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A low-background detection system using a BGO detector for sensitive hydrogen analysis with the 1H(15N, αγ)12C reaction
252
Nuclear
Instruments
and Methods
in Physics
A LOW-BACKGROUND DETECTION SYSTEM USING A BGO DETECTOR HYDROGEN ANALYSIS WITH THE ‘H( =N, cty)% REACTION
Research
B45 (1990) 252-255 North-Holland
FOR SENSITIVE
D. KUHN, F. RAUCH and H. BAUMANN Institut ftirKernphysik der Johann Wolfgang Goethe Uniuersitiit, 6000 Frankfurt am Main, FRG
A geometrically compact detection system with a low cosmic-radiation background rate, 0.55 cts./min in the energy range 3.8-4.8 MeV, and good detection efficiency (14%) for the 4.4 MeV reaction y-rays, is described. It consists of a 3 in. X 3 in. BGO detector, a plastic-detector anticoincidence shield and a Pb shield. The detection sensitivity of this system is 5 20 at.ppm. The performance of BGO detectors in comparision to NaI detectors for measuring 4.4 MeV y-rays is discussed.
1. Introduction Since more than a decade, hydrogen analysis by resonant nuclear reactions is an important tool in many branches of science and technology. Most frequently the reaction ‘H(15N, cuy)“C at the 6.39 MeV resonance [l] and the reaction *H(19F, (wy)160 at the 16.44 MeV resonance [2] are used. For measuring of the y-rays from these reactions, usually NaI detectors, with dimensions of a few to several inches, are employed. Only recently also BGO detectors have come into use; they are attractive because for a given size they have a higher efficiency than NaI detectors. The detection sensitivity reached with such detectors is in the order of lo3 at.ppm. Cosmic-ray background (CRB) is the main factor limiting the sensitivity. A first, simple step for reducing the CRB rate is to surround the detector with a shield of bulk material, e.g. lead or concrete, which mainly effects the background from the soft component of the cosmic radiation. For more complete reduction, anticoincidence shielding has to be added which suppresses the events due to muons and other charged particles. Such a low-level system (LLS), containing also shielding against neutron-induced background, has been built by Damjantschitsch et al. [3]; its main detector is a large NaI crystal, the sensitivity is lo-100 at.ppm. In our laboratory, we have for several years used unshielded NaI detectors for hydrogen profiling with the “N technique, see e.g. refs. [4,.5]. For an experimental program on the analysis of hydrogen in minerals for which high sensitivity is mandatory, we have tested a LLS which utilizes the advantageous properties of a BGO detector and has an anticoincidence shield plus a Pb shield. The setup is rather compact, which was necessary because of the limited space in the experimental area. Another requirement was high detection efficiency: since for some minerals hydrogen becomes 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
mobile by irradiation and some samples are very small, it is essential that a large fraction of the y-rays produced in the sample can be detected. In the next chapter we compare the properties of (unshielded) BGO and NaI detectors. The LLS is described in chapter 3 and the detection sensitivity is discussed in chapter 4. We concentrate here on the measurement of 4.43 MeV y-rays from the iH(15N, oly)l’C reaction, but the results and conclusions obtained are also useful for H analysis with the ‘H(l’F, oly)160 reaction and for other measurements involving detection of y-rays in the 4-10 MeV range.
2. BGO versus NaI For the LLS a 3 in. x 3 in. BGO detector is used. In this chapter we compare it with regard to the CRB rate and detection efficiency with a 5 in. x 4 in. NaI detector because it has about the same efficiency. This comparison is instructive for considerations on the design of a LLS and furthermore shows the nice properties of BGO in H-analysis measurements with unshielded detectors. The CRB rates and efficiency values reported are related to an energy region of interest (ROI) in the pulse-height spectrum which, besides the full-energy peak of the 4.43 MeV y-rays, encloses the single-escape peak and for the NaI also the double-escape peak (see below). The efficiency is defined here as the integral number of pulses in the ROI, normalized to the 47r yield of 4.43 MeV y-rays produced in the sample at the resonance energy. Efficiency values were determined in measurements with a NH&l pellet or a H-implanted Si wafer as calibration samples. The 471 yield was calculated using for the product of resonance cross section and width the value o,F = 2820 eV b [6].
253
D. Kuhn et al. /A low-background detection system
The detector position for all measurements is 0” relative to the beam direction. This not only takes advantage of the strongly anisotropic angular distribution of the reaction at the 6.39 MeV resonance [7], but also allows to minimize the sample-detector distance. Since for this detector position it is difficult to obtain good shielding against beam-induced 4.43 MeV y-rays from the beam collimator, we use a heatable Ta collimator. It was found that at temperatures of about 1000 o C the H concentrations in the Ta bulk and in the surface layer become unmeasurably small. Pulse-height spectra of 4.43 MeV y-rays taken with the BGO and a 5 in. X 4 in. NaI are shown in fig. 1. One sees that the line shape is different, the BGO spectrum having a larger fraction of pulses in the full-energy peak; the double-escape peak is very small. This better line shape is a consequence of the higher y-ray attenuation coefficient of BGO compared to NaI. Therefore, the ROI can be made narrower for the BGO detector with little loss in efficiency, thus improving the signalto-background ratio. We have chosen as ROI the range 3.8-4.8 MeV, and for the NaI the range 3.5-4.8 MeV. It should be noted that the width of the peaks is not only determined by the energy resolution of the detectors but also by Doppler broadening; furthermore, in the O” detector position the mean energy of the y-rays is Doppler-shifted by about 130 keV. In fig. 2 the measured efficiency values for various detector-target distances are plotted. The target for the measurement at the smallest distance was the hydrogen
800600.
3"x3" BGO ,
Distance [mm1
Fig. 2. Plot of the efficiency values of the 3 in. x 3 in. BGO and the 5 in. X 4 in. NaI detectors as a function of the target-detector distance. The lines are to guide the eye.
in the surface contamination layer of a thin foil; it was checked that the areal density of hydrogen did not change during the measurement. It is seen that both detectors have about the same efficiency; the BGO detector is more effective at small distances, due to its smaller absorption length for y-radiation. At 5 mm distance the BGO efficiency is 148, which leads to 0.67 cts./pC “N*+ for 100 at.ppm H in Si. Thus, considering the efficiency values, a 3 in. X 3 in. BGO and a 5 in. X 4 in. NaI detector are about equally well suited in the analysis of samples in which the signal-to-background ratio is large. However, when this ratio is small, the BGO is superior as will become clear from the CRB rates of the different detectors. The CRB measurements were performed in the basement of our accelerator building near the location where i5N profiling is done. The basement has a concrete ceiling with an average thickness of 50 cm. Measurements on the ground floor for bare detectors and for various shielding conditions showed a CRB rate of about a factor 2.5 higher than in the basement.
Energy [MeVI
0 a
5"x4"NaI withoutshield 3'*~3" BGOwithoutshield
a
3'*~3" BGO ~;;,i;$~ea; s$ild,
160
1
2
3
4
5
Energy[PINI
Fig. 1. Comparison of pulse-height spectra of the 3 in. x 3 in. BGO
and the 5 in. x 4 in. NaI detectors. The black areas indicate the region of interest (ROI).
)
i
i
k
i
,;
Energy [MeVI
Fig. 3. Comparison of background spectra measured with unshielded detectors and with different shielding conditions. III. EQUIPMENT
254
l
&8;<0'
BGO
oNa
1' I'
1'
/' ,' ,,*&4"
Fig. 5. Cross sections of the veto detector: (1) preamplifier and voltage divider, (2) photomultiplier, (3) plastic scintillator, (4) detector housing. Fig. 4. Plot of the background rate in the energy range from 3 to 5 MeV of various unshielded detectors as a faction of detector volume. The line is to guide the eye.
Background spectra for the 3 in. x 3 in. BGO and the 5 in. x 4 in. NaI detectors are displayed in fig. 3; together with two other spectra to be discussed in the next chapter. In the low-energy region, the peaks due to y-rays from 4oK (1.46 Mev) and uzTh (2.62 MeV) are prominent. The high-energy part of the spectra is only due to cosmic-radiation-induced events. Of main concern is the CRB rate in the ROI for H analysis. These values are 11.2 cts./min for the NaI and 4.3 cts./min for the BGO detector. For contemplating detection systems with larger detectors, the increase of the CRB rate with detector size is of interest. Measured values for several NaI detectors and the 3 in. x 3 in. BGO one are plotted in fig. 4 as a function of the detector volume, yielding a roughly linear relationship. This dependence is expected to hold for BGO detectors as well. The 8 in. x 8 in. NaI detector was realized by putting two 8 in. X 4 in. NaI detectors close together (face by face) and summing the pulses from both detectors. All detectors were positioned with the axes being horizontal.
For cons~cting the veto detector, an 8 in. x 8 in. plastic scintillator was at our disposal. The inner part was removed to obtain a central well of 4 in. diameter; on the outside a flat area was machined for mounting a 4 in, photomultiplier (see fig. 5). A detector housing was made from 2 mm steel sheet; as lining for the well 5 mm steel sheet was used. The light collection efficiency of various parts of the detector differ by less then a factor 2 as checked with a =Na source. The full setup is shown in fig. 6. The veto detector is encased by 10 cm Pb from all sides, except for gaps for the photomultiplier tubes of the detectors and for the beam tube. The threshold for anticoincidence is set to about 100 keV deposited energy in the veto detector. With each veto pulse, the gate of the multichannel analyzer registrating the BGO pulses is closed for 15 ps. This long time window is necessary for rejecting events in the BGO produced by the decay products of stopped muons (2.2 ps half-life). These are y-rays from $ + e+ + v, + c,ande++e--+2y,andneutronsfromp-+p-,n+ q. Due to the weak y-ray absorption of plastic scintillators, even during the analysis of samples with high H content, the count rate in the veto detector is so low
3. The low-level system In the LLS that we have developed and tested, the 3 in. X 3 in. BGO detector is enclosed by a plastic scintilIator as veto detector. This assembly is surrounded by a 10 cm Pb shield. Although the veto detector is of simple design and does not frilly enclose the BGO, good CRB reduction is achieved and the results obtained for this system can provide the basis for an improved design. The small size of the BGO compared to a NaI detector of the same efficiency is of advantage here: both the anticoincidence shield and the passive shield can be kept small so that the setup is not too bnlky and in addition the shielding is less costly.
Fig. 6. Cross section of the low-level system: (1) Pb shield, (2) plastic scintillator, (3) beam tube, (4) sample, (5) BGO detector.
D. Kuhn et ai. /A
how-backgrounddetection system
that no deadtime correction is necessary; also losses in the single-escape peak of the BGO pulse-height spectrum due to 511 keV y-rays being detected in the veto detector are negligible. The count rate of the veto detector from cosmic-radiation events of about 20 cts./s does not require deadtime correction either. A background spectrum taken w&h the LLS is shown in fig. 3, together with a spectrum taken with the veto detector turned off and the spectra of the unshielded detectors already discussed. It is seen that the Pb shield alone reduces the 40R and 232Th peaks strongly; the peak due to *14Bi becomes more prominent. The CRB rate in the high-energy range is reduced by about a factor 2; in the ROI for H analysis it is 2.44 cts./min. A much better suppression is obtained with the veto detector being active; the count rate in the ROI is reduced to the small value of OS.5 cts./min. The remaining background is m&&y due to incomplete shielding of the soft component and the muon component of the cosmic radiation. A further reduction would be possible by increasing the thickness of the Pb shield and closing the gaps in the Pb shield and anticoincidence shield. At about 7 MeV there appears a broad peak which is due to y-rays from the capture of thermal neutrons in the BGO itself and the material surrounding it. These y-rays contribute to the CRB rate in the ROI through Compton-scattering events. Taking into account the capture cross sections and amounts of the various nuclides in the LLS and estimating the relative detection probabilities in the BGO, the strongest contributions come from 56Fe (7.6 MeV) and ‘07Pb (7.4 MeV). As inferred from the almost unchanged CRB rate in a me~~ement with a 15 cm neutron shield (polyethylene with 15% H,BO,) enclosing the LLS at all sides, the neutrons are produced mostly in the LLS itself. This is in accordance with results obtained by Heusser [S].
4. Detection sensitivity and improvements The detection sensitivity obtainable with the LLS depends, for fixed CRB rate (0.55 cts./min) and efficiency (14%), on the available beam current and measuring time and on the desired precision; also the sample material enters through its stopping power. Assuming reasonable values of 100 n.4 15N2+ and 60 min, a hydrogen concentration of 20 at.ppm H in Si can be detected with an error s 25%. We have disregarded here beam-induced background from off-resonance reactions in the H-bearing surface layer of the sample and from reactions with hydrogen in the beam collimator (compare section 2). A possible improvement of the LLS with regard to
255
the CRB rate will be to put the anticoincidence shield outside the Pb shield. This makes it easier to close the gaps in the Pb shield, thus reducing the remaining background from the soft component, and still the amount of Pb is smaller than in the present setup, which diminishes the background from stopped muons. For a new LLS that we are planning, a 4 in. X 4 in. BGO detector with a central end well of 3 cm diameter and 5 cm depth will be used. The sample can be located near the center of the crystal which leads to an increased efficiency of 35%. This value was calculated with a Monte Carlo simulation program [9] by which the 4.4 MeV y-ray line shapes and efficiencies of the 3 in. X 3 in. BGO and the 5 in. X 4 in. NaI detectors could be well reproduced. (Details of these calculations and results for other detectors will be published separately.) Although for the bare 4 in. X 4 in. BGO detector the CRB rate will be about twice that of the 3 in. X 3 in. BGO one (see fig. 3), the CRB rate for the planned LLS with the shielding improvements discussed above is expected to be as low as for the present one. Altogether, it should be possible to increase the sensitivity by a factor 2-3. The ideal state of complete CRB suppression could be approached with a more sophisticated shielding system. We have performed a measurement with the 3 in. x 3 in. BGO located inside the LLS of the Heidelberg group [3] in which a CRB rate of 0.19 cts./min was found. We want to thank M. Weiser and S. Jans of the group of Dr. S. Kalbitzer for their kind support in the background m~smement at Heidelberg.
References t11 W.A. Lanford, Nucl. In&. and Meth. 149 (1978) 1. [21 D.A. Leich and T.A. Tombrello, Nucl. Ins@. and Meth. 108 (1973) 67. M. Weiser, G. Heusser, S. Kalbitzer 131 H. D~j~~c~tsch, and H. Mannsberger, Nucl. Instr. and Me& 218 (1983) 218. [41 P. March and F. Rauch, Nucl. Instr. and Me& B15 (1986) 516. H. Baumann and F. Rauch, Nucl. Instr. and t51 B. Hoffmm, Meth. B15 (1986) 361. 161 S. Gorodetzky, J.C. Adloff, F. Brochard, P. Chevallier, D. Disdier, Ph. Gorodetzky, R. Modjtabed-Zadeh and F. Scheibling, Nucl. Phys. A113 (1968) 221. t71 A.A. Krauss, A.P. French, W.A. Fowler and CC. Lauritsen, Phys. Rev. 89 (1953) 299. PI G. Heusser, Nucl. Instr. and Meth. B17 (1986) 418. t91 R. Brun, F. Bruyant, M. Maire, A.C. McPherson and P. Zanarini, GEANT3 User Guide (CERN, Geneva, 1987).
III. EQUIPMENT
Nuclear
Instruments
and Methods
in Physics
A LOW-BACKGROUND DETECTION SYSTEM USING A BGO DETECTOR HYDROGEN ANALYSIS WITH THE ‘H( =N, cty)% REACTION
Research
B45 (1990) 252-255 North-Holland
FOR SENSITIVE
D. KUHN, F. RAUCH and H. BAUMANN Institut ftirKernphysik der Johann Wolfgang Goethe Uniuersitiit, 6000 Frankfurt am Main, FRG
A geometrically compact detection system with a low cosmic-radiation background rate, 0.55 cts./min in the energy range 3.8-4.8 MeV, and good detection efficiency (14%) for the 4.4 MeV reaction y-rays, is described. It consists of a 3 in. X 3 in. BGO detector, a plastic-detector anticoincidence shield and a Pb shield. The detection sensitivity of this system is 5 20 at.ppm. The performance of BGO detectors in comparision to NaI detectors for measuring 4.4 MeV y-rays is discussed.
1. Introduction Since more than a decade, hydrogen analysis by resonant nuclear reactions is an important tool in many branches of science and technology. Most frequently the reaction ‘H(15N, cuy)“C at the 6.39 MeV resonance [l] and the reaction *H(19F, (wy)160 at the 16.44 MeV resonance [2] are used. For measuring of the y-rays from these reactions, usually NaI detectors, with dimensions of a few to several inches, are employed. Only recently also BGO detectors have come into use; they are attractive because for a given size they have a higher efficiency than NaI detectors. The detection sensitivity reached with such detectors is in the order of lo3 at.ppm. Cosmic-ray background (CRB) is the main factor limiting the sensitivity. A first, simple step for reducing the CRB rate is to surround the detector with a shield of bulk material, e.g. lead or concrete, which mainly effects the background from the soft component of the cosmic radiation. For more complete reduction, anticoincidence shielding has to be added which suppresses the events due to muons and other charged particles. Such a low-level system (LLS), containing also shielding against neutron-induced background, has been built by Damjantschitsch et al. [3]; its main detector is a large NaI crystal, the sensitivity is lo-100 at.ppm. In our laboratory, we have for several years used unshielded NaI detectors for hydrogen profiling with the “N technique, see e.g. refs. [4,.5]. For an experimental program on the analysis of hydrogen in minerals for which high sensitivity is mandatory, we have tested a LLS which utilizes the advantageous properties of a BGO detector and has an anticoincidence shield plus a Pb shield. The setup is rather compact, which was necessary because of the limited space in the experimental area. Another requirement was high detection efficiency: since for some minerals hydrogen becomes 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers
B.V.
mobile by irradiation and some samples are very small, it is essential that a large fraction of the y-rays produced in the sample can be detected. In the next chapter we compare the properties of (unshielded) BGO and NaI detectors. The LLS is described in chapter 3 and the detection sensitivity is discussed in chapter 4. We concentrate here on the measurement of 4.43 MeV y-rays from the iH(15N, oly)l’C reaction, but the results and conclusions obtained are also useful for H analysis with the ‘H(l’F, oly)160 reaction and for other measurements involving detection of y-rays in the 4-10 MeV range.
2. BGO versus NaI For the LLS a 3 in. x 3 in. BGO detector is used. In this chapter we compare it with regard to the CRB rate and detection efficiency with a 5 in. x 4 in. NaI detector because it has about the same efficiency. This comparison is instructive for considerations on the design of a LLS and furthermore shows the nice properties of BGO in H-analysis measurements with unshielded detectors. The CRB rates and efficiency values reported are related to an energy region of interest (ROI) in the pulse-height spectrum which, besides the full-energy peak of the 4.43 MeV y-rays, encloses the single-escape peak and for the NaI also the double-escape peak (see below). The efficiency is defined here as the integral number of pulses in the ROI, normalized to the 47r yield of 4.43 MeV y-rays produced in the sample at the resonance energy. Efficiency values were determined in measurements with a NH&l pellet or a H-implanted Si wafer as calibration samples. The 471 yield was calculated using for the product of resonance cross section and width the value o,F = 2820 eV b [6].
253
D. Kuhn et al. /A low-background detection system
The detector position for all measurements is 0” relative to the beam direction. This not only takes advantage of the strongly anisotropic angular distribution of the reaction at the 6.39 MeV resonance [7], but also allows to minimize the sample-detector distance. Since for this detector position it is difficult to obtain good shielding against beam-induced 4.43 MeV y-rays from the beam collimator, we use a heatable Ta collimator. It was found that at temperatures of about 1000 o C the H concentrations in the Ta bulk and in the surface layer become unmeasurably small. Pulse-height spectra of 4.43 MeV y-rays taken with the BGO and a 5 in. X 4 in. NaI are shown in fig. 1. One sees that the line shape is different, the BGO spectrum having a larger fraction of pulses in the full-energy peak; the double-escape peak is very small. This better line shape is a consequence of the higher y-ray attenuation coefficient of BGO compared to NaI. Therefore, the ROI can be made narrower for the BGO detector with little loss in efficiency, thus improving the signalto-background ratio. We have chosen as ROI the range 3.8-4.8 MeV, and for the NaI the range 3.5-4.8 MeV. It should be noted that the width of the peaks is not only determined by the energy resolution of the detectors but also by Doppler broadening; furthermore, in the O” detector position the mean energy of the y-rays is Doppler-shifted by about 130 keV. In fig. 2 the measured efficiency values for various detector-target distances are plotted. The target for the measurement at the smallest distance was the hydrogen
800600.
3"x3" BGO ,
Distance [mm1
Fig. 2. Plot of the efficiency values of the 3 in. x 3 in. BGO and the 5 in. X 4 in. NaI detectors as a function of the target-detector distance. The lines are to guide the eye.
in the surface contamination layer of a thin foil; it was checked that the areal density of hydrogen did not change during the measurement. It is seen that both detectors have about the same efficiency; the BGO detector is more effective at small distances, due to its smaller absorption length for y-radiation. At 5 mm distance the BGO efficiency is 148, which leads to 0.67 cts./pC “N*+ for 100 at.ppm H in Si. Thus, considering the efficiency values, a 3 in. X 3 in. BGO and a 5 in. X 4 in. NaI detector are about equally well suited in the analysis of samples in which the signal-to-background ratio is large. However, when this ratio is small, the BGO is superior as will become clear from the CRB rates of the different detectors. The CRB measurements were performed in the basement of our accelerator building near the location where i5N profiling is done. The basement has a concrete ceiling with an average thickness of 50 cm. Measurements on the ground floor for bare detectors and for various shielding conditions showed a CRB rate of about a factor 2.5 higher than in the basement.
Energy [MeVI
0 a
5"x4"NaI withoutshield 3'*~3" BGOwithoutshield
a
3'*~3" BGO ~;;,i;$~ea; s$ild,
160
1
2
3
4
5
Energy[PINI
Fig. 1. Comparison of pulse-height spectra of the 3 in. x 3 in. BGO
and the 5 in. x 4 in. NaI detectors. The black areas indicate the region of interest (ROI).
)
i
i
k
i
,;
Energy [MeVI
Fig. 3. Comparison of background spectra measured with unshielded detectors and with different shielding conditions. III. EQUIPMENT
254
l
&8;<0'
BGO
oNa
1' I'
1'
/' ,' ,,*&4"
Fig. 5. Cross sections of the veto detector: (1) preamplifier and voltage divider, (2) photomultiplier, (3) plastic scintillator, (4) detector housing. Fig. 4. Plot of the background rate in the energy range from 3 to 5 MeV of various unshielded detectors as a faction of detector volume. The line is to guide the eye.
Background spectra for the 3 in. x 3 in. BGO and the 5 in. x 4 in. NaI detectors are displayed in fig. 3; together with two other spectra to be discussed in the next chapter. In the low-energy region, the peaks due to y-rays from 4oK (1.46 Mev) and uzTh (2.62 MeV) are prominent. The high-energy part of the spectra is only due to cosmic-radiation-induced events. Of main concern is the CRB rate in the ROI for H analysis. These values are 11.2 cts./min for the NaI and 4.3 cts./min for the BGO detector. For contemplating detection systems with larger detectors, the increase of the CRB rate with detector size is of interest. Measured values for several NaI detectors and the 3 in. x 3 in. BGO one are plotted in fig. 4 as a function of the detector volume, yielding a roughly linear relationship. This dependence is expected to hold for BGO detectors as well. The 8 in. x 8 in. NaI detector was realized by putting two 8 in. X 4 in. NaI detectors close together (face by face) and summing the pulses from both detectors. All detectors were positioned with the axes being horizontal.
For cons~cting the veto detector, an 8 in. x 8 in. plastic scintillator was at our disposal. The inner part was removed to obtain a central well of 4 in. diameter; on the outside a flat area was machined for mounting a 4 in, photomultiplier (see fig. 5). A detector housing was made from 2 mm steel sheet; as lining for the well 5 mm steel sheet was used. The light collection efficiency of various parts of the detector differ by less then a factor 2 as checked with a =Na source. The full setup is shown in fig. 6. The veto detector is encased by 10 cm Pb from all sides, except for gaps for the photomultiplier tubes of the detectors and for the beam tube. The threshold for anticoincidence is set to about 100 keV deposited energy in the veto detector. With each veto pulse, the gate of the multichannel analyzer registrating the BGO pulses is closed for 15 ps. This long time window is necessary for rejecting events in the BGO produced by the decay products of stopped muons (2.2 ps half-life). These are y-rays from $ + e+ + v, + c,ande++e--+2y,andneutronsfromp-+p-,n+ q. Due to the weak y-ray absorption of plastic scintillators, even during the analysis of samples with high H content, the count rate in the veto detector is so low
3. The low-level system In the LLS that we have developed and tested, the 3 in. X 3 in. BGO detector is enclosed by a plastic scintilIator as veto detector. This assembly is surrounded by a 10 cm Pb shield. Although the veto detector is of simple design and does not frilly enclose the BGO, good CRB reduction is achieved and the results obtained for this system can provide the basis for an improved design. The small size of the BGO compared to a NaI detector of the same efficiency is of advantage here: both the anticoincidence shield and the passive shield can be kept small so that the setup is not too bnlky and in addition the shielding is less costly.
Fig. 6. Cross section of the low-level system: (1) Pb shield, (2) plastic scintillator, (3) beam tube, (4) sample, (5) BGO detector.
D. Kuhn et ai. /A
how-backgrounddetection system
that no deadtime correction is necessary; also losses in the single-escape peak of the BGO pulse-height spectrum due to 511 keV y-rays being detected in the veto detector are negligible. The count rate of the veto detector from cosmic-radiation events of about 20 cts./s does not require deadtime correction either. A background spectrum taken w&h the LLS is shown in fig. 3, together with a spectrum taken with the veto detector turned off and the spectra of the unshielded detectors already discussed. It is seen that the Pb shield alone reduces the 40R and 232Th peaks strongly; the peak due to *14Bi becomes more prominent. The CRB rate in the high-energy range is reduced by about a factor 2; in the ROI for H analysis it is 2.44 cts./min. A much better suppression is obtained with the veto detector being active; the count rate in the ROI is reduced to the small value of OS.5 cts./min. The remaining background is m&&y due to incomplete shielding of the soft component and the muon component of the cosmic radiation. A further reduction would be possible by increasing the thickness of the Pb shield and closing the gaps in the Pb shield and anticoincidence shield. At about 7 MeV there appears a broad peak which is due to y-rays from the capture of thermal neutrons in the BGO itself and the material surrounding it. These y-rays contribute to the CRB rate in the ROI through Compton-scattering events. Taking into account the capture cross sections and amounts of the various nuclides in the LLS and estimating the relative detection probabilities in the BGO, the strongest contributions come from 56Fe (7.6 MeV) and ‘07Pb (7.4 MeV). As inferred from the almost unchanged CRB rate in a me~~ement with a 15 cm neutron shield (polyethylene with 15% H,BO,) enclosing the LLS at all sides, the neutrons are produced mostly in the LLS itself. This is in accordance with results obtained by Heusser [S].
4. Detection sensitivity and improvements The detection sensitivity obtainable with the LLS depends, for fixed CRB rate (0.55 cts./min) and efficiency (14%), on the available beam current and measuring time and on the desired precision; also the sample material enters through its stopping power. Assuming reasonable values of 100 n.4 15N2+ and 60 min, a hydrogen concentration of 20 at.ppm H in Si can be detected with an error s 25%. We have disregarded here beam-induced background from off-resonance reactions in the H-bearing surface layer of the sample and from reactions with hydrogen in the beam collimator (compare section 2). A possible improvement of the LLS with regard to
255
the CRB rate will be to put the anticoincidence shield outside the Pb shield. This makes it easier to close the gaps in the Pb shield, thus reducing the remaining background from the soft component, and still the amount of Pb is smaller than in the present setup, which diminishes the background from stopped muons. For a new LLS that we are planning, a 4 in. X 4 in. BGO detector with a central end well of 3 cm diameter and 5 cm depth will be used. The sample can be located near the center of the crystal which leads to an increased efficiency of 35%. This value was calculated with a Monte Carlo simulation program [9] by which the 4.4 MeV y-ray line shapes and efficiencies of the 3 in. X 3 in. BGO and the 5 in. X 4 in. NaI detectors could be well reproduced. (Details of these calculations and results for other detectors will be published separately.) Although for the bare 4 in. X 4 in. BGO detector the CRB rate will be about twice that of the 3 in. X 3 in. BGO one (see fig. 3), the CRB rate for the planned LLS with the shielding improvements discussed above is expected to be as low as for the present one. Altogether, it should be possible to increase the sensitivity by a factor 2-3. The ideal state of complete CRB suppression could be approached with a more sophisticated shielding system. We have performed a measurement with the 3 in. x 3 in. BGO located inside the LLS of the Heidelberg group [3] in which a CRB rate of 0.19 cts./min was found. We want to thank M. Weiser and S. Jans of the group of Dr. S. Kalbitzer for their kind support in the background m~smement at Heidelberg.
References t11 W.A. Lanford, Nucl. In&. and Meth. 149 (1978) 1. [21 D.A. Leich and T.A. Tombrello, Nucl. Ins@. and Meth. 108 (1973) 67. M. Weiser, G. Heusser, S. Kalbitzer 131 H. D~j~~c~tsch, and H. Mannsberger, Nucl. Instr. and Me& 218 (1983) 218. [41 P. March and F. Rauch, Nucl. Instr. and Me& B15 (1986) 516. H. Baumann and F. Rauch, Nucl. Instr. and t51 B. Hoffmm, Meth. B15 (1986) 361. 161 S. Gorodetzky, J.C. Adloff, F. Brochard, P. Chevallier, D. Disdier, Ph. Gorodetzky, R. Modjtabed-Zadeh and F. Scheibling, Nucl. Phys. A113 (1968) 221. t71 A.A. Krauss, A.P. French, W.A. Fowler and CC. Lauritsen, Phys. Rev. 89 (1953) 299. PI G. Heusser, Nucl. Instr. and Meth. B17 (1986) 418. t91 R. Brun, F. Bruyant, M. Maire, A.C. McPherson and P. Zanarini, GEANT3 User Guide (CERN, Geneva, 1987).
III. EQUIPMENT