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Line features in the background γ-ray spectrum in the range 0.05–10 MeV at balloon altitudes
Nuclear Instruments and Methods in Physics Research 220 (1984) 549-560 North-Holland, Amsterdam
549
L I N E F E A T U R E S IN T H E B A C K G R O U N D y-RAY S P E C T R U M IN T H E R A N G E 0.05-10 MeV AT BALLOON ALTITUDES C.A. A Y R E , P.N. B H A T *, Y.Q. M A **, R . M . M Y E R S an d M . G . T H O M P S O N
Department of Physics, University of Durham, Durham, England Received 4 July 1983
An actively shielded high resolution -/-ray spectrometer has been flown as a balloon payload on two occasions, and most recently in June 1981, to search for -/-ray features in the spectra of astrophysical sources. A comprehensive table of background lines recorded on the June 1981 flight is given and there is evidence for -/-rays from several members of the natural uranium radioactive series; the origin of which is uncertain. The main nuclide detected in the series is 214Bi and there is no evidence that this originates in the spectrometer material therefore an atmospheric origin is suggested.
1. Introduction Over the last decade there have been numerous theoretical predictions of the intensities of `/-ray lines emanating from astrophysical sources. Potential sources are supernovae and novae; neutron stars and black holes, and nuclear y-rays will also arise as a consequence of interactions of particles and cosmic rays with the interstellar medium. Many of the line features will have narrow widths (i.e. few keV at energies of 1 MeV) and with the development of reasonably sized coaxial solid state detectors, characterised by high detection efficiences and line resolution typically 2-3 keV over the energy range of interest, it is reasonable to expect nuclear y-rays will be unambiguously detected shortly. Nuclear -/-rays are mostly in the range 0-10 MeV, although some occur at higher energies, but measurements of such rays can only be made at high altitudes where the attentuation of the -/-rays by the earth's atmosphere is reduced. In this paper measurements are reported on background -/-ray lines found at balloon altitudes. The detector which is described in sect. 2 is an intrinsic germanium crystal surrounded by a NaI(T1) anticoincidence shield. A knowledge of the background -/-ray line features and an understanding of the origin of the features is necessary if data from -/-ray spectrometers are to be interpreted correctly. Also reported are measurements of several nuclides of the naturally occurring radioactive series which have been made with the instrument. The origin of these nuclides has not been * On leave from the Tata Institute of Fundamental Research, Bombay, India. ** On leave from the Institute of High Energy Physics, Beijing, China. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
unequivocally established but it is believed the decaying nuclides are external to the instrument.
2. The spectrometer
2.1. Outline of the instrument The spectrometer is basically a high resolution solid state detector contained within an active shield. The high resolution detector is a hyperpure germanium crystal, Ge(Hp), and collimation and background rejection for this detector are provided by a three-piece NaI(T1) anticoincidence shield. One of the shield elements is further sub-divided so as to constitute a polarimeter. Events which Compton scatter a photon from the Ge(Hp) detector into one of the polarimeter elements and which is detected, are also recorded. In normal operation of the spectrometer, the data from each Ge(Hp) event are recorded along with information relating to any energy coincidentally deposited in one or more of the shield elements. Event selection, i.e. between coincident and non-coincident events, is made subsequently software-wise from the raw data available from the spectrometer. The spectrometer, which has cylindrical symmetry is shown in fig. 1. Covering the aperture of the spectrometer is a 3 mm thick charged particle detector. Events which are coincident with pulses from this detector are not recorded. The pulses from the Ge(Hp) detector, in the range 0-10 V and equivalent to 0-10 MeV, are analysed using an A D C having 214 channels, giving a resolution of 0.65 keV/channel. The low energy threshold of the detector is - 50 keV; hence the spectrometer covers the energy range 0.05-10 MeV. The electronic system is capable of
550
CA. Ayre et aL / Line features in background y- ray spectrum
Charged
Parfic[e Defec for
NoI(TL) Annulus (A1) Viewed By Three Phofomu[fiptiers
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NoI (Tt) Pot ori mete (Pl - P~ ) Each -Viewed By One
Phofomu~ Plug (A3) By Three ~u[fip[iers
10cm Fig. 1. Schemmaticdiagram of the -/-ray spectrometer.
analysing two coincident pulses occurring within the spectrometer, the instrumental resolving time being 1 #s. The pulses chosen for analysis are selected following a specified hierarchy. In addition to the analysis of a pulse from the Ge(Hp) detector, a coincident pulse from one of the shield elements, P1, P2, P3, P4, A2, A3, Aa, is also analysed; the order given being the order of precedence and the elements being as shown in fig. 1. The threshold for the shield pulses is typically 50 keV. However, for those events for which there appears to be no pulse coincident with a Ge(Hp) detector pulse from any shield element, as defined by the discriminators associated with the shield, the summed pulse from all the shield elements is analysed by the second ADC channel. In this way the shield threshold is reduced and the shield sensitivity improved. The counting rates of
the shields and the Ge(Hp) detector are continually monitored. The experimental package is on an alt-azimuthal mounting, which is capable of being pointed with an accuracy of +0.25 ° in altitude and +0.5 ° in azimuth. Altitude control is achieved using two precision orthogonal inclinometers, and azimuthal control using a pair of zero-field seeking magnetometers which can be set to the required azimuthal angle. Both the altitude and azimuth of the telescope axis are checked using sub-systems additional to and independent of those actually involved in the steering control systems. In the case of the azimuth this is achieved using two orthogonal pairs of magnetometers and a digital resolver which measures the azimuthal pointing direction of the telescope to approximately 0.1 °, whilst in the case of the altitude,
551
C.A. Ayre et aL / Line features in background "/-ray spectrum
two precise geared pendulums monitor the direction of pointing to approximately 0.2 ° . 2.2. The spectrometer detectors
The detector at the heart of the spectrometer is a single crystal of hyperpure germanium of volume 86 cm3 in a closed-end coaxial configuration. The crystal, having diameter of 50 mm and length 44 mm is situated at the end of a cold finger 50 cm long attached to a 15 1 dewar containing liquid nitrogen. Under normal atmospheric pressure the liquid nitrogen consumption is 1.6 1/day. For the balloon flight the pressure of the liquid nitrogen is controlled using a relative pressure relief valve set to open at a differential pressure of 0.4 bar. This ensures that the temperature of the nitrogen remains above - 2 1 0 ° C and therefore the nitrogen remains in the liquid state. The Ge(Hp) detector has an efficiency of 23% at 1.33 MeV (relative to a standard 3" NaI(T1) crystal) and an energy resolution of 2.12 keV at 1.33 MeV and 0.985 keV at 122 keV. In order that the NaI(T1) shield fitted closely around the Ge(Hp) detector the preamplifier of the latter was moved from the normal position near to the detector crystal to a point adjacent to the dewar, and therefore - 50 cm from the crystal head. Contrary to expectation and inexplicably, the energy resolution
~-5
and noise of the detector improved as a result of this change. The energy resolution and absolute photopeak efficiency of the Ge(Hp) detector have been measured in the laboratory using coaxially placed radioactive sources and the results are shown in fig. 2. The peak efficiency is defined as the number of events that occur in the photopeak divided by the number of -/-rays incident on the detector. The figure also shows the best possible resolution that could be achieved for the detector using an ideal preamplifier assuming a Fano factor of 0.13 [1]. The solid state detector is surrounded by a collimation crystal of thickness 12.5 cm, A 2, and a rear plug of thickness 14 cm, A3, both crystals having a diameter of 30 cm. The unit A 3 is viewed by three photomultipliers and a necessary requirement is that at least two of the three photomultipliers detect > 50 keV deposited in the unit. With this threshold the unit counts at 5.3 × 10 3 s -1 at an atmospheric depth of 5.4 g cm -2. The collimation unit A 2 has four polarisation measuring crystal segments each of thickness 6 cm contained within it. These segments are in the form of four quadrants 9 cm tall and the upper face of the Ge(Hp) crystal is in the same plane as the upper end of the annulus which the quadrants form. Each polarimeter segment is viewed by a single photomultiplier. The main collimation crystal A2, is viewed by four photomultipliers in an identical manner to those o n m3, and the counting rate of A 2 is typically 16 × 103 s -1 at an atmospheric depth of 5.4 g cm -2, whilst that of each polarimeter segment is 7 × 102 s -1. The energy resolutions (fwhm) of the crystals A 2 and A 3 measured in the spectrometer are given in fig. 3. At 662 keV the resolu-
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552
C.A. Ayre et a L / Line features in background y-ray spectrum
tion (fwhm) of the individual crystals A 2 and m 3 are 12.7% and 10.2% respectively. In the case of the four polarimeter segments it is not possible to distinguish photopeaks in y-ray spectra of laboratory sources, due to the surrounding scattering material. However the resolution of the polarimeter segments is estimated to be - 25%. The third shielding element of the spectrometer, A~, is an annular crystal, also of NaI(TI), of diameter 22 cm and thickness 15 cm. This element is viewed by three photomultipliers and the necessary and sufficient condition for the detection of a gamma-ray is that at least two of the tubes detect > 50 keV deposited in the crystal. At balloon altitudes this segment counts at 4.2 × 10 3 s -1. The purpose of this element A 1 is to limit the aperture of the spectrometer, and the geometric aperture of the spectrometer incorporating A~ is 4.8 ° (fwhm). The active shield in total weighs - 180 kg.
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3. Predicted spectrometer performance
There is a strong possibility that cosmic y-ray production leads to polarised photons. The possibility of incorporating a simple polarimeter in the spectrometer arose when it was realised that an annulus of NaI(TI) could be bored out of the main collimation shield A 2, segmented, optically isolated from the body of A 2 and replaced within it. Monte Carlo calculations have been carried out in which polarised photons of various energies were allowed to strike the front face of the Ge(Hp) detector and to be Compton scattered into the quadrants of NaI(Tl) surrounding the detector. The results of the calculations are given in fig. 4, in which is plotted the ratio of the counts in neighbouring quadrants coincident with a pulse from the Ge(Hp) detector. In the model it is assumed the y-rays are 100% polarized with their electric vector normal to one of the quadrants as shown in the figure inset, this is clearly a favourable situation. The figure also shows the percentage of the v-rays which are incident on the Ge(Hp) detector and which contribute to the polarisation measurement. Clearly the potential of the spectrometer for polarisation studies is dependent upon the incident v-ray flux. Due to the general falling v-ray intensity with increasing energy, and the energy dependence of the percentage of incident photons contributing to a polarisation measurement, such measurements are most accurately made over a restricted range of energy. Fig. 4 shows the result of considering a typical y-ray spectrum, varying as E - 1.5 and demonstrates that there is a major contribution to the count rate around 200 keV, and therefore potentially the most precise polarisation measurements are at this energy. However, from the figure polarisation stud-
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Fig. 4. The ratio of counting rates (P2 + P4)Ge/(P~ + P3) "Ge as a function of energy for a 100% polarised incident beam. The parameter for each point is the percentage of the incident y-rays which contribute to the polarisation measurement. For a typical spectrum Nv( > E)tx E~-l.5, the relative contributions of photons to polarisation measurements are shown as a function of energy.
ies are a possibility over the range 100 keV < Ev < 500 keV. 3.2. Spectrometer response to diffuse and point sources
The acceptance factor of the telescope is defined by 0M
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where Aefr is the effective area of the telescope and 0 M is the maximum angle off the telescope axis which photons reaching the germanium detector can have. 0 M is energy dependent and the value for which the acceptance factor is half of its maximum value, 0rwhm, varies from 4.8 ° at 500 keV to 6.04 ° at 3 MeV, Of,hm being shown in fig. 5 and this figure also shows the acceptance factor. For a diffuse source the photon flux is obtained by dividing the detector counting rate by the acceptance factor. For a point source the response of the telescope is dependent upon the type of observations. For an 'on-source'/'off-source' type of observation, the source flux is obtained by dividing the detector counting rate, after background subtraction, by the effective area. If however the source drifts through the field of view crossing the axis of the telescope, then the effective area
C.A. Ayre et al. / Line features in background 7- ray spectrum
when the spectrometer was launched at 12.35 Universal Time, (07.35 local time). The spectrometer remained at an atmospheric depth of 5.4 + 0.1 mb for - 24 h, and the performance of the instrument is considered here in the light of the data obtained during the 1981 balloon flight.
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is a function of the angle between the telescope axis and the direction of the source. This dependency is shown in fig. 6, in the form of the dependence of the effective area on the angles 0 and ~, (these being defined in the figure inset) normalised to the value 19.63 cm2 at 0 = 0 °"
4. Measured spectrometer performance and observations The spectrometer has been flown on two occasions from NSBF, Palestine, Texas. The first flight of the spectrometer took place on 28th August 1979, the balloon reaching a float altitude of 3.9 mb at 16.30 Universal Time. A second flight occurred on June 6th 1981
4.1. Background y-ray spectrum
Data have been analysed and a detailed study of the 511 keV positron annihilation line shows that the system stability was excellent and that the drift in gain of the detector was essential zero (<< 0.2 keV around 511 keV) during the 24 h period of the experiment. The background y-ray spectrum of the detector is shown in fig. 7, for an exposure of 5.06 × 104 s. The programme Hypermet (Phillips and Marlow [2]) has been used to recognise features in the continuum spectrum, and table 1 lists such features (having a predicted count of > 2.00 only) and their probable origin. Fig. 8 shows the frequency distribution for the significance of the intensity of the feature found by Hypermet for Ev < 2 MeV, and indicates that positive identification of the features is good for N / o > 2.0. Indeed only two features out of the 50 in this region are not satisfactorily explained and both of these have line widths which are much wider than the nearly features. They are therefore considered to be unreliable features as are all having N / o < 1.0. The data of Lederer et al. [3] and Erdtman and Soyka [4] were used in the feature identification. Many of the features correspond to the decay of members of the uranium, thorium and actinium radioactive series. Table 2 lists the member of the uranium series identified and the relative intensities to be expected according to the above references and the intensities found in the present experiments. The tabulated intensities from Lederer et al. are the number of y-rays emitted per 100 decaying nuclei. The experimental data have been normalised to the values by equating the total number of observed y-rays in all the detected lines of the decaying nucleus with the total number expected from the same corresponding lines for 100 decaying radioactive nuclei. No corrections have been made to the data for any absorption of the y-rays in the instrument prior to their being detected by the Ge(Hp) detector. The correlation between the measured and expected intensities is impressive. For example in the case of 214Bi the system identifies the 14 most intense lines from the total spectrum comprising 20 lines, and a similar situation occurred for most of the nuclides of the table. The experimental data have been used to estimate the spectrometer's line sensitivity which is shown in fig. 9, following the treatment of Jacobson et al. [5], corresponding to an exposure time of 2 h (1 h on source one on background) and a significance of 30 in the detected features. Clearly the active nature of the shield consider-
53.32 + 0.16 66.53 + 0.12 92.62 + 0.38 139.64 + 0.08 142.58 + 0.18 158.88 + 0.42 175.06 _+0.39 186.67 + 0.59 198.11 + 0.09 200.94 _+0.34 212.14 + 0.20 238.20_+.+_ 0.30 274.02 + 0.77 303.28 + 0.67 351.61 + 0.57 438.51 + 0.38 472.41 _+ 0.26 498.19 + 0.40 510.29 + 0.16 512.52 _+0.25 535.99 + 0.17 582,89 + 0.15 609.51 + 0.10 643.77 + 0.33 655.87 + 0.04 693.60_.+ 0.25 697.06 + 0.38 708.86 + 0.06 740.08 + 0.18
Measured energy (kev)
351.99 438.0 472.3 498.05 511.0, 510.72 511.0 536.09 583.139 609.312 641.0 654.4 691.3 696.07 709.31 739.48
53.437 66.73 92.80, 93.35 139.68 143.762 159.71 174.9 185.72, 186.0 198.3 202.84 212.19 238.63 272.97
Tabulated energy (keV) 6694 + 1641 4269 + 451 348 4-_ 111 3749 + 488 923 + 171 307 _+ 118 334 + 115 180 + 90 9084 + 774 1585 + 341 117 + 36 156 + 72 109 + 42 153 + 58 134 + 61 122 + 51 665 __+ 99 141 + 41 1172 + 175 774 + 148 40 + 16 56 + 21 310 _+ 49 61 _+ 27 190 + 35 200 _+ 65 187 _+ 61 492 + 115 31 _+ 11
Counts 1.91 2.34 2.15 1.72 1.86 2.10 1.95 2.33 2.08 2.08 1.16 1.62 3.49 3.51 1.96 1.21 2.11 2.35 2.35 2.35 0.89 0.87 1.80 1.68 0.87 2.34 2.34 8.54 1.01
_ 0.27 + 0.16 + 0.76 + 0.22 + 0.28 + 0.96 +_ 0.38 + 0.87 + 0.23 + 0.32 + 0.58 + 0.81 + 0.79 _+ 0.80 + 0.61 + 1.1 + 0.36 _+ 0.43 _+ 0.22 + 0.3 4- 0.18 _+ 0.17 + 0.38 -F 0.70 + 0.09 ± 0.50 + 0.50 + 1.49 ± 0.28
Line width (keV)
Table 1 Features in the spectrum of 50613 s detected by Hypermet in the range 50 keV < Ev < 4000 keV.
82pb214 12Mg23 llNa24m 51SbH5 e + / e -, 81T12°9 e+/e 53113° SlT12°8 83Bi 214 33As72 33ASS2 32Ge72 33AsSem 33ASS2 531130
32Ge 75m ~2U235 32Ge 77m 33As71 92U235, 33As71 32Ge 71m 531127, 54Xe127 531121 82pb212 32Ge66
9°Th234,92U235
32Ge73m, 3 3 A s 7 3 32Ge 73m
Possible nuclides
double escape peak of 1718.07 keV double escape peak of 1731.31 keV 531129(n .y)
fission 32Ge72(n,n'y)
positron annihilation, nat. 92 Th 232 positron annihilation 531129(n,y ) nat.9°Th232 nat. 92U 238 32GeTZ(p,n)
nat. 92U 238 tlNa23(p,n) 11Na23(n,y), 13A127(n,or)
nat.9OTh 232
32Ge74(n,y), 32Ge76(n,2n). nat. 92U 235 32Ge76(n,y ) 32GeT°(p -/) nat. 92U 235 3eGeTO(p, y) 32GeV°(n,T), 32Ge7°(d,p), 32Ge72(p,d) 53II27(n,n'y)' 531127(y,-y'), 531127(p,n)
32Ge72(n,y), 32GeTa(p,d), 32Ge73(p,p')
Origin/comments
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766.33 + 0.19 842.33 _+0.3 910.48 _+0.58 970.65 _+0.85 1032 _+0.15 1120.84 + 0.52 1238.60 _+0.20 1294.51 _+0.33 1375.99 _+0.51 1406.99 _+0.32 1432.46 _+0.21 1460,84 _+0.26 1507.14 + 0.19 1541.29 -+ 0.23 1583 -+ 1.97 1660.83 -+ 0.25 1730.93 _+0.66 1764.62 _+0.12 1779.15 _+0.29 1810.71 _+ 1.08 1847.77 _+0.35 2020.30 _+0.23 2094.18 _+ 3.27 2128.86 _+0.24 2205.26 _+0.57 2223.24 _+0.28 2450.49 _+2.00 2616.57 _+ 0.40 2664.97 _+0.39 3053.76 _+ 1.2 3156.05 _+ 1.5 3442.80 _+ 1.5 3525.38 _+ 1.4
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768.356 843.76 911.22 968.83 1029.9 1120.287 1238.11 1293.538 1377.669 1407.98 1434.08,1434.06 1460.75 1509.228,1509.0 1540.9 1581.8 1661.28 1729.60,1731.31 1764.50 1778.7 63 + 167 + 120 + 119_+ 37 _+ 215 _+ 95 _+ 57 _+ 52 _+ 57 _+ 22 _+ 102 _+ 25 _+ 23 _+ 33_+ 25 + 67 + 256 + 93 -+ 44-+ 26 -+ 15+ 37 _+ 18 _+ 94 _+ 202 _+ 39 _+ 129 _+ 29 _+ 23 + 28 + 26 + 30 -+
31 65 36 32 15 67 18 21 20 20 9 14 10 10 15 8 18 19 15 16 10 5 17 7 17 23 16 16 12 10 12 12 12 0.88 + 0.23 1.57 + 0.73 2.36 + 1.04 5.13 + 1.46 1.01 + 0.23 2.82 ___0.61 2.58 _+0.42 2.41 _+0.69 2.64 _+0.74 2.73 _+ 1.26 1.20 _+ 0.29 3.21 _+ 0.49 1.24 + 0.27 1.25 _+ 0.32 5.08 + 1.3 1.37 _+0.24 5.52 + 0.82 3.05 + 0.13 3.05 + 0.27 5.52 _+ 1.07 1.44 + 0.38 1.48 + 0.27 10.24 + 4.3 1.52 + 0.34 6.29 + 0.64 6.29 + 0.41 7.78 + 4.09 7.03 + 0.49 7.03 + 1.58 7.76 + 1.95 7.98 + 2.08 8.40 + 2.11 8.40 + 1.80 lHl(n,-t) nat. 92U 238 nat.92Th 232
fission
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GeT°(p,n) single escape peak of 2603.8 keV double escape peak of 3149.5 keV nat. 92U 238
nat. 92U 238
nat.92U 238, single escape peak of 2020.0 keV fission double escape peak of 2603.8 keV nat.O2u 238 nat.92U 23s, fission nat. 92U 238 13A127(n,),)
nat. 92U 238 nat. 92U 238
single escape peak of 1540.9 keV nat.92U 238 nat.92U 238
nat.92U 238 13A127(n,n,) nat.9°Th232 nat.9OTh 232
38Bi2X4 33As7O 33As82m 33As82m 83Bi214 1D2 83Bi 214 81T1208
82Bi214 83Bi214 24Cr52 ' 25Mn52m 19K4O 83Bi 2x4, 33As7O 33As82m 33As82m 83Bi214 83Bi214 ' 33ASS2 83Bi214 13AI2S
51Sb116
83Bi214 12Mg27 ' 13A127 89Ac 228 89Ac 228 33As82m 83Bi214 83Bi214
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C.A. Ayre et al. / Line features in background y- ray spectrum
557
Table 2 Lines detected in three of the four radioactive series. The normal±sat±on factor used to compare the measured line intensities with the tabulated values, excludes consideration of the features * (see column 4). Nuclide
Uranium series 214Bi
214pb
226Ra 234Th Thorium series 2oaT1
Actinium series 235 U
Tabulated energy (keV)
609.31 665.45 768.36 806.17 934.06 1120.29 1155.19 1238.11 1280.96 1377.67 1401.50 1407.98 1509.23 1661.28 1729.60 1764.50 1847.42 2118.55 2204.21 2447.81 74.81 77.11 241.92 295.22 351.99 186.18 92.38. 92.80 }
Tabulated intensity (number of ),-rays / 1 0 0 decaying nuclei) 46 1.56 4.88 1.22 3.17 15.00 1.69 5.92 1.47 4.02 1.39 2.48 2.19 1.15 3.05 15.92 2.12 1.21 4.99 1.55 6.3 10.7 7.5 19.2 37.1 3.3 2.57. 3.00 )
Measured energy (keV)
609.51" ± 0.10 Undetected 766.33 ± 0.19 Undetected 934.17 ± 0.20 1120.84+0.52 Undetected 1238.6±0.20 Undetected 1375.99 ± 0.51 Undetected 1406.99±0.32 1507.14 ± 0.19 1660.83 ± 0.25 1730.93 ± 0.66 1764.62±0.12 1847.77±0.35 Undetected 2205.26 ± 0.77 2450.49 ± 2.00 Undetected 77.69 ± 0.73 Undetected 295.92 ± 0.33 351.61 ±0.57 186.67 ± 0.59
19.5 ± 3.1 4.0 ± 2.0 2.8 ± 1.7 13.5± 4.2 6.0+ 1.1 3.3 + 1.3 3.6± 1.6 ± 1.6 ± 4.2 ± 16.1 ± 1.6±
1.3 0.6 0.5 1.1 1.2 0.6
5.9 ± 1.1 2.5 + 1.0 20.1 ± 14.3 15.9 ± 10.1 31.0± 14.1
92.62 ± 0.38
74.87 277.3 510.72 583.14 860.37 2614.47
3.5 6.5 22.5 86.5 12.0 100
Undetected Undetected 510.29.16 582.89 ± 0.15 Undetected 2616.57 ± 0.40
89.96 93.35 105.00 109.14 143.76 163.35 185.72 202.12 205.31
1.5 2.5 1.0 1.5 10.5 4.7 54.0 1.0 4.7
Undetected 92.62+0.38 104.56 + 0.29 Undetected 142.58 + 0.18 162.51 +0.54 186.67___ 0.59* 200.94±0.34* Undetected
a b l y i m p r o v e s the sensitivity o f the i n s t r u m e n t . T h e v a r i a t i o n s o f several of the lines in the b a c k g r o u n d s p e c t r u m h a v e b e e n s t u d i e d as a f u n c t i o n of
Normalised intensity
Counts Count error
6.3 2.0 1.7 3.2 5.3 2.6 2.9 2.5 3.1 3.7 13.5 2.6 5.5 2.5 1.4 1.56 2.20 2.0 3.14
lost in e ÷ / e 56.3 ± 21.1 129.7 ± 16.0 4.3+ 0.9 + 11.7 + 2.2+ 2.2 + 20.1 ± -
1.4 0.4 2.8 1.4 1.2 4.3
2.67 8.06 3.13 1.97 5.39 1.60 2.0 4.6 -
a t m o s p h e r i c d e p t h , a n d the v a r i a t i o n s of the 511 keV, 198 keV, 139 keV a n d 53 keV lines are p r e s e n t e d in fig. 10. T h e sensitivity o f the s p e c t r o m e t e r is n o t as g o o d as
558
C.A. Ayre et al. / Line features in background 3"-ray spectrum I
I
i
100
,,
,
15
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(lJ (::7"
,= LL
. 94 3 1 2 Line intehsity / Error in
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Fig. 8. The frequency distribution of the significance of the line features found by Hypermet. [] Probable origin of feature identified. [] Unidentified feature.
~u 10-1
~E 01
'
'
I
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'
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I
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Fig. 9. The sensitivity of the spectrometer for a 1 h 'on-source' 1 h 'off-source' observation and 3~ significance.
indicated in fig. 9 near to the background line feature. For example at 511 MeV the line sensitivity of the spectrometer is 3.5 x 10 -3 photons cm -2 s - l . The variation of the intensity of the 511 keV line with altitude
,
I , ,, I , 101 102 Atmospheric Depth (g cm"z} ,,
,
103
Fig. 10. The measured variation of the intensities of the 53, 67, 139, 198 and 511 keV lines with atmospheric depth.
differs markedly from that of the other four, this reflecting their different origin. The latter lines are principally due to n - G e interactions and the variation of the intensity is dependent upon the neutron spectrum incident on the detector. The neutron intensity falls with altitude above the Pfotzer maximum and it might be expected that the intensities of the d e p e n d e n t lines fall also. This is seen not to be the case and it is believed that neutrons -are produced in the ant±coincidence shield of mean thickness 65 g c m - 2 which then contribute to the background as though the detector was at a d e p t h of 65 g cm -2. The present results are compared with those of M a h o n e y et al. [6] and Wormack and Overbeck [7] in table 3, the results being normalised according to the areas of the Ge detectors in the various spectrometers. The ant±coincidence shield of the detector of Mahoney et al. had a thickness of 6.5 cm (CsI) and a collimated aperture of - 20 ° (gwhm) compared with the 12.5 cm (NaI) and 5 ° aperture of the present detector. The present results presented in table 3 are consistent with those of Mahoney et al. and Womack and Overbeck, considering the different float depths of the experiments and the different thickness of passive material around the Ge crystals in the three spectrometers.
Table 3 Intensities of some line features from the present experiment at 5.4 g cm 2 compared with measurements of Mahoney et al. [6] at 3 g cm-2 and Womack and Overbeck [7] at 4.7 g cm-2. The intensities * are corrected for the large difference between the crystal length of Womack et al., 1.8 cm, and that of the other groups, - 4 cm. Energy
Feature intensity (10-3 3' c m - 2 s - 1)
(keV)
Mahoney et al.: area 40 cm2, length 4 cm
Present results: area 19.6 cm2, length 4.4 cm
Womack and Overbeck: area 15.3 cm2, length 1.8 cm
53.~ 66.73 139.68 198.30 511.00
13.8 5.6 6.1 12.0 3.1
9.7±2.3 6.2±0.7 5.5±0.7 13.1±1.1 2.9±0.3
10 ±2.3 5.8±1.5 3.3±0.1" 8.0±0.4* 3.3±0.3*
C.A. Ayre et al. / Line features, in background "t- ray spectrum
The relative intensities of the higher energy y-rays of 2a4Bi of table 2 are in good agreement with the tabulated expected values. However the intensity of the 609.32 keV radiation is less than half its expected value. If it is postulated that the origin of the radiation is external to the NaI(TI) crystals which form the spectrometer shield then these shield elements will attenuate the v-rays, and in particular the lower energy more than the higher energy radiation. Considering the y radiation to be attenuated by 15 cm of NaI gives excellent agreement between the intensities of all the observed "y-rays and those expected: Thus the 214Bi originates outside the spectrometer elements. Laboratory experiments with the germanium detector, a spherical lead shield and various ancilliary lead shielding proved that the observed radiations from members of the uranium series do not originate from the materials of which the spectrometer is constructed. The origin of the 214Bi is presumably the air around the detector: at 36 km the attenuation length is 20 km horizontally and 8 km vertically for E v = 1764 keV and assuming the radiation is isotropic then 2~4Bi activity concentrations of the order of 5.5 p C / m 3 must be present in the air at these altitudes. This activity is possibly × 200 greater than any reasonable extrapolation of measurements on 222Rn and 214pb made up to altitudes of 20 km (see Jacobi and Andre [8], and Feeby and Seitz [9]). 2xaBi nuclei in the atmosphere arise naturally as a consequence of the decay of 222Rn, which issues from the ground and is quoted as having an activity of 1-100 p C / m 3 near the ground and perhaps 500 p C / m 3 within buildings [10]. The spectrometer was sealed some 12 h before launch in a thin aluminium vessel of diameter 64 cm. Any radon gas trapped within this volume would then decay and within 3-4 h the concentration of 214Bi nuclei would be sensibly constant for the duration of the balloon flight. If this is the origin of 214Bi then the measured activity corresponds to an activity in air at ground level of 105pC/era 3 which is a factor of × 1000 above the generally accepted values. The intensity of 222Rn issuing from the ground is very dependent upon local conditions and it is not impossible that at Palestine, Texas the concentration of Rn is greatly in excess of that in other parts of the world, and perhaps there is a correlation between helium availability and radon concentration, Further calculations show that the 214Bi is unlikely to be present in the helium in the balloon, as the concentration required would be greatly in excess of that acceptable e.g. - 86/~C/m 3. The origin of the 2a4Bi and other nuclides of the radiactive series has not been unambiguously established, but their presence could have design repercussions on high resolution v-ray spectrometers and suggests that the use of a pressurised vessel to house the electrons or spectrometer itself is possibly undesirable.
559
5. Conclusions The high resolution spectrometer was successfully flown on two occasions and in addition to investigating several astrophysical objects the background spectrum at 5.4 g cm-2 has been studied in some detail. Clearly the greater the knowledge and understanding of the background lines the more credible will be any suggestion of line features from point sources. The "t-rays contributing to the background feature cannot be isotropic and, since the spectrometer shield is more efficient at shielding the main detector for y-rays from a certain direction, as the zenith angle of the spectrometer changes so should the intensities of the background lines. Near a background line the detection of a feature of astrophysical origin will be most difficult and it is considered that the techniques of drift or raster scanning are preferable to the alternative of long periods of source tracking during which time the payload zenith angle would change considerably and the corresponding background line intensity. The aforementioned preferred techniques take frequent averages of the background line intensity and analysis techniques can be devised which capitalise on the frequent recording of the background spectrum. Gamma-rays from Z14Bi and the uranium series in general appear to be present in the high altitude spectrum and the total spectrum from all the members of the series can be estimated based upon the measured concentration of Za4Bi. An attempt to ascertain if there was any altitude dependence of the 214Bi emanations was inconclusive due to the relatively few events recorded during the ascent of the spectrometer and the consequent poor statistics. Nevertheless y-rays from the decay of members of the naturally occurring radioactive series do complicate the interpretation of data obtained using a high resolution spectrometer, and the flight of either a larger instrument or an instrument similar to that discussed here under controlled ascent conditions would either prove or disprove the presence in the atmosphere at very high altitudes of members of the natural radioactive series. The Science and Engineering Research Council is thanked for the provision of a research grant to cover the major part of the work and for providing a research studentship for R.M.M. The staff at the National Scientific Balloon Facility in Palestine, Texas are acknowledged for an excellent launch and payload recovery, and for their help during the pre-lannch assembly.
References [1] R.H. Pehl, Phys. Today 30 (1977) 50. [2] G.W. Phillips and N.W. Marlow, NRL memor, report 3198 (1976).
560
C.A. Ayre et al. / Line features in background "/- ray spectrum
[3] C.M. Lederer and V.S. Shirley, Table of isotopes (Wiley, New York, 1978). [4] G. Erdtman and W. Soyka, The gamma rays of the radionuclides (Verlay Chemie, Weinham, New York, 1979). [5] A.S. Jacobson, R.J. Bishop, G.W. Culp, L. Lung, W.A. Mahoney and J.B. Willett, Nucl. Instr. and Meth. 127 (1975) 115. [6] W.A. Mahoney, J.C. Ling, J.B. Willett and A.S. Jacobson, N A S A tech. memor. 79619 (1978) p. 462.
[7] E.A. Womack and J.W. Overbeck, J. Geophys. Res. 75 (1970) 1811. [8] W. Jacobi and K. Andre, J. Geophys. Res. 68 (1963) 3799. [9] H.W. Freeby and H. Seitz, J. Geophys. Res. 75 (1970) 2885. [10] M. Eisenbud, Environmental radioactivity (Academic Press, New York, 1973) p. 159.
549
L I N E F E A T U R E S IN T H E B A C K G R O U N D y-RAY S P E C T R U M IN T H E R A N G E 0.05-10 MeV AT BALLOON ALTITUDES C.A. A Y R E , P.N. B H A T *, Y.Q. M A **, R . M . M Y E R S an d M . G . T H O M P S O N
Department of Physics, University of Durham, Durham, England Received 4 July 1983
An actively shielded high resolution -/-ray spectrometer has been flown as a balloon payload on two occasions, and most recently in June 1981, to search for -/-ray features in the spectra of astrophysical sources. A comprehensive table of background lines recorded on the June 1981 flight is given and there is evidence for -/-rays from several members of the natural uranium radioactive series; the origin of which is uncertain. The main nuclide detected in the series is 214Bi and there is no evidence that this originates in the spectrometer material therefore an atmospheric origin is suggested.
1. Introduction Over the last decade there have been numerous theoretical predictions of the intensities of `/-ray lines emanating from astrophysical sources. Potential sources are supernovae and novae; neutron stars and black holes, and nuclear y-rays will also arise as a consequence of interactions of particles and cosmic rays with the interstellar medium. Many of the line features will have narrow widths (i.e. few keV at energies of 1 MeV) and with the development of reasonably sized coaxial solid state detectors, characterised by high detection efficiences and line resolution typically 2-3 keV over the energy range of interest, it is reasonable to expect nuclear y-rays will be unambiguously detected shortly. Nuclear -/-rays are mostly in the range 0-10 MeV, although some occur at higher energies, but measurements of such rays can only be made at high altitudes where the attentuation of the -/-rays by the earth's atmosphere is reduced. In this paper measurements are reported on background -/-ray lines found at balloon altitudes. The detector which is described in sect. 2 is an intrinsic germanium crystal surrounded by a NaI(T1) anticoincidence shield. A knowledge of the background -/-ray line features and an understanding of the origin of the features is necessary if data from -/-ray spectrometers are to be interpreted correctly. Also reported are measurements of several nuclides of the naturally occurring radioactive series which have been made with the instrument. The origin of these nuclides has not been * On leave from the Tata Institute of Fundamental Research, Bombay, India. ** On leave from the Institute of High Energy Physics, Beijing, China. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
unequivocally established but it is believed the decaying nuclides are external to the instrument.
2. The spectrometer
2.1. Outline of the instrument The spectrometer is basically a high resolution solid state detector contained within an active shield. The high resolution detector is a hyperpure germanium crystal, Ge(Hp), and collimation and background rejection for this detector are provided by a three-piece NaI(T1) anticoincidence shield. One of the shield elements is further sub-divided so as to constitute a polarimeter. Events which Compton scatter a photon from the Ge(Hp) detector into one of the polarimeter elements and which is detected, are also recorded. In normal operation of the spectrometer, the data from each Ge(Hp) event are recorded along with information relating to any energy coincidentally deposited in one or more of the shield elements. Event selection, i.e. between coincident and non-coincident events, is made subsequently software-wise from the raw data available from the spectrometer. The spectrometer, which has cylindrical symmetry is shown in fig. 1. Covering the aperture of the spectrometer is a 3 mm thick charged particle detector. Events which are coincident with pulses from this detector are not recorded. The pulses from the Ge(Hp) detector, in the range 0-10 V and equivalent to 0-10 MeV, are analysed using an A D C having 214 channels, giving a resolution of 0.65 keV/channel. The low energy threshold of the detector is - 50 keV; hence the spectrometer covers the energy range 0.05-10 MeV. The electronic system is capable of
550
CA. Ayre et aL / Line features in background y- ray spectrum
Charged
Parfic[e Defec for
NoI(TL) Annulus (A1) Viewed By Three Phofomu[fiptiers
Jl
-I Annulus (A2) By Four multipliers
NoI (Tt) Pot ori mete (Pl - P~ ) Each -Viewed By One
Phofomu~ Plug (A3) By Three ~u[fip[iers
10cm Fig. 1. Schemmaticdiagram of the -/-ray spectrometer.
analysing two coincident pulses occurring within the spectrometer, the instrumental resolving time being 1 #s. The pulses chosen for analysis are selected following a specified hierarchy. In addition to the analysis of a pulse from the Ge(Hp) detector, a coincident pulse from one of the shield elements, P1, P2, P3, P4, A2, A3, Aa, is also analysed; the order given being the order of precedence and the elements being as shown in fig. 1. The threshold for the shield pulses is typically 50 keV. However, for those events for which there appears to be no pulse coincident with a Ge(Hp) detector pulse from any shield element, as defined by the discriminators associated with the shield, the summed pulse from all the shield elements is analysed by the second ADC channel. In this way the shield threshold is reduced and the shield sensitivity improved. The counting rates of
the shields and the Ge(Hp) detector are continually monitored. The experimental package is on an alt-azimuthal mounting, which is capable of being pointed with an accuracy of +0.25 ° in altitude and +0.5 ° in azimuth. Altitude control is achieved using two precision orthogonal inclinometers, and azimuthal control using a pair of zero-field seeking magnetometers which can be set to the required azimuthal angle. Both the altitude and azimuth of the telescope axis are checked using sub-systems additional to and independent of those actually involved in the steering control systems. In the case of the azimuth this is achieved using two orthogonal pairs of magnetometers and a digital resolver which measures the azimuthal pointing direction of the telescope to approximately 0.1 °, whilst in the case of the altitude,
551
C.A. Ayre et aL / Line features in background "/-ray spectrum
two precise geared pendulums monitor the direction of pointing to approximately 0.2 ° . 2.2. The spectrometer detectors
The detector at the heart of the spectrometer is a single crystal of hyperpure germanium of volume 86 cm3 in a closed-end coaxial configuration. The crystal, having diameter of 50 mm and length 44 mm is situated at the end of a cold finger 50 cm long attached to a 15 1 dewar containing liquid nitrogen. Under normal atmospheric pressure the liquid nitrogen consumption is 1.6 1/day. For the balloon flight the pressure of the liquid nitrogen is controlled using a relative pressure relief valve set to open at a differential pressure of 0.4 bar. This ensures that the temperature of the nitrogen remains above - 2 1 0 ° C and therefore the nitrogen remains in the liquid state. The Ge(Hp) detector has an efficiency of 23% at 1.33 MeV (relative to a standard 3" NaI(T1) crystal) and an energy resolution of 2.12 keV at 1.33 MeV and 0.985 keV at 122 keV. In order that the NaI(T1) shield fitted closely around the Ge(Hp) detector the preamplifier of the latter was moved from the normal position near to the detector crystal to a point adjacent to the dewar, and therefore - 50 cm from the crystal head. Contrary to expectation and inexplicably, the energy resolution
~-5
and noise of the detector improved as a result of this change. The energy resolution and absolute photopeak efficiency of the Ge(Hp) detector have been measured in the laboratory using coaxially placed radioactive sources and the results are shown in fig. 2. The peak efficiency is defined as the number of events that occur in the photopeak divided by the number of -/-rays incident on the detector. The figure also shows the best possible resolution that could be achieved for the detector using an ideal preamplifier assuming a Fano factor of 0.13 [1]. The solid state detector is surrounded by a collimation crystal of thickness 12.5 cm, A 2, and a rear plug of thickness 14 cm, A3, both crystals having a diameter of 30 cm. The unit A 3 is viewed by three photomultipliers and a necessary requirement is that at least two of the three photomultipliers detect > 50 keV deposited in the unit. With this threshold the unit counts at 5.3 × 10 3 s -1 at an atmospheric depth of 5.4 g cm -2. The collimation unit A 2 has four polarisation measuring crystal segments each of thickness 6 cm contained within it. These segments are in the form of four quadrants 9 cm tall and the upper face of the Ge(Hp) crystal is in the same plane as the upper end of the annulus which the quadrants form. Each polarimeter segment is viewed by a single photomultiplier. The main collimation crystal A2, is viewed by four photomultipliers in an identical manner to those o n m3, and the counting rate of A 2 is typically 16 × 103 s -1 at an atmospheric depth of 5.4 g cm -2, whilst that of each polarimeter segment is 7 × 102 s -1. The energy resolutions (fwhm) of the crystals A 2 and A 3 measured in the spectrometer are given in fig. 3. At 662 keV the resolu-
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Fig. 2. The energy resolution and photopeak efficiency of the spectrometer. The ultimate obtainable resolution for a Fano factor of 0.13 is shown following Pehl [1].
I
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l'.Z~
E~'Z(MeV)-~z Fig. 3. The energy resolution (fwhm) of the shielding elements A 2 and A 3 as a function of incident photon energy.
552
C.A. Ayre et a L / Line features in background y-ray spectrum
tion (fwhm) of the individual crystals A 2 and m 3 are 12.7% and 10.2% respectively. In the case of the four polarimeter segments it is not possible to distinguish photopeaks in y-ray spectra of laboratory sources, due to the surrounding scattering material. However the resolution of the polarimeter segments is estimated to be - 25%. The third shielding element of the spectrometer, A~, is an annular crystal, also of NaI(TI), of diameter 22 cm and thickness 15 cm. This element is viewed by three photomultipliers and the necessary and sufficient condition for the detection of a gamma-ray is that at least two of the tubes detect > 50 keV deposited in the crystal. At balloon altitudes this segment counts at 4.2 × 10 3 s -1. The purpose of this element A 1 is to limit the aperture of the spectrometer, and the geometric aperture of the spectrometer incorporating A~ is 4.8 ° (fwhm). The active shield in total weighs - 180 kg.
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3. Predicted spectrometer performance
There is a strong possibility that cosmic y-ray production leads to polarised photons. The possibility of incorporating a simple polarimeter in the spectrometer arose when it was realised that an annulus of NaI(TI) could be bored out of the main collimation shield A 2, segmented, optically isolated from the body of A 2 and replaced within it. Monte Carlo calculations have been carried out in which polarised photons of various energies were allowed to strike the front face of the Ge(Hp) detector and to be Compton scattered into the quadrants of NaI(Tl) surrounding the detector. The results of the calculations are given in fig. 4, in which is plotted the ratio of the counts in neighbouring quadrants coincident with a pulse from the Ge(Hp) detector. In the model it is assumed the y-rays are 100% polarized with their electric vector normal to one of the quadrants as shown in the figure inset, this is clearly a favourable situation. The figure also shows the percentage of the v-rays which are incident on the Ge(Hp) detector and which contribute to the polarisation measurement. Clearly the potential of the spectrometer for polarisation studies is dependent upon the incident v-ray flux. Due to the general falling v-ray intensity with increasing energy, and the energy dependence of the percentage of incident photons contributing to a polarisation measurement, such measurements are most accurately made over a restricted range of energy. Fig. 4 shows the result of considering a typical y-ray spectrum, varying as E - 1.5 and demonstrates that there is a major contribution to the count rate around 200 keV, and therefore potentially the most precise polarisation measurements are at this energy. However, from the figure polarisation stud-
2
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to
1-0 Phofon Energy (HeY)
10
Fig. 4. The ratio of counting rates (P2 + P4)Ge/(P~ + P3) "Ge as a function of energy for a 100% polarised incident beam. The parameter for each point is the percentage of the incident y-rays which contribute to the polarisation measurement. For a typical spectrum Nv( > E)tx E~-l.5, the relative contributions of photons to polarisation measurements are shown as a function of energy.
ies are a possibility over the range 100 keV < Ev < 500 keV. 3.2. Spectrometer response to diffuse and point sources
The acceptance factor of the telescope is defined by 0M
S$2(cm2sr) = f 0
Aeff2~r sin 0d0,
where Aefr is the effective area of the telescope and 0 M is the maximum angle off the telescope axis which photons reaching the germanium detector can have. 0 M is energy dependent and the value for which the acceptance factor is half of its maximum value, 0rwhm, varies from 4.8 ° at 500 keV to 6.04 ° at 3 MeV, Of,hm being shown in fig. 5 and this figure also shows the acceptance factor. For a diffuse source the photon flux is obtained by dividing the detector counting rate by the acceptance factor. For a point source the response of the telescope is dependent upon the type of observations. For an 'on-source'/'off-source' type of observation, the source flux is obtained by dividing the detector counting rate, after background subtraction, by the effective area. If however the source drifts through the field of view crossing the axis of the telescope, then the effective area
C.A. Ayre et al. / Line features in background 7- ray spectrum
when the spectrometer was launched at 12.35 Universal Time, (07.35 local time). The spectrometer remained at an atmospheric depth of 5.4 + 0.1 mb for - 24 h, and the performance of the instrument is considered here in the light of the data obtained during the 1981 balloon flight.
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is a function of the angle between the telescope axis and the direction of the source. This dependency is shown in fig. 6, in the form of the dependence of the effective area on the angles 0 and ~, (these being defined in the figure inset) normalised to the value 19.63 cm2 at 0 = 0 °"
4. Measured spectrometer performance and observations The spectrometer has been flown on two occasions from NSBF, Palestine, Texas. The first flight of the spectrometer took place on 28th August 1979, the balloon reaching a float altitude of 3.9 mb at 16.30 Universal Time. A second flight occurred on June 6th 1981
4.1. Background y-ray spectrum
Data have been analysed and a detailed study of the 511 keV positron annihilation line shows that the system stability was excellent and that the drift in gain of the detector was essential zero (<< 0.2 keV around 511 keV) during the 24 h period of the experiment. The background y-ray spectrum of the detector is shown in fig. 7, for an exposure of 5.06 × 104 s. The programme Hypermet (Phillips and Marlow [2]) has been used to recognise features in the continuum spectrum, and table 1 lists such features (having a predicted count of > 2.00 only) and their probable origin. Fig. 8 shows the frequency distribution for the significance of the intensity of the feature found by Hypermet for Ev < 2 MeV, and indicates that positive identification of the features is good for N / o > 2.0. Indeed only two features out of the 50 in this region are not satisfactorily explained and both of these have line widths which are much wider than the nearly features. They are therefore considered to be unreliable features as are all having N / o < 1.0. The data of Lederer et al. [3] and Erdtman and Soyka [4] were used in the feature identification. Many of the features correspond to the decay of members of the uranium, thorium and actinium radioactive series. Table 2 lists the member of the uranium series identified and the relative intensities to be expected according to the above references and the intensities found in the present experiments. The tabulated intensities from Lederer et al. are the number of y-rays emitted per 100 decaying nuclei. The experimental data have been normalised to the values by equating the total number of observed y-rays in all the detected lines of the decaying nucleus with the total number expected from the same corresponding lines for 100 decaying radioactive nuclei. No corrections have been made to the data for any absorption of the y-rays in the instrument prior to their being detected by the Ge(Hp) detector. The correlation between the measured and expected intensities is impressive. For example in the case of 214Bi the system identifies the 14 most intense lines from the total spectrum comprising 20 lines, and a similar situation occurred for most of the nuclides of the table. The experimental data have been used to estimate the spectrometer's line sensitivity which is shown in fig. 9, following the treatment of Jacobson et al. [5], corresponding to an exposure time of 2 h (1 h on source one on background) and a significance of 30 in the detected features. Clearly the active nature of the shield consider-
53.32 + 0.16 66.53 + 0.12 92.62 + 0.38 139.64 + 0.08 142.58 + 0.18 158.88 + 0.42 175.06 _+0.39 186.67 + 0.59 198.11 + 0.09 200.94 _+0.34 212.14 + 0.20 238.20_+.+_ 0.30 274.02 + 0.77 303.28 + 0.67 351.61 + 0.57 438.51 + 0.38 472.41 _+ 0.26 498.19 + 0.40 510.29 + 0.16 512.52 _+0.25 535.99 + 0.17 582,89 + 0.15 609.51 + 0.10 643.77 + 0.33 655.87 + 0.04 693.60_.+ 0.25 697.06 + 0.38 708.86 + 0.06 740.08 + 0.18
Measured energy (kev)
351.99 438.0 472.3 498.05 511.0, 510.72 511.0 536.09 583.139 609.312 641.0 654.4 691.3 696.07 709.31 739.48
53.437 66.73 92.80, 93.35 139.68 143.762 159.71 174.9 185.72, 186.0 198.3 202.84 212.19 238.63 272.97
Tabulated energy (keV) 6694 + 1641 4269 + 451 348 4-_ 111 3749 + 488 923 + 171 307 _+ 118 334 + 115 180 + 90 9084 + 774 1585 + 341 117 + 36 156 + 72 109 + 42 153 + 58 134 + 61 122 + 51 665 __+ 99 141 + 41 1172 + 175 774 + 148 40 + 16 56 + 21 310 _+ 49 61 _+ 27 190 + 35 200 _+ 65 187 _+ 61 492 + 115 31 _+ 11
Counts 1.91 2.34 2.15 1.72 1.86 2.10 1.95 2.33 2.08 2.08 1.16 1.62 3.49 3.51 1.96 1.21 2.11 2.35 2.35 2.35 0.89 0.87 1.80 1.68 0.87 2.34 2.34 8.54 1.01
_ 0.27 + 0.16 + 0.76 + 0.22 + 0.28 + 0.96 +_ 0.38 + 0.87 + 0.23 + 0.32 + 0.58 + 0.81 + 0.79 _+ 0.80 + 0.61 + 1.1 + 0.36 _+ 0.43 _+ 0.22 + 0.3 4- 0.18 _+ 0.17 + 0.38 -F 0.70 + 0.09 ± 0.50 + 0.50 + 1.49 ± 0.28
Line width (keV)
Table 1 Features in the spectrum of 50613 s detected by Hypermet in the range 50 keV < Ev < 4000 keV.
82pb214 12Mg23 llNa24m 51SbH5 e + / e -, 81T12°9 e+/e 53113° SlT12°8 83Bi 214 33As72 33ASS2 32Ge72 33AsSem 33ASS2 531130
32Ge 75m ~2U235 32Ge 77m 33As71 92U235, 33As71 32Ge 71m 531127, 54Xe127 531121 82pb212 32Ge66
9°Th234,92U235
32Ge73m, 3 3 A s 7 3 32Ge 73m
Possible nuclides
double escape peak of 1718.07 keV double escape peak of 1731.31 keV 531129(n .y)
fission 32Ge72(n,n'y)
positron annihilation, nat. 92 Th 232 positron annihilation 531129(n,y ) nat.9°Th232 nat. 92U 238 32GeTZ(p,n)
nat. 92U 238 tlNa23(p,n) 11Na23(n,y), 13A127(n,or)
nat.9OTh 232
32Ge74(n,y), 32Ge76(n,2n). nat. 92U 235 32Ge76(n,y ) 32GeT°(p -/) nat. 92U 235 3eGeTO(p, y) 32GeV°(n,T), 32Ge7°(d,p), 32Ge72(p,d) 53II27(n,n'y)' 531127(y,-y'), 531127(p,n)
32Ge72(n,y), 32GeTa(p,d), 32Ge73(p,p')
Origin/comments
2
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3, :x
766.33 + 0.19 842.33 _+0.3 910.48 _+0.58 970.65 _+0.85 1032 _+0.15 1120.84 + 0.52 1238.60 _+0.20 1294.51 _+0.33 1375.99 _+0.51 1406.99 _+0.32 1432.46 _+0.21 1460,84 _+0.26 1507.14 + 0.19 1541.29 -+ 0.23 1583 -+ 1.97 1660.83 -+ 0.25 1730.93 _+0.66 1764.62 _+0.12 1779.15 _+0.29 1810.71 _+ 1.08 1847.77 _+0.35 2020.30 _+0.23 2094.18 _+ 3.27 2128.86 _+0.24 2205.26 _+0.57 2223.24 _+0.28 2450.49 _+2.00 2616.57 _+ 0.40 2664.97 _+0.39 3053.76 _+ 1.2 3156.05 _+ 1.5 3442.80 _+ 1.5 3525.38 _+ 1.4
3149.5
1847.42 2020.0 2092.8 2127.50 2204.21 2223.35 2447.81 2614.47
768.356 843.76 911.22 968.83 1029.9 1120.287 1238.11 1293.538 1377.669 1407.98 1434.08,1434.06 1460.75 1509.228,1509.0 1540.9 1581.8 1661.28 1729.60,1731.31 1764.50 1778.7 63 + 167 + 120 + 119_+ 37 _+ 215 _+ 95 _+ 57 _+ 52 _+ 57 _+ 22 _+ 102 _+ 25 _+ 23 _+ 33_+ 25 + 67 + 256 + 93 -+ 44-+ 26 -+ 15+ 37 _+ 18 _+ 94 _+ 202 _+ 39 _+ 129 _+ 29 _+ 23 + 28 + 26 + 30 -+
31 65 36 32 15 67 18 21 20 20 9 14 10 10 15 8 18 19 15 16 10 5 17 7 17 23 16 16 12 10 12 12 12 0.88 + 0.23 1.57 + 0.73 2.36 + 1.04 5.13 + 1.46 1.01 + 0.23 2.82 ___0.61 2.58 _+0.42 2.41 _+0.69 2.64 _+0.74 2.73 _+ 1.26 1.20 _+ 0.29 3.21 _+ 0.49 1.24 + 0.27 1.25 _+ 0.32 5.08 + 1.3 1.37 _+0.24 5.52 + 0.82 3.05 + 0.13 3.05 + 0.27 5.52 _+ 1.07 1.44 + 0.38 1.48 + 0.27 10.24 + 4.3 1.52 + 0.34 6.29 + 0.64 6.29 + 0.41 7.78 + 4.09 7.03 + 0.49 7.03 + 1.58 7.76 + 1.95 7.98 + 2.08 8.40 + 2.11 8.40 + 1.80 lHl(n,-t) nat. 92U 238 nat.92Th 232
fission
33As82m
GeT°(p,n) single escape peak of 2603.8 keV double escape peak of 3149.5 keV nat. 92U 238
nat. 92U 238
nat.92U 238, single escape peak of 2020.0 keV fission double escape peak of 2603.8 keV nat.O2u 238 nat.92U 23s, fission nat. 92U 238 13A127(n,),)
nat. 92U 238 nat. 92U 238
single escape peak of 1540.9 keV nat.92U 238 nat.92U 238
nat.92U 238 13A127(n,n,) nat.9°Th232 nat.9OTh 232
38Bi2X4 33As7O 33As82m 33As82m 83Bi214 1D2 83Bi 214 81T1208
82Bi214 83Bi214 24Cr52 ' 25Mn52m 19K4O 83Bi 2x4, 33As7O 33As82m 33As82m 83Bi214 83Bi214 ' 33ASS2 83Bi214 13AI2S
51Sb116
83Bi214 12Mg27 ' 13A127 89Ac 228 89Ac 228 33As82m 83Bi214 83Bi214
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C.A. Ayre et al. / Line features in background y- ray spectrum
557
Table 2 Lines detected in three of the four radioactive series. The normal±sat±on factor used to compare the measured line intensities with the tabulated values, excludes consideration of the features * (see column 4). Nuclide
Uranium series 214Bi
214pb
226Ra 234Th Thorium series 2oaT1
Actinium series 235 U
Tabulated energy (keV)
609.31 665.45 768.36 806.17 934.06 1120.29 1155.19 1238.11 1280.96 1377.67 1401.50 1407.98 1509.23 1661.28 1729.60 1764.50 1847.42 2118.55 2204.21 2447.81 74.81 77.11 241.92 295.22 351.99 186.18 92.38. 92.80 }
Tabulated intensity (number of ),-rays / 1 0 0 decaying nuclei) 46 1.56 4.88 1.22 3.17 15.00 1.69 5.92 1.47 4.02 1.39 2.48 2.19 1.15 3.05 15.92 2.12 1.21 4.99 1.55 6.3 10.7 7.5 19.2 37.1 3.3 2.57. 3.00 )
Measured energy (keV)
609.51" ± 0.10 Undetected 766.33 ± 0.19 Undetected 934.17 ± 0.20 1120.84+0.52 Undetected 1238.6±0.20 Undetected 1375.99 ± 0.51 Undetected 1406.99±0.32 1507.14 ± 0.19 1660.83 ± 0.25 1730.93 ± 0.66 1764.62±0.12 1847.77±0.35 Undetected 2205.26 ± 0.77 2450.49 ± 2.00 Undetected 77.69 ± 0.73 Undetected 295.92 ± 0.33 351.61 ±0.57 186.67 ± 0.59
19.5 ± 3.1 4.0 ± 2.0 2.8 ± 1.7 13.5± 4.2 6.0+ 1.1 3.3 + 1.3 3.6± 1.6 ± 1.6 ± 4.2 ± 16.1 ± 1.6±
1.3 0.6 0.5 1.1 1.2 0.6
5.9 ± 1.1 2.5 + 1.0 20.1 ± 14.3 15.9 ± 10.1 31.0± 14.1
92.62 ± 0.38
74.87 277.3 510.72 583.14 860.37 2614.47
3.5 6.5 22.5 86.5 12.0 100
Undetected Undetected 510.29.16 582.89 ± 0.15 Undetected 2616.57 ± 0.40
89.96 93.35 105.00 109.14 143.76 163.35 185.72 202.12 205.31
1.5 2.5 1.0 1.5 10.5 4.7 54.0 1.0 4.7
Undetected 92.62+0.38 104.56 + 0.29 Undetected 142.58 + 0.18 162.51 +0.54 186.67___ 0.59* 200.94±0.34* Undetected
a b l y i m p r o v e s the sensitivity o f the i n s t r u m e n t . T h e v a r i a t i o n s o f several of the lines in the b a c k g r o u n d s p e c t r u m h a v e b e e n s t u d i e d as a f u n c t i o n of
Normalised intensity
Counts Count error
6.3 2.0 1.7 3.2 5.3 2.6 2.9 2.5 3.1 3.7 13.5 2.6 5.5 2.5 1.4 1.56 2.20 2.0 3.14
lost in e ÷ / e 56.3 ± 21.1 129.7 ± 16.0 4.3+ 0.9 + 11.7 + 2.2+ 2.2 + 20.1 ± -
1.4 0.4 2.8 1.4 1.2 4.3
2.67 8.06 3.13 1.97 5.39 1.60 2.0 4.6 -
a t m o s p h e r i c d e p t h , a n d the v a r i a t i o n s of the 511 keV, 198 keV, 139 keV a n d 53 keV lines are p r e s e n t e d in fig. 10. T h e sensitivity o f the s p e c t r o m e t e r is n o t as g o o d as
558
C.A. Ayre et al. / Line features in background 3"-ray spectrum I
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indicated in fig. 9 near to the background line feature. For example at 511 MeV the line sensitivity of the spectrometer is 3.5 x 10 -3 photons cm -2 s - l . The variation of the intensity of the 511 keV line with altitude
,
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Fig. 10. The measured variation of the intensities of the 53, 67, 139, 198 and 511 keV lines with atmospheric depth.
differs markedly from that of the other four, this reflecting their different origin. The latter lines are principally due to n - G e interactions and the variation of the intensity is dependent upon the neutron spectrum incident on the detector. The neutron intensity falls with altitude above the Pfotzer maximum and it might be expected that the intensities of the d e p e n d e n t lines fall also. This is seen not to be the case and it is believed that neutrons -are produced in the ant±coincidence shield of mean thickness 65 g c m - 2 which then contribute to the background as though the detector was at a d e p t h of 65 g cm -2. The present results are compared with those of M a h o n e y et al. [6] and Wormack and Overbeck [7] in table 3, the results being normalised according to the areas of the Ge detectors in the various spectrometers. The ant±coincidence shield of the detector of Mahoney et al. had a thickness of 6.5 cm (CsI) and a collimated aperture of - 20 ° (gwhm) compared with the 12.5 cm (NaI) and 5 ° aperture of the present detector. The present results presented in table 3 are consistent with those of Mahoney et al. and Womack and Overbeck, considering the different float depths of the experiments and the different thickness of passive material around the Ge crystals in the three spectrometers.
Table 3 Intensities of some line features from the present experiment at 5.4 g cm 2 compared with measurements of Mahoney et al. [6] at 3 g cm-2 and Womack and Overbeck [7] at 4.7 g cm-2. The intensities * are corrected for the large difference between the crystal length of Womack et al., 1.8 cm, and that of the other groups, - 4 cm. Energy
Feature intensity (10-3 3' c m - 2 s - 1)
(keV)
Mahoney et al.: area 40 cm2, length 4 cm
Present results: area 19.6 cm2, length 4.4 cm
Womack and Overbeck: area 15.3 cm2, length 1.8 cm
53.~ 66.73 139.68 198.30 511.00
13.8 5.6 6.1 12.0 3.1
9.7±2.3 6.2±0.7 5.5±0.7 13.1±1.1 2.9±0.3
10 ±2.3 5.8±1.5 3.3±0.1" 8.0±0.4* 3.3±0.3*
C.A. Ayre et al. / Line features, in background "t- ray spectrum
The relative intensities of the higher energy y-rays of 2a4Bi of table 2 are in good agreement with the tabulated expected values. However the intensity of the 609.32 keV radiation is less than half its expected value. If it is postulated that the origin of the radiation is external to the NaI(TI) crystals which form the spectrometer shield then these shield elements will attenuate the v-rays, and in particular the lower energy more than the higher energy radiation. Considering the y radiation to be attenuated by 15 cm of NaI gives excellent agreement between the intensities of all the observed "y-rays and those expected: Thus the 214Bi originates outside the spectrometer elements. Laboratory experiments with the germanium detector, a spherical lead shield and various ancilliary lead shielding proved that the observed radiations from members of the uranium series do not originate from the materials of which the spectrometer is constructed. The origin of the 214Bi is presumably the air around the detector: at 36 km the attenuation length is 20 km horizontally and 8 km vertically for E v = 1764 keV and assuming the radiation is isotropic then 2~4Bi activity concentrations of the order of 5.5 p C / m 3 must be present in the air at these altitudes. This activity is possibly × 200 greater than any reasonable extrapolation of measurements on 222Rn and 214pb made up to altitudes of 20 km (see Jacobi and Andre [8], and Feeby and Seitz [9]). 2xaBi nuclei in the atmosphere arise naturally as a consequence of the decay of 222Rn, which issues from the ground and is quoted as having an activity of 1-100 p C / m 3 near the ground and perhaps 500 p C / m 3 within buildings [10]. The spectrometer was sealed some 12 h before launch in a thin aluminium vessel of diameter 64 cm. Any radon gas trapped within this volume would then decay and within 3-4 h the concentration of 214Bi nuclei would be sensibly constant for the duration of the balloon flight. If this is the origin of 214Bi then the measured activity corresponds to an activity in air at ground level of 105pC/era 3 which is a factor of × 1000 above the generally accepted values. The intensity of 222Rn issuing from the ground is very dependent upon local conditions and it is not impossible that at Palestine, Texas the concentration of Rn is greatly in excess of that in other parts of the world, and perhaps there is a correlation between helium availability and radon concentration, Further calculations show that the 214Bi is unlikely to be present in the helium in the balloon, as the concentration required would be greatly in excess of that acceptable e.g. - 86/~C/m 3. The origin of the 2a4Bi and other nuclides of the radiactive series has not been unambiguously established, but their presence could have design repercussions on high resolution v-ray spectrometers and suggests that the use of a pressurised vessel to house the electrons or spectrometer itself is possibly undesirable.
559
5. Conclusions The high resolution spectrometer was successfully flown on two occasions and in addition to investigating several astrophysical objects the background spectrum at 5.4 g cm-2 has been studied in some detail. Clearly the greater the knowledge and understanding of the background lines the more credible will be any suggestion of line features from point sources. The "t-rays contributing to the background feature cannot be isotropic and, since the spectrometer shield is more efficient at shielding the main detector for y-rays from a certain direction, as the zenith angle of the spectrometer changes so should the intensities of the background lines. Near a background line the detection of a feature of astrophysical origin will be most difficult and it is considered that the techniques of drift or raster scanning are preferable to the alternative of long periods of source tracking during which time the payload zenith angle would change considerably and the corresponding background line intensity. The aforementioned preferred techniques take frequent averages of the background line intensity and analysis techniques can be devised which capitalise on the frequent recording of the background spectrum. Gamma-rays from Z14Bi and the uranium series in general appear to be present in the high altitude spectrum and the total spectrum from all the members of the series can be estimated based upon the measured concentration of Za4Bi. An attempt to ascertain if there was any altitude dependence of the 214Bi emanations was inconclusive due to the relatively few events recorded during the ascent of the spectrometer and the consequent poor statistics. Nevertheless y-rays from the decay of members of the naturally occurring radioactive series do complicate the interpretation of data obtained using a high resolution spectrometer, and the flight of either a larger instrument or an instrument similar to that discussed here under controlled ascent conditions would either prove or disprove the presence in the atmosphere at very high altitudes of members of the natural radioactive series. The Science and Engineering Research Council is thanked for the provision of a research grant to cover the major part of the work and for providing a research studentship for R.M.M. The staff at the National Scientific Balloon Facility in Palestine, Texas are acknowledged for an excellent launch and payload recovery, and for their help during the pre-lannch assembly.
References [1] R.H. Pehl, Phys. Today 30 (1977) 50. [2] G.W. Phillips and N.W. Marlow, NRL memor, report 3198 (1976).
560
C.A. Ayre et al. / Line features in background "/- ray spectrum
[3] C.M. Lederer and V.S. Shirley, Table of isotopes (Wiley, New York, 1978). [4] G. Erdtman and W. Soyka, The gamma rays of the radionuclides (Verlay Chemie, Weinham, New York, 1979). [5] A.S. Jacobson, R.J. Bishop, G.W. Culp, L. Lung, W.A. Mahoney and J.B. Willett, Nucl. Instr. and Meth. 127 (1975) 115. [6] W.A. Mahoney, J.C. Ling, J.B. Willett and A.S. Jacobson, N A S A tech. memor. 79619 (1978) p. 462.
[7] E.A. Womack and J.W. Overbeck, J. Geophys. Res. 75 (1970) 1811. [8] W. Jacobi and K. Andre, J. Geophys. Res. 68 (1963) 3799. [9] H.W. Freeby and H. Seitz, J. Geophys. Res. 75 (1970) 2885. [10] M. Eisenbud, Environmental radioactivity (Academic Press, New York, 1973) p. 159.