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Prompt gamma emissions in the reaction 115In(n,γ)116In for neutrons around the 1.45 eV absorption resonance
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 1711–1715
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Prompt gamma emissions in the reaction the 1.45 eV absorption resonance
115
Inðn; gÞ116 In for neutrons around
A. Tartaglione a,b,, J.J. Blostein a,b, R.E. Mayer a a b
´mico Bariloche and Instituto Balseiro, GAEN, Comisio ´n Nacional de Energı´a Ato ´mica, Universidad Nacional de Cuyo, R8402AGP S.C. de Bariloche, Argentina Centro Ato Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), Argentina
a r t i c l e in fo
abstract
Article history: Received 12 June 2008 Received in revised form 3 February 2009 Accepted 18 March 2009
The relative intensities of different gamma emissions produced after the reaction 115 Inðn; gÞ116 In were analyzed for the particular case of incident neutron energies around the 1.45 eV indium absorption resonance. For this purpose, a pulsed neutron source in combination with the time-of-flight method for selecting the incident neutron energy range was employed. For neutrons around the mentioned absorption resonance the prompt gamma spectrum was extended to energies below 273 keV, and the intensities of gamma emissions not reported in the literature for epithermal neutrons were determined. & 2009 Elsevier Ltd. All rights reserved.
PACS: 28.20.Fc 29.30.Kv 24.30.v 29.87.+g 25.40.h 28.20.v Keywords: PGNAA 115 Inðn; gÞ116 In Neutron absorption Nuclear reactions Measured Eg , Ig at resonances
1. Introduction The prompt gamma neutron activation analysis technique (PGNAA) basically consists of spectra analysis of the prompt gamma emissions produced after a neutron absorption. During the last decades PGNAA has been implemented by using neutron fluxes at research reactors (Hanna et al., 1981; Anderson et al., 1981). Other kind of facilities, for example electron linear accelerators (LINACs), and more recently spallation neutron sources have also been employed as pulsed sources for PGNAA selecting the neutron energy by the time-of-flight method (TOF) (Fenstermacher et al., 1957). At the beginning NaI(Tl) scintillators were employed as gamma detectors, later Ge(Li) detectors were used, and more recently the resolution in gamma spectroscopy was further improved through the introduction of high purity germanium (HPGe) detectors (Re´vay et al., 2004). Being the PGNAA technique very useful for Corresponding author at: Centro Ato´mico Bariloche and Instituto Balseiro, GAEN, Comisio´n Nacional de Energı´a Ato´mica, Universidad Nacional de Cuyo, R8402AGP S.C. de Bariloche, Argentina. E-mail address: [email protected] (A. Tartaglione).
0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.03.078
non-destructive elemental analysis its development continues nowadays, and for thermal neutrons very complete data tables have been published listing the prompt gamma emission intensities of all known isotopes (International Atomic Energy Agency, 2007). However, in the epithermal neutron energy range the absorption cross-section of several isotopes presents resonances and therefore the emitted prompt gamma spectra are strongly dependent on the absorbed neutron energy. For this reason, in the epithermal neutron energy range the tables containing the intensities of gamma lines are incomplete. Concerning the particular case of the 115 Inðn; gÞ116 In reaction, initial works employed reactor thermal neutrons and NaI(Tl) scintillators (Hamermesh and Hummel, 1952; Draper, 1958). This nuclear reaction was also studied for neutrons with energies around the 1.45, 3.85, and 9.07 eV resonances employing an electron LINAC as pulsed neutron source and NaI(Tl) scintillators (Draper et al., 1958). In the latter the gamma spectra were analyzed for energies up to 280 keV. In Wharer et al. (1970) the gamma resolution was improved employing a Ge(Li) detector, observing gamma emissions from 273 to 6400 keV. The high energy gamma range of the mentioned reaction (for neutrons
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A. Tartaglione et al. / Applied Radiation and Isotopes 67 (2009) 1711–1715
Fig. 1. Experimental setup scheme. An indium foil sample is placed in the pulsed neutron beam. Near to the sample position (out of the neutron beam) a Ge(Li) detector is employed to detect gamma emissions. Lead and iron shielding was used around the detector in order to reduce prompt and activation gamma from the surroundings.
energies in the thermal range and around indium resonances) was analyzed in Lone et al. (1970) and Corvi and Stefanon (1974). A complete data table from thermal neutron absorptions can be found in the work of Blachot (2001) where the low energy prompt gamma data from Rabenstein et al. (1972) and Alexeev et al. (1976) were compiled. The mentioned works have been of great importance to determine nuclear levels and associated properties like spins and parities. The neutron absorption cross-section of the mentioned reaction has been recently studied in a wide range of neutron energies (Yoon et al., 2002) without information reported about the relative intensities of different prompt gamma emissions. Tables for the capture of neutrons with energies around 1.45 eV are not recent and the minimum gamma energy reported is 273 keV (Wharer et al., 1970). In this work we study the nuclear reaction 115 Inðn; gÞ116 In for neutrons with energies around the 1.45 eV absorption resonance. Employing an electron LINAC as pulsed neutron source and a Ge(Li) semiconductor as gamma detector we have extended the gamma energy range below 273 keV and the relative intensities of the observed prompt gamma emissions were determined. The intensities obtained in this work are different from those known for incident neutrons restricted to the thermal energy range. In this context, more complete tables of prompt gamma intensities for different neutron energies would supply useful information to extend the use of PGNAA to the epithermal neutron energy range, and also it would provide data relevant to nuclear physics.
2. Experimental setup The experiments were performed at the Bariloche electron linear accelerator facility (Argentina). It operated at 100 Hz frequency (with electron pulse widths of 1 ms), and a 25 mA mean electron current. When the 25 MeV electron pulse impinges on a lead target the bremsstrahlung photons produce fast neutrons by photonuclear effect. The neutrons were moderated in a 40 mm thick polyethylene slab, and the beam was collimated by means of a conical convergent setup, reducing its diameter from 8 in at the moderator position to 2 in diameter at the end of the extraction tube, through 1.5 m thick concrete shielding and a 12 cm added wall composed of a solid mixture of boric acid dissolved in
Fig. 2. Schematic of the data acquisition setup, composed by two independent detection lines of detection: Ge(Li) gamma detector and gamma flash. The first line is the main signal of the experiment, the second one is used as cero time signal (for time gate generation, as well as for TOF acquisitions). A TSCA is used for pulse height discrimination of Ge(Li) pulses. Only pulses not discriminated by the TSCA and produced during the selected time gate are acquired in the PHA and TOF spectra.
paraffin. A cadmium sheet 1.12 mm thick can be placed at the end of the extraction tube to let only epithermal neutrons emerge from the beam. By removing this cadmium sheet also neutrons in the thermal energy range can reach the sample. The sample consisted of a square foil of natural isotopic composition indium, 99.99% purity, 10 cm side and 0.95 mm thick, placed in the neutron beam, 726 cm from the neutron source. The gamma emissions were detected with a 30 cm3 Canberra coaxial Ge(Li) detector (model 7229) placed 14.3 cm below the sample position, out of the neutron beam. A 10 cm lead shadow placed on an iron table provided reduction of prompt and activation gammas from the surroundings. Fig. 1 shows a scheme of the experimental setup. Time-of-flight as well as pulse height analysis (PHA) spectra were acquired with this experimental setup employing the electronics schematized in Fig. 2. The TOF spectra were recorded in 4096 channels, 2 ms width each.
3. Experimental procedure and results An essential part of this work is the time gate selection that allows to choose the incident neutron energy range. At the
ARTICLE IN PRESS
2
272.1
+
511 β
472.9
415.5 (d) 433.7
375.5 384.6 (m)
+
433.7
384.8
335.7
511 β
234.3 (m)
284.9 297.7 320.4 (m) 334.7 283.1 297.6
Time Gate B
470.6
7
234.6
125.5 139.6
14
Time Gate A
273.0
60.3 85.2 95.5
1.45 eV
4
1
0 x10-5 21
c (Eγ) (a.u.)
c(t) (a.u.)
3.85 eV
2
186.0
Thermal
9.07 eV
3
173.1
0.01
202.4
x10-3
0.1
c (Eγ) (a.u.)
6
10
84.8 (m) 94.5 (m)
En (eV) 10
1713
125.6 139.9 (d) 161.8 (d) 173.2 (m) 186.0 201.8
A. Tartaglione et al. / Applied Radiation and Isotopes 67 (2009) 1711–1715
0 50 100 150 200 250 300 350 400 450 500 550 Eγ (keV) Gate A
140
1000
8000
Time-of-flight (μs) Fig. 3. Neutron TOF spectrum obtained using gamma pulses as stop signal for the time encoder, during a wide time gate. The lower indium resonances are distinguished as well as a Maxwellian component in the neutron thermal energy range (higher time-of-flight). The indicated time gates A and B, were selected in order to acquire PHA spectra within energy ranges correspondent with those of TOF intervals.
beginning, a TOF spectrum was acquired during a wide time gate after the accelerator pulse. In Fig. 3 the nuclear resonances of indium at 9.07, 3.85, and 1.45 eV are observed. For long times a nearly Maxwellian shape is observed, associated with kinetic energies within the approximate equilibrium distribution of neutron energies in the moderator at room temperature . After this identification of different neutron energy ranges by the TOF method, two time gates were selected for PHA analysis of the 115 Inðn; gÞ116 In reaction gamma spectrum. On one hand a gate from 700 to 6980 ms (gate A) allowed to acquire prompt gamma spectra of reactions produced by thermal neutrons (0.0056–0.57 eV). On the other hand, a gate from 346 to 562 ms (gate B) allowed to analyze the reaction for neutrons with energies around the 1.45 eV absorption resonance (0.9–2.43 eV), (see Fig. 3). For measurements employing gate B the cadmium sheet was placed between the moderator and the indium sample removing the thermal component of the neutron beam. The background contribution was determined by periodically removing the sample from the neutron beam and subtracted from the sample spectra. An independent neutron detector placed near the neutron source was employed as a monitor and used to normalize the spectra taking into account fluctuations in the neutron production. All the PHA spectra were corrected considering the energy dependent efficiency of the detector. The gamma ray detection system was calibrated in energy using 133 Ba, 137 Cs, and 60 Co gamma sources. The energy of these calibration sources were obtained from Firestone and Shirley (1996). This calibration was performed with the LINAC in normal operation, removing the sample from the beam. The behavior of the system was extremely linear from 50 to 1500 keV, being the main range of interest from 50 to 500 keV. Fig. 4 shows the obtained PHA spectra for gates A and B. The intensities of prompt gamma emissions were obtained from the peak areas of the PHA spectra shown in Fig. 4. The Compton heel counts underlying the peaks were subtracted by the area fitting method. All the peak intensities of this work are reported as relative to the intensity of the emission of indium at 273 keV. This line was selected as reference because this is the most intense indium emission for thermal neutrons as well as for neutrons with energies around 1.45 eV. In Fig. 4, the lines indicated with (d) correspond to gamma emissions produced
Fig. 4. Upper frame: PHA spectrum of prompt gamma emissions measured during a time gate that spans the absorption of thermal neutrons (gate A). The lines indicated with (d) correspond to b decay emissions. The lines indicated with (m) correspond to unresolved multiplets reported in the literature. Lower frame: PHA spectrum of prompt gamma emissions during time gate B, which takes into account those gammas generated by the absorption of neutrons with energies around 1.45 eV.
120 Relative Intensity (%)
Gate B
0
This work Rabenstein (1972) Alexeev (1976)
100 80 60 40 20 0 50
100 150 200 250 300 350 400 450 500 Eγ (keV)
Fig. 5. Thermal neutron absorption prompt gamma. This figure compares obtained relative intensities with other published data (Rabenstein et al., 1972; Alexeev et al., 1976). The good agreement observed between the obtained results and tabulated values partially validates the implemented spectrometry technique as well as the method employed for peak area determinations.
after the b decay of the 116 In nucleus (International Atomic Energy Agency, 2007). The lines indicated with (m) correspond to unresolved multiplets reported in the literature for thermal neutrons (International Atomic Energy Agency, 2007). For gate B, it is worth to note that the decay emissions have a relatively low intensity and therefore are not observed, mainly because the cadmium sheet strongly reduced the neutron absorption rate, and also because of different time amplitude of this gate relative to that of gate A. For this reason the peaks observed at 139.9, 161.8, and 415.5 keV for gate A disappeared for gate B. In Fig. 5, peak intensities due to thermal neutron absorptions in indium are compared with tabulated values (Rabenstein et al., 1972; Alexeev et al., 1976). These tables were created employing data from higher resolution gamma detectors. To compare our results, we condensed the cited data by adding the intensities of those peaks not resolved by our spectrometer. Gamma attenuation corrections in the sample were applied (Saloman et al., 1988) taking also into account the attenuation of the neutron beam at different energies. The 95.4 keV peak intensity was corrected by attenuation at the mentioned gamma energy. We also performed an extra calculation of the attenuation
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Table 1
Relative Intensity (%)
120
This work Wahrer (1970)
Intensities of prompt gamma emissions of the reaction 115 Inðn; gÞ116 In for thermal and around 1.45 eV neutron energies.
100 Thermal
80 60
0 50
100 150 200 250 300 350 400 450 500 Eγ (keV)
Fig. 6. Prompt gamma from 1.45 eV absorbed neutrons in indium. Calculated peak relative intensities are in good agreement with the previous work of Wharer et al. (1970). Minimum energy gamma peak reported by cited reference is 273 keV, nevertheless present work shows peak intensities from 60 keV.
coefficient taking into account that this peak is an unresolved multiplet (considering the energies and relative intensities reported by Alexeev et al. (1976). The absorption coefficient thus obtained is about 1% greater than the obtained considering only one emission at 95.4 keV. Being this difference lower than the experimental uncertainty ð4%Þ, we show the results considering the attenuation correction for only one single emission at 95.4 keV. The central result of this work is an extension of the prompt gamma emission spectrum below 273 keV due to absorption of neutrons with energies around 1.45 eV (time gate B). In Fig. 6 we show the obtained peak intensities; these results are compared with the previous results from other authors (Wharer et al., 1970). The same correction of gamma and neutron attenuation explained before was applied to these data. The relative obtained intensities, for thermal and 1.45 eV neutrons are given in Table 1. Those data were converted to absolute values in the scale of photons per 1000 neutron captures. For the thermal neutron energy range (gate A) we employed as reference the absolute peak area of the 273 keV gamma-ray reported in Rabenstein et al. (1972) (204 10 photons per 1000 neutron captures). The lines indicated with (m) in Table 1 correspond to emissions reported as multiplets in the references for thermal neutrons, not resolved in this work. In these cases we report the total intensity observed for the unresolved multiplets. For neutrons around the 1.45 eV resonance (gate B) we calculated the absorption rate in terms of the absorption rate in gate A. For such purpose, we measured the incident neutron spectrum shape employing the TOF technique with a 3 He proportional counter of known efficiency placed at the sample position. The filling gas pressure of this detector was verified by neutron transmission experiments (Kropff et al., 1976) employing the mentioned detector as sample. The incident spectrum shape was corrected by dead time effects (Cuello et al., 1993), as well as by emission time in the neutron source. The flight path and electronic delay were calibrated with different resonant filters placed in the incident beam. The background contribution was carefully determined and subtracted from the spectra placing the neutron detector in different positions out of the incident beam. The background contribution was also verified by means of a polyethylene beam stop covered with cadmium placed at the end of the neutron extraction tube. The incident spectrum shape thus obtained was successfully verified by means of Monte Carlo simulations starting from the 25 MeV electron pulses and
Rel. Ig
Abs. Ig a
Rel. Ig
Abs. Ig
–
– 0.770(34)
– 157(10)
60.3(15) 85.2(15)
0.726(53) 0.955(34)
110(16) 145(20)
0.749(31)
152(9)
95.5(11)
0.924(25)
140(19)
139:9ð19ÞðdÞ
0.093(29) –
19(6) –
125.5(14) 139.6(17)
0.170(18) 0.166(11)
25(4) 25(3)
161:8ð14ÞðdÞ
–
–
–
–
–
173:2ð26ÞðmÞ 186.0(15) 201.8(15)
0.216(26)
44(5)
173.1(26)
0.322(49)
49(10)
0.704(27) 0.067(27) 0.128(28)
143(8) 13(5) 26(6)
186.0(15) 202.4(11) 234.6(12)
0.838(23) 0.155(54) 0.187(24)
127(17) 23(8) 28(5)
1.000(29) 0.121(30) 0.374(31) 0.103(32)
204(11) 24(6) 76(7) 21(6)
273.0(17) 283.1(23) 297.6(28) –
1.00(3) 0.111(20) 0.385(37) –
152(21) 16(3) 58(9) –
384:6ð21ÞðmÞ
0.340(33) 0.114(34) 0.308(36)
69(7) 23(7) 62(8)
335.7(27) – 384.8(24)
0.446(35) – 0.337(19)
68(10) – 51(7)
415:5ð20ÞðdÞ 433.7(18) 472.9(25)
–
–
–
–
–
0.223(39) 0.1940(47)
45(8) 39(2)
433.9(21) 470.6(42)
0.232(44) 0.304(32)
35(8) 46(7)
95:4ð12ÞðmÞ 125.6(16)
20
Eg (keV)
Eg (keV)
84:8ð10ÞðmÞ
40
1.45 eV res. a
234:3ð16ÞðmÞ 272.1(18) 284.9(13) 297.7(21) 320:4ð34ÞðmÞ 334.7(30) 375.5(30)
The obtained intensities are presented relative to the 273 keV line as well as in absolute units. b decay emissions are indicated with (d). The lines indicated with (m) correspond to emissions reported as multiplets in the references for thermal neutrons, not resolved in this work. In these cases we report the total intensity observed for the unresolved multiplets. a Intensity per 1000 neutron captures employing as reference the peak area of the 273 keV g-ray reported in Rabenstein et al. (1972).
carefully considering the lead target and the polyethylene moderator characteristics (Granada et al., 1987). Employing the latter result, the 115 In absorption cross-section of reference ENDF (2005), and the sample effective thickness formerly mentioned we calculated the relative absorption rates of the 115 Inðn; gÞ116 In reaction for gates A and B. The mentioned ratio between absorption rates is NCaptures B =N Captures A ¼ 0:092 0:011. The uncertainty of this ratio takes into account the uncertainties of dead time and background corrections, the employed detector efficiency, the sample thickness, and the neutron energy calibration used to determine the limits of gates A and B.
4. Analysis of results and conclusions The validation of the spectroscopic technique employed in this work and the data treatment is brought about by the favorable comparison of the results presented in Fig. 5 with previously published works for neutrons in the thermal energy range and, in Fig. 6, by the agreement with the results of Wharer et al. (1970) for gamma energies greater than 273 keV. For absorbed neutrons around the 1.45 eV, nine prompt gamma emissions were observed below 273 keV and their relative intensities were determined (see Fig. 6 and Table 1). Three of these peaks (60.3, 95.5, and 186 keV) were reported many years ago by Draper et al. (1958) using a lower resolution gamma detector, while the other six gamma lines (at 85.2, 125.5, 139.6, 173.1, 202.4, and 234.6 keV) were not found in the reviewed literature for epithermal neutrons. Differences between gamma emissions arising from the absorption of thermal neutrons and from resonant epithermal neutrons are evident in Table 1.
ARTICLE IN PRESS A. Tartaglione et al. / Applied Radiation and Isotopes 67 (2009) 1711–1715
It is worth to note that all gamma energies we have found in this work for neutrons around 1.45 eV are also present with different relative intensities in prompt gamma tables for absorbed thermal neutrons. The relevant information presented in this work concerning indium will be useful in order to extend the use of PGNAA technique in a wide incident neutron energy range. The results presented in this work confirm the convenience of employing pulsed neutron sources in combination with the TOF method for nuclear physics studies.
Acknowledgments We thank ANPCyT, CONICET, CNEA, and UNCuyo for financial support in carrying out this work. Also we would like to thank L. Capararo for technical support and, to M. Schneebeli and P. D’Avanzo for LINAC general maintenance and operation. The authors wish to mention this work was carried out in the frame of IAEA Research Agreement 13726/R0. References Alexeev, V.L., Emelianov, B.A., Kaminker, D.M., Khazov, Yu.L., Kondurov, I.A., Loginov, Yu.E., Rumiantsev, V.L., Sakharov, S.L., Smirnov, A.I., 1976. The properties of 116 In excited states. Nucl. Phys. A 262, 19–51. Anderson, D.L., Failey, M.P., Zoller, W.H., Walters, W.B., Gordon, G.E., Lindstrom, R.M., 1981. Facility for non-destructive analysis for major and trace elements using neutron capture gamma-ray spectrometry. J. Radioanal. Chem. 63, 97. Blachot, J., 2001. Nuclear data sheets for A ¼ 116 . Nucl. Data Sheets 92, 455–622. Corvi, F., Stefanon, M., 1974. Gamma rays from resonance neutron capture in 115 In. Nucl. Phys. A 233, 185–216.
1715
Cuello, G., Prado, P., Dawidowski, J., 1993. Dead-time effects in time-of-flight measurements. Nucl. Instrum. Methods Phys. Res. A 325, 309–313. Draper, J., 1958. 0–600 keV gamma-ray spectra from thermal neutron capture in the region A ¼ 104 to 198. Phys. Rev. 114, 268–278. Draper, J.E., Fenstermacher, C.A., Schultz, H.L., 1958. Variations in spectra of resonance neutron capture gamma rays in indium. Phys. Rev. 111, 906–912. ENDF/B-VI, 2005. data file for 115 In (Mat ¼ 4931). Firestone, R., Shirley, V. (Eds.), 1996. Table of Isotopes, eighth ed. WileyInterscience, New York. Fenstermacher, C.A., Bennet, R.G., Walters, A.E., Bockelman, C.K., Schultz, H.L., 1957. Absolute intensities of resonance neutron capture gamma rays from Cd, Te, and Sm. Phys. Rev. 107, 1650–1654. Granada, J.R., Dawidowski, J., Mayer, R.E., Gillette, V.H., 1987. Thermal neutron cross section and transport properties of polyethylene. Nucl. Instrum. Methods Phys. Res. A 261, 573–578. Hamermesh, B., Hummel, V., 1952. Neutron capture g-ray spectra from the elements Z ¼ 17–30 and Z ¼ 45–57. Phys. Rev. 88, 916–919. Hanna, A.G., Brugger, R.M., Glascock, M.D., 1981. The prompt gamma neutron activation analysis facility at MURR. Nucl. Instrum. Methods 188, 619. International Atomic Energy Agency. Database of prompt gamma rays from slow neutron capture for elemental analysis, Viena, 2007. Kropff, F., Granada, J.R., Remez, L.A., Vasile, A., 1976. Total cross-section of chromium at low neutron energies. Ann. Nucl. Energy 3, 55–59. Lone, M.A., Earle, E.D., Bartholomew, G.A., 1970. The 115 Inðn; gÞ116 In reaction. Nucl. Phys. A 156, 113–132. Re´vay, Zs., Belgya, T., Kasztovszky, Zs., Weil, J.L., Molna´r, G.L., 2004. Cold neutron PGAA facility at Budapest. Nucl. Instrum. Methods Phys. Res. B 213, 385–388. Rabenstein, D., Harrach, D., Vonach, H., Dussel, G.G., Perazzo, R.P.I., 1972. 116 In level scheme and p–n configurations. Nucl. Phys. A 197, 129–162. Saloman, E.B., Hubbell, J.H., Scofield, J.H., 1988. X-ray attenuation cross sections for energies 100 eV to 100 keV and elements Z ¼ 1 to Z ¼ 92. At. Data Nucl. Data Tables 38, 1–196. Wharer, R.E., King, J.D., Kukoc, H., Pan, P.J., Taylor, H.W., 1970. A study of the states of 116 In through radiative resonance neutron capture in 115 In. Nucl. Phys. A 154, 467–481. Yoon, J., Ro, T., Lee, S., Yamamoto, S., Kobayashi, K., 2002. Measurement of neutron capture cross-section of indium in the energy region from 0.003 eV to 30 keV. Ann. Nucl. Energy 29, 1157–1169.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Prompt gamma emissions in the reaction the 1.45 eV absorption resonance
115
Inðn; gÞ116 In for neutrons around
A. Tartaglione a,b,, J.J. Blostein a,b, R.E. Mayer a a b
´mico Bariloche and Instituto Balseiro, GAEN, Comisio ´n Nacional de Energı´a Ato ´mica, Universidad Nacional de Cuyo, R8402AGP S.C. de Bariloche, Argentina Centro Ato Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), Argentina
a r t i c l e in fo
abstract
Article history: Received 12 June 2008 Received in revised form 3 February 2009 Accepted 18 March 2009
The relative intensities of different gamma emissions produced after the reaction 115 Inðn; gÞ116 In were analyzed for the particular case of incident neutron energies around the 1.45 eV indium absorption resonance. For this purpose, a pulsed neutron source in combination with the time-of-flight method for selecting the incident neutron energy range was employed. For neutrons around the mentioned absorption resonance the prompt gamma spectrum was extended to energies below 273 keV, and the intensities of gamma emissions not reported in the literature for epithermal neutrons were determined. & 2009 Elsevier Ltd. All rights reserved.
PACS: 28.20.Fc 29.30.Kv 24.30.v 29.87.+g 25.40.h 28.20.v Keywords: PGNAA 115 Inðn; gÞ116 In Neutron absorption Nuclear reactions Measured Eg , Ig at resonances
1. Introduction The prompt gamma neutron activation analysis technique (PGNAA) basically consists of spectra analysis of the prompt gamma emissions produced after a neutron absorption. During the last decades PGNAA has been implemented by using neutron fluxes at research reactors (Hanna et al., 1981; Anderson et al., 1981). Other kind of facilities, for example electron linear accelerators (LINACs), and more recently spallation neutron sources have also been employed as pulsed sources for PGNAA selecting the neutron energy by the time-of-flight method (TOF) (Fenstermacher et al., 1957). At the beginning NaI(Tl) scintillators were employed as gamma detectors, later Ge(Li) detectors were used, and more recently the resolution in gamma spectroscopy was further improved through the introduction of high purity germanium (HPGe) detectors (Re´vay et al., 2004). Being the PGNAA technique very useful for Corresponding author at: Centro Ato´mico Bariloche and Instituto Balseiro, GAEN, Comisio´n Nacional de Energı´a Ato´mica, Universidad Nacional de Cuyo, R8402AGP S.C. de Bariloche, Argentina. E-mail address: [email protected] (A. Tartaglione).
0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.03.078
non-destructive elemental analysis its development continues nowadays, and for thermal neutrons very complete data tables have been published listing the prompt gamma emission intensities of all known isotopes (International Atomic Energy Agency, 2007). However, in the epithermal neutron energy range the absorption cross-section of several isotopes presents resonances and therefore the emitted prompt gamma spectra are strongly dependent on the absorbed neutron energy. For this reason, in the epithermal neutron energy range the tables containing the intensities of gamma lines are incomplete. Concerning the particular case of the 115 Inðn; gÞ116 In reaction, initial works employed reactor thermal neutrons and NaI(Tl) scintillators (Hamermesh and Hummel, 1952; Draper, 1958). This nuclear reaction was also studied for neutrons with energies around the 1.45, 3.85, and 9.07 eV resonances employing an electron LINAC as pulsed neutron source and NaI(Tl) scintillators (Draper et al., 1958). In the latter the gamma spectra were analyzed for energies up to 280 keV. In Wharer et al. (1970) the gamma resolution was improved employing a Ge(Li) detector, observing gamma emissions from 273 to 6400 keV. The high energy gamma range of the mentioned reaction (for neutrons
ARTICLE IN PRESS 1712
A. Tartaglione et al. / Applied Radiation and Isotopes 67 (2009) 1711–1715
Fig. 1. Experimental setup scheme. An indium foil sample is placed in the pulsed neutron beam. Near to the sample position (out of the neutron beam) a Ge(Li) detector is employed to detect gamma emissions. Lead and iron shielding was used around the detector in order to reduce prompt and activation gamma from the surroundings.
energies in the thermal range and around indium resonances) was analyzed in Lone et al. (1970) and Corvi and Stefanon (1974). A complete data table from thermal neutron absorptions can be found in the work of Blachot (2001) where the low energy prompt gamma data from Rabenstein et al. (1972) and Alexeev et al. (1976) were compiled. The mentioned works have been of great importance to determine nuclear levels and associated properties like spins and parities. The neutron absorption cross-section of the mentioned reaction has been recently studied in a wide range of neutron energies (Yoon et al., 2002) without information reported about the relative intensities of different prompt gamma emissions. Tables for the capture of neutrons with energies around 1.45 eV are not recent and the minimum gamma energy reported is 273 keV (Wharer et al., 1970). In this work we study the nuclear reaction 115 Inðn; gÞ116 In for neutrons with energies around the 1.45 eV absorption resonance. Employing an electron LINAC as pulsed neutron source and a Ge(Li) semiconductor as gamma detector we have extended the gamma energy range below 273 keV and the relative intensities of the observed prompt gamma emissions were determined. The intensities obtained in this work are different from those known for incident neutrons restricted to the thermal energy range. In this context, more complete tables of prompt gamma intensities for different neutron energies would supply useful information to extend the use of PGNAA to the epithermal neutron energy range, and also it would provide data relevant to nuclear physics.
2. Experimental setup The experiments were performed at the Bariloche electron linear accelerator facility (Argentina). It operated at 100 Hz frequency (with electron pulse widths of 1 ms), and a 25 mA mean electron current. When the 25 MeV electron pulse impinges on a lead target the bremsstrahlung photons produce fast neutrons by photonuclear effect. The neutrons were moderated in a 40 mm thick polyethylene slab, and the beam was collimated by means of a conical convergent setup, reducing its diameter from 8 in at the moderator position to 2 in diameter at the end of the extraction tube, through 1.5 m thick concrete shielding and a 12 cm added wall composed of a solid mixture of boric acid dissolved in
Fig. 2. Schematic of the data acquisition setup, composed by two independent detection lines of detection: Ge(Li) gamma detector and gamma flash. The first line is the main signal of the experiment, the second one is used as cero time signal (for time gate generation, as well as for TOF acquisitions). A TSCA is used for pulse height discrimination of Ge(Li) pulses. Only pulses not discriminated by the TSCA and produced during the selected time gate are acquired in the PHA and TOF spectra.
paraffin. A cadmium sheet 1.12 mm thick can be placed at the end of the extraction tube to let only epithermal neutrons emerge from the beam. By removing this cadmium sheet also neutrons in the thermal energy range can reach the sample. The sample consisted of a square foil of natural isotopic composition indium, 99.99% purity, 10 cm side and 0.95 mm thick, placed in the neutron beam, 726 cm from the neutron source. The gamma emissions were detected with a 30 cm3 Canberra coaxial Ge(Li) detector (model 7229) placed 14.3 cm below the sample position, out of the neutron beam. A 10 cm lead shadow placed on an iron table provided reduction of prompt and activation gammas from the surroundings. Fig. 1 shows a scheme of the experimental setup. Time-of-flight as well as pulse height analysis (PHA) spectra were acquired with this experimental setup employing the electronics schematized in Fig. 2. The TOF spectra were recorded in 4096 channels, 2 ms width each.
3. Experimental procedure and results An essential part of this work is the time gate selection that allows to choose the incident neutron energy range. At the
ARTICLE IN PRESS
2
272.1
+
511 β
472.9
415.5 (d) 433.7
375.5 384.6 (m)
+
433.7
384.8
335.7
511 β
234.3 (m)
284.9 297.7 320.4 (m) 334.7 283.1 297.6
Time Gate B
470.6
7
234.6
125.5 139.6
14
Time Gate A
273.0
60.3 85.2 95.5
1.45 eV
4
1
0 x10-5 21
c (Eγ) (a.u.)
c(t) (a.u.)
3.85 eV
2
186.0
Thermal
9.07 eV
3
173.1
0.01
202.4
x10-3
0.1
c (Eγ) (a.u.)
6
10
84.8 (m) 94.5 (m)
En (eV) 10
1713
125.6 139.9 (d) 161.8 (d) 173.2 (m) 186.0 201.8
A. Tartaglione et al. / Applied Radiation and Isotopes 67 (2009) 1711–1715
0 50 100 150 200 250 300 350 400 450 500 550 Eγ (keV) Gate A
140
1000
8000
Time-of-flight (μs) Fig. 3. Neutron TOF spectrum obtained using gamma pulses as stop signal for the time encoder, during a wide time gate. The lower indium resonances are distinguished as well as a Maxwellian component in the neutron thermal energy range (higher time-of-flight). The indicated time gates A and B, were selected in order to acquire PHA spectra within energy ranges correspondent with those of TOF intervals.
beginning, a TOF spectrum was acquired during a wide time gate after the accelerator pulse. In Fig. 3 the nuclear resonances of indium at 9.07, 3.85, and 1.45 eV are observed. For long times a nearly Maxwellian shape is observed, associated with kinetic energies within the approximate equilibrium distribution of neutron energies in the moderator at room temperature . After this identification of different neutron energy ranges by the TOF method, two time gates were selected for PHA analysis of the 115 Inðn; gÞ116 In reaction gamma spectrum. On one hand a gate from 700 to 6980 ms (gate A) allowed to acquire prompt gamma spectra of reactions produced by thermal neutrons (0.0056–0.57 eV). On the other hand, a gate from 346 to 562 ms (gate B) allowed to analyze the reaction for neutrons with energies around the 1.45 eV absorption resonance (0.9–2.43 eV), (see Fig. 3). For measurements employing gate B the cadmium sheet was placed between the moderator and the indium sample removing the thermal component of the neutron beam. The background contribution was determined by periodically removing the sample from the neutron beam and subtracted from the sample spectra. An independent neutron detector placed near the neutron source was employed as a monitor and used to normalize the spectra taking into account fluctuations in the neutron production. All the PHA spectra were corrected considering the energy dependent efficiency of the detector. The gamma ray detection system was calibrated in energy using 133 Ba, 137 Cs, and 60 Co gamma sources. The energy of these calibration sources were obtained from Firestone and Shirley (1996). This calibration was performed with the LINAC in normal operation, removing the sample from the beam. The behavior of the system was extremely linear from 50 to 1500 keV, being the main range of interest from 50 to 500 keV. Fig. 4 shows the obtained PHA spectra for gates A and B. The intensities of prompt gamma emissions were obtained from the peak areas of the PHA spectra shown in Fig. 4. The Compton heel counts underlying the peaks were subtracted by the area fitting method. All the peak intensities of this work are reported as relative to the intensity of the emission of indium at 273 keV. This line was selected as reference because this is the most intense indium emission for thermal neutrons as well as for neutrons with energies around 1.45 eV. In Fig. 4, the lines indicated with (d) correspond to gamma emissions produced
Fig. 4. Upper frame: PHA spectrum of prompt gamma emissions measured during a time gate that spans the absorption of thermal neutrons (gate A). The lines indicated with (d) correspond to b decay emissions. The lines indicated with (m) correspond to unresolved multiplets reported in the literature. Lower frame: PHA spectrum of prompt gamma emissions during time gate B, which takes into account those gammas generated by the absorption of neutrons with energies around 1.45 eV.
120 Relative Intensity (%)
Gate B
0
This work Rabenstein (1972) Alexeev (1976)
100 80 60 40 20 0 50
100 150 200 250 300 350 400 450 500 Eγ (keV)
Fig. 5. Thermal neutron absorption prompt gamma. This figure compares obtained relative intensities with other published data (Rabenstein et al., 1972; Alexeev et al., 1976). The good agreement observed between the obtained results and tabulated values partially validates the implemented spectrometry technique as well as the method employed for peak area determinations.
after the b decay of the 116 In nucleus (International Atomic Energy Agency, 2007). The lines indicated with (m) correspond to unresolved multiplets reported in the literature for thermal neutrons (International Atomic Energy Agency, 2007). For gate B, it is worth to note that the decay emissions have a relatively low intensity and therefore are not observed, mainly because the cadmium sheet strongly reduced the neutron absorption rate, and also because of different time amplitude of this gate relative to that of gate A. For this reason the peaks observed at 139.9, 161.8, and 415.5 keV for gate A disappeared for gate B. In Fig. 5, peak intensities due to thermal neutron absorptions in indium are compared with tabulated values (Rabenstein et al., 1972; Alexeev et al., 1976). These tables were created employing data from higher resolution gamma detectors. To compare our results, we condensed the cited data by adding the intensities of those peaks not resolved by our spectrometer. Gamma attenuation corrections in the sample were applied (Saloman et al., 1988) taking also into account the attenuation of the neutron beam at different energies. The 95.4 keV peak intensity was corrected by attenuation at the mentioned gamma energy. We also performed an extra calculation of the attenuation
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Table 1
Relative Intensity (%)
120
This work Wahrer (1970)
Intensities of prompt gamma emissions of the reaction 115 Inðn; gÞ116 In for thermal and around 1.45 eV neutron energies.
100 Thermal
80 60
0 50
100 150 200 250 300 350 400 450 500 Eγ (keV)
Fig. 6. Prompt gamma from 1.45 eV absorbed neutrons in indium. Calculated peak relative intensities are in good agreement with the previous work of Wharer et al. (1970). Minimum energy gamma peak reported by cited reference is 273 keV, nevertheless present work shows peak intensities from 60 keV.
coefficient taking into account that this peak is an unresolved multiplet (considering the energies and relative intensities reported by Alexeev et al. (1976). The absorption coefficient thus obtained is about 1% greater than the obtained considering only one emission at 95.4 keV. Being this difference lower than the experimental uncertainty ð4%Þ, we show the results considering the attenuation correction for only one single emission at 95.4 keV. The central result of this work is an extension of the prompt gamma emission spectrum below 273 keV due to absorption of neutrons with energies around 1.45 eV (time gate B). In Fig. 6 we show the obtained peak intensities; these results are compared with the previous results from other authors (Wharer et al., 1970). The same correction of gamma and neutron attenuation explained before was applied to these data. The relative obtained intensities, for thermal and 1.45 eV neutrons are given in Table 1. Those data were converted to absolute values in the scale of photons per 1000 neutron captures. For the thermal neutron energy range (gate A) we employed as reference the absolute peak area of the 273 keV gamma-ray reported in Rabenstein et al. (1972) (204 10 photons per 1000 neutron captures). The lines indicated with (m) in Table 1 correspond to emissions reported as multiplets in the references for thermal neutrons, not resolved in this work. In these cases we report the total intensity observed for the unresolved multiplets. For neutrons around the 1.45 eV resonance (gate B) we calculated the absorption rate in terms of the absorption rate in gate A. For such purpose, we measured the incident neutron spectrum shape employing the TOF technique with a 3 He proportional counter of known efficiency placed at the sample position. The filling gas pressure of this detector was verified by neutron transmission experiments (Kropff et al., 1976) employing the mentioned detector as sample. The incident spectrum shape was corrected by dead time effects (Cuello et al., 1993), as well as by emission time in the neutron source. The flight path and electronic delay were calibrated with different resonant filters placed in the incident beam. The background contribution was carefully determined and subtracted from the spectra placing the neutron detector in different positions out of the incident beam. The background contribution was also verified by means of a polyethylene beam stop covered with cadmium placed at the end of the neutron extraction tube. The incident spectrum shape thus obtained was successfully verified by means of Monte Carlo simulations starting from the 25 MeV electron pulses and
Rel. Ig
Abs. Ig a
Rel. Ig
Abs. Ig
–
– 0.770(34)
– 157(10)
60.3(15) 85.2(15)
0.726(53) 0.955(34)
110(16) 145(20)
0.749(31)
152(9)
95.5(11)
0.924(25)
140(19)
139:9ð19ÞðdÞ
0.093(29) –
19(6) –
125.5(14) 139.6(17)
0.170(18) 0.166(11)
25(4) 25(3)
161:8ð14ÞðdÞ
–
–
–
–
–
173:2ð26ÞðmÞ 186.0(15) 201.8(15)
0.216(26)
44(5)
173.1(26)
0.322(49)
49(10)
0.704(27) 0.067(27) 0.128(28)
143(8) 13(5) 26(6)
186.0(15) 202.4(11) 234.6(12)
0.838(23) 0.155(54) 0.187(24)
127(17) 23(8) 28(5)
1.000(29) 0.121(30) 0.374(31) 0.103(32)
204(11) 24(6) 76(7) 21(6)
273.0(17) 283.1(23) 297.6(28) –
1.00(3) 0.111(20) 0.385(37) –
152(21) 16(3) 58(9) –
384:6ð21ÞðmÞ
0.340(33) 0.114(34) 0.308(36)
69(7) 23(7) 62(8)
335.7(27) – 384.8(24)
0.446(35) – 0.337(19)
68(10) – 51(7)
415:5ð20ÞðdÞ 433.7(18) 472.9(25)
–
–
–
–
–
0.223(39) 0.1940(47)
45(8) 39(2)
433.9(21) 470.6(42)
0.232(44) 0.304(32)
35(8) 46(7)
95:4ð12ÞðmÞ 125.6(16)
20
Eg (keV)
Eg (keV)
84:8ð10ÞðmÞ
40
1.45 eV res. a
234:3ð16ÞðmÞ 272.1(18) 284.9(13) 297.7(21) 320:4ð34ÞðmÞ 334.7(30) 375.5(30)
The obtained intensities are presented relative to the 273 keV line as well as in absolute units. b decay emissions are indicated with (d). The lines indicated with (m) correspond to emissions reported as multiplets in the references for thermal neutrons, not resolved in this work. In these cases we report the total intensity observed for the unresolved multiplets. a Intensity per 1000 neutron captures employing as reference the peak area of the 273 keV g-ray reported in Rabenstein et al. (1972).
carefully considering the lead target and the polyethylene moderator characteristics (Granada et al., 1987). Employing the latter result, the 115 In absorption cross-section of reference ENDF (2005), and the sample effective thickness formerly mentioned we calculated the relative absorption rates of the 115 Inðn; gÞ116 In reaction for gates A and B. The mentioned ratio between absorption rates is NCaptures B =N Captures A ¼ 0:092 0:011. The uncertainty of this ratio takes into account the uncertainties of dead time and background corrections, the employed detector efficiency, the sample thickness, and the neutron energy calibration used to determine the limits of gates A and B.
4. Analysis of results and conclusions The validation of the spectroscopic technique employed in this work and the data treatment is brought about by the favorable comparison of the results presented in Fig. 5 with previously published works for neutrons in the thermal energy range and, in Fig. 6, by the agreement with the results of Wharer et al. (1970) for gamma energies greater than 273 keV. For absorbed neutrons around the 1.45 eV, nine prompt gamma emissions were observed below 273 keV and their relative intensities were determined (see Fig. 6 and Table 1). Three of these peaks (60.3, 95.5, and 186 keV) were reported many years ago by Draper et al. (1958) using a lower resolution gamma detector, while the other six gamma lines (at 85.2, 125.5, 139.6, 173.1, 202.4, and 234.6 keV) were not found in the reviewed literature for epithermal neutrons. Differences between gamma emissions arising from the absorption of thermal neutrons and from resonant epithermal neutrons are evident in Table 1.
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It is worth to note that all gamma energies we have found in this work for neutrons around 1.45 eV are also present with different relative intensities in prompt gamma tables for absorbed thermal neutrons. The relevant information presented in this work concerning indium will be useful in order to extend the use of PGNAA technique in a wide incident neutron energy range. The results presented in this work confirm the convenience of employing pulsed neutron sources in combination with the TOF method for nuclear physics studies.
Acknowledgments We thank ANPCyT, CONICET, CNEA, and UNCuyo for financial support in carrying out this work. Also we would like to thank L. Capararo for technical support and, to M. Schneebeli and P. D’Avanzo for LINAC general maintenance and operation. The authors wish to mention this work was carried out in the frame of IAEA Research Agreement 13726/R0. References Alexeev, V.L., Emelianov, B.A., Kaminker, D.M., Khazov, Yu.L., Kondurov, I.A., Loginov, Yu.E., Rumiantsev, V.L., Sakharov, S.L., Smirnov, A.I., 1976. The properties of 116 In excited states. Nucl. Phys. A 262, 19–51. Anderson, D.L., Failey, M.P., Zoller, W.H., Walters, W.B., Gordon, G.E., Lindstrom, R.M., 1981. Facility for non-destructive analysis for major and trace elements using neutron capture gamma-ray spectrometry. J. Radioanal. Chem. 63, 97. Blachot, J., 2001. Nuclear data sheets for A ¼ 116 . Nucl. Data Sheets 92, 455–622. Corvi, F., Stefanon, M., 1974. Gamma rays from resonance neutron capture in 115 In. Nucl. Phys. A 233, 185–216.
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Cuello, G., Prado, P., Dawidowski, J., 1993. Dead-time effects in time-of-flight measurements. Nucl. Instrum. Methods Phys. Res. A 325, 309–313. Draper, J., 1958. 0–600 keV gamma-ray spectra from thermal neutron capture in the region A ¼ 104 to 198. Phys. Rev. 114, 268–278. Draper, J.E., Fenstermacher, C.A., Schultz, H.L., 1958. Variations in spectra of resonance neutron capture gamma rays in indium. Phys. Rev. 111, 906–912. ENDF/B-VI, 2005. data file for 115 In (Mat ¼ 4931). Firestone, R., Shirley, V. (Eds.), 1996. Table of Isotopes, eighth ed. WileyInterscience, New York. Fenstermacher, C.A., Bennet, R.G., Walters, A.E., Bockelman, C.K., Schultz, H.L., 1957. Absolute intensities of resonance neutron capture gamma rays from Cd, Te, and Sm. Phys. Rev. 107, 1650–1654. Granada, J.R., Dawidowski, J., Mayer, R.E., Gillette, V.H., 1987. Thermal neutron cross section and transport properties of polyethylene. Nucl. Instrum. Methods Phys. Res. A 261, 573–578. Hamermesh, B., Hummel, V., 1952. Neutron capture g-ray spectra from the elements Z ¼ 17–30 and Z ¼ 45–57. Phys. Rev. 88, 916–919. Hanna, A.G., Brugger, R.M., Glascock, M.D., 1981. The prompt gamma neutron activation analysis facility at MURR. Nucl. Instrum. Methods 188, 619. International Atomic Energy Agency. Database of prompt gamma rays from slow neutron capture for elemental analysis, Viena, 2007. Kropff, F., Granada, J.R., Remez, L.A., Vasile, A., 1976. Total cross-section of chromium at low neutron energies. Ann. Nucl. Energy 3, 55–59. Lone, M.A., Earle, E.D., Bartholomew, G.A., 1970. The 115 Inðn; gÞ116 In reaction. Nucl. Phys. A 156, 113–132. Re´vay, Zs., Belgya, T., Kasztovszky, Zs., Weil, J.L., Molna´r, G.L., 2004. Cold neutron PGAA facility at Budapest. Nucl. Instrum. Methods Phys. Res. B 213, 385–388. Rabenstein, D., Harrach, D., Vonach, H., Dussel, G.G., Perazzo, R.P.I., 1972. 116 In level scheme and p–n configurations. Nucl. Phys. A 197, 129–162. Saloman, E.B., Hubbell, J.H., Scofield, J.H., 1988. X-ray attenuation cross sections for energies 100 eV to 100 keV and elements Z ¼ 1 to Z ¼ 92. At. Data Nucl. Data Tables 38, 1–196. Wharer, R.E., King, J.D., Kukoc, H., Pan, P.J., Taylor, H.W., 1970. A study of the states of 116 In through radiative resonance neutron capture in 115 In. Nucl. Phys. A 154, 467–481. Yoon, J., Ro, T., Lee, S., Yamamoto, S., Kobayashi, K., 2002. Measurement of neutron capture cross-section of indium in the energy region from 0.003 eV to 30 keV. Ann. Nucl. Energy 29, 1157–1169.