be found, for example, in refs. [11,14]. So far, none of these isotopes could be identified, but it is clear that they, as an ensemble, contribute noticeably to the background. They are mostly formed in an excited state and therefore also contribute together with the stable p--capture products through their de-excitation gamma rays. Production rates via fast muons are probably of similar magnitude WI.
at 15 m w.e.
Target
Radionuelide
Production rate (atoms kg-’ min -‘I
5. Discussion
Fe
54Mll 56Mn
0.013 0.013
Ge
72Ga “Ga 74Ga
0.012 0.013 0.0065
Pb
*04Tl t0q.l m7T1
0.0085 0.018 0.017
In our laboratory more neutrons are produced in the shield itself than outside because of the higher density and higher Z (larger WC and m). As a consequence, the shielding thickness of a detector should be chosen not thicker than necessary to absorb external gamma rays. Any additional high-Z material adds to the neutron producing target. Since only a few percent of the neutrons are not of cosmogenie origin, a neutron shield on the outside would not be effective. Placing the neutron II. UNDERGROUND
LABORATORIES
422
G. Hewer
/ Background components of Ge low-level spectrometers
shield around the detector would strongly increase the whole shield (target) because of its necessarily bulky size. A better solution would be an anticoincidence counter covering the outside of the lead shield which effectively eliminates incoming muons. The veto time has to be chosen long enough (a few ten ps) to also cancel delayed excitation radiation. It can be better met when using gas proportional or Geiger-Mtiller counters than scintillation detectors which, because of their larger gamma sensitivity, cause a longer dead time. The lower spectrum in fig. 4 was recorded with a provisional veto shield of proportional counters around the lead shield. It reduced the neutron-induced 847 keV 56Fe peak by a factor of 4 + 1.5. The activation component can only be avoided by placing the detector deep underground. Beyond a depth of about 1000 m w.e. the cosmic-ray-produced neutron flux would be less than 1% of that from (QL,n) reactions and fission in our laboratory. Here, an outside neutron shield would be helpful if the neutroninduced background component would still be disturbing.
References (11 A gallium solar neutrino experiment will be performed by the international GALLEX Collaboration: MPI Kernphysik Heidelberg, KFK Karlsruhe, TU Munich, CEN Saclay, University of Nice, University of Milano, Univer-
sity of Rome, and WIS Rehovot. See for example, W. Hampel, AIP Conf. Proc. no. 26, Homestake (1984) p. 162. PI G. Heusser, W. Hampel and M. Hiibner, Proc. 2nd Int. Conf. on Low Level Counting, High Tatras 1980, Phys. Appl. 8 (1982) 127. [31 G. Heusser, these Proceedings (Low-level Counting) Nucl. Instr. and Meth. B17 (1986) 423. B.A. Magurno and R. [41 J.R. Stehn, M.D. Goldberg, Wiener-Chasman, BNL 325, 2nd ed., suppl. 2, ~01s. l-3 (1964). [51 NCRP Report no. 45 (1975). L.D. Stephens and H.W. Patterson, J. [61 M. Yamashita, Geophys. Res. 71 (1966) 3817. 171 Strippa Project SKB Techn. Report no. 85-06. PI Y. Feige, B.G. Oltman and J. Kastner, J. Geophys. Res. 73 (1968) 3135. 191 M.W. Kuhn, S.N. Davis and H.W. Bentley, Geophys. Res. Lett. 11 (1984) 607. WI R. Zito, D.J. Donahue, S.N. Davis, H.W. Bentley and P. Fritz, Geophys. Res. Lett. 7 (1980) 235. 1111 T. Kirsten and W. Hampel, Proc. 1st Int. Conf. on Low Level Counting, High Tatras 1975, Bratislava (1977) p. 427. Nucl. Phys. Al66 (1971) 145. WI S. Charalambus, S. Diaz, S. Kaplan and R. Tyle, Phys. P31 B. MacDonald, Rev. 139 (1965) B1253. u41 G. Heusser and T. Kirsten, Nucl. Phys. Al95 (1972) 369. U. Jahnke, K.H. Lindenberger, G. 1151 W.U. Schroder, Roschert, R. Engfer and H.K. Walter, Z. Phys. 268 (1974) 57.