ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 594 (2008) 56– 60
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
LaBr3:Ce scintillators for in-beam gamma-ray spectroscopy with fast beams of rare isotopes D. Weisshaar a,, M.S. Wallace c, P. Adrich a, D. Bazin a, C.M. Campbell a,b, J.M. Cook a,b, S. Ettenauer a, A. Gade a,b, T. Glasmacher a,b, S. McDaniel a,b, A. Obertelli a, A. Ratkiewicz a,b, A.M. Rogers a,b, K. Siwek a,b, S.R. Tornga c a
National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA c Los Alamos National Laboratory, Los Alamos, NM 87545, USA b
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 March 2008 Accepted 10 June 2008 Available online 17 June 2008
The scintillator material LaBr3:Ce, which was discovered in 2001, has grown in popularity for a variety of applications in homeland security and nuclear medicine. In this paper we investigate its use for nuclear science spectroscopy applications in the context of in-beam gamma-ray spectroscopy with fast ion beams. In such experiments, the Doppler broadening of the gamma-ray line measured with a finite-size detector in the laboratory fundamentally limits the achievable energy resolution. For a typical experiment this effect is of the order of 3%. With the intrinsic energy resolution of LaBr3:Ce better than 3% FWHM at 662 keV, and its favorable stopping power, it could be a nearly ideal scintillator for gammaray spectroscopy applications with fast beams. Results from in-beam gamma-ray spectroscopy measurements using two LaBr3:Ce scintillators are presented and compared to results from segmented germanium detectors. Results from these measurements suggest that LaBr3:Ce based scintillation arrays can provide a viable tool for many experiments. Additionally, we show that the excellent timing properties of LaBr3:Ce can significantly reduce background events in the gamma-ray spectra. & 2008 Elsevier B.V. All rights reserved.
Keywords: LaBr3:Ce Gamma spectroscopy Radioactive ion beam
1. Introduction In-beam gamma-ray spectroscopy has become a widely-used tool for nuclear structure and nuclear astrophysics experiments at rare-isotope facilities with in-flight separated beams [1–9]. Inbeam gamma-ray spectroscopy with fast beams differs from lowenergy nuclear spectroscopy in that the excited states in the nuclei of interest decay while in motion as opposed to almost at rest. For rare-isotope facilities based on in-flight separation these nuclei are moving with velocities b ¼ v=c 0:3–0.6. At these velocities the gamma-ray energies emitted by the decaying nuclear states are significantly Doppler shifted in the laboratory frame. The energy resolution achieved for the reconstructed gamma-ray energy in the rest system of the gamma-ray emitting nucleus is therefore dependent upon not only the intrinsic energy resolution of the gamma-ray detector, but also on the considerable Doppler broadening of the full-energy peak. The main contributions to this broadening are the spatial resolution of the detector, for the determination of the detection angle y with
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respect to the trajectory of the gamma-emitting particle, and the uncertainty of the particle’s velocity at the time of the gamma-ray emission. This uncertainty is affected by the momentum spread of the incoming beam and the momentum change in the target. Typical target thicknesses are a few hundred mg/cm2 in order to maximize the number of reaction centers and thus the gammaray yield. The momentum loss in such targets is the major contribution to the velocity uncertainty as the depths of the nuclear reaction and gamma decay within the target are unknown. For typical experimental conditions (v=c 0:4, a target thickness of 300 mg=cm2 and a momentum spread of 1% of the incoming beam) the uncertainty in b contributes dEb =E 223% to the total energy resolution. For a scintillation detector a reasonable spatial resolution in y is 60–70 mrad and yields dEy =E ¼ 223%. Therefore, the overall Doppler broadening of in-flight emitted gamma rays in such experiments is of the order of dE=E 3%. An efficient detector system having an intrinsic energy resolution which matches the Doppler broadening is desirable for in-beam gamma-ray spectroscopy experiments. Well-known scintillators like NaI(Tl), CsI(Tl), and CsI(Na) do not fulfill this requirement as these materials provide typical energy resolutions of about 7% at 662 keV. Currently the best detector systems used
ARTICLE IN PRESS D. Weisshaar et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 56–60
for these types of experiments consist of segmented high-purity germanium detectors [10–15]. The segmentation establishes the spatial resolution needed for reducing the Doppler broadening. There are some downsides to germanium detectors as they are expensive, need to be operated at cryogenic temperatures, do not provide good timing, and their excellent intrinsic energy resolution of 0.2% at 1 MeV cannot be fully exploited in experiments with fast beams. The recently discovered scintillator material LaBr3:Ce [16] could provide the needed energy resolution while also providing fast timing. Here, we present the results of operating two LaBr3:Ce detectors in an experiment with in-flight separated beams of rare isotopes and we discuss the use of LaBr3:Ce scintillators in such in-beam gamma-ray spectroscopy experiments.
2. LaBr3:Ce scintillators LaBr3:Ce is relatively dense at 5:29 g=cm3 and has a larger stopping power than germanium because LaBr3:Ce (Z La ¼ 57 and Z Br ¼ 35) has a higher atomic number than germanium (Z Ge ¼ 32) at almost the same density. Its peak emission wavelength of 380 nm is well matched to Bi-alkali photocathodes. The light output of 61 photons/keV is significantly higher than the light output of other materials, e.g. NaI(Tl). The material’s primary 1=e decay time is 16 ns for 5% Ce doping. This should allow high-rate experiments and provide good timing properties. LaBr3:Ce is very hygroscopic similar to NaI(Tl). Several detectors were purchased by Los Alamos National Laboratory from Saint Gobain Crystals. The commercial name of s these detectors is BrilLanCe 380 [17]. The crystals in the detector assemblies were 25 mm tall cylinders with a diameter of 25 mm. The detectors used in the in-beam experiment came sealed with XP2060B photomultiplier tubes (PMT) from Photonis. Fig. 1 shows energy resolution measurements taken with several gamma-ray calibration sources. All detectors using the XP2060B PMT had similar energy resolutions. An energy resolution of 1.6% fullwidth-half-maximum (FWHM) at 1408 keV and 2.8% FWHM at 662 keV was observed for the gamma-ray calibration sources. The coincident resolution timing (CRT) between two identical detectors using the XP2060B PMT was 400 ps FWHM at 511 keV. Additionally, tests were done with H6533 tubes from Hamamatsu. A CRT of 237 ps FWHM at 511 keV was achieved when using the
12 LaBr3 :Ce energy resolution
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Fig. 1. Energy resolution (FWHM) of the LaBr3:Ce scintillators of 25 mm diameter and 25 mm length measured with several gamma-ray calibration sources at rest.
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faster H6533 PMT. These PMTs do not provide the same quality energy spectrum and were not used for the in-beam experiments. A drawback of the LaBr3:Ce material is its internal contamination due to the alpha-emitter 227Ac [18,19]. The decays produce broad peaks in the gamma-ray energy spectra between 1.7 and 3.5 MeV. Through advances in selection and purification of the raw materials the decay rate of this alpha contamination has been reduced by two orders of magnitude from 1.4 Bq/cm3 to 0.02 Bq/ cm3. With these reduced contamination levels, radioactive decays of 138La become apparent. 138La is present in natural La at an isotopic abundance of 0.09%. It has a half-life of 1:05 1011 years and decays by electron capture and beta decay. The decays of 138La produce background up to 1.46 MeV in the gamma-ray spectra. For experiments with in-flight separated beams these contaminations are irrelevant as gamma rays are measured in prompt coincidence with particles and this condition is not met for internal decays. The detectors used in the experiment described below were fabricated prior to these improvements in crystal purity and therefore contain contamination due to the decay of 227Ac.
3. LaBr3:Ce for in-beam gamma-ray spectroscopy An in-beam experiment has been performed at the Coupled Cyclotron Facility [20] of the National Superconducting Cyclotron Laboratory at Michigan State University. Two of the 25 mm diameter by 25 mm thick cylindrical LaBr3:Ce scintillators were added to an experiment which was dedicated to the testing of the performance of the GRETINA prototype gamma-ray tracking detector [21,22]. The LaBr3:Ce detectors, as well the GRETINA detector, were mounted in the frame of the Segmented Germanium Array (SeGA) [10]. The remaining positions were equipped with the 32-fold segmented germanium detectors of SeGA. The LaBr3:Ce detectors were positioned at 37 and 143 with respect to the beam axis at a distance of 25 cm from the target. At these positions the opening angles of the LaBr3:Ce detectors were twice as large as the spatial resolution of the SeGA detectors. Fig. 2 shows an image of the experimental setup. The estimated uncertainty dE=dy due to the opening angles is expected to be 2.5% for the detector at 37 and 1.5% at 143 . At these positions the efficiency of the two scintillators was two orders of magnitude less than the efficiency of the remaining SeGA array based on small size of the LaBr3:Ce crystals compared to the SeGA detectors. The scintillators were read out with the SeGA electronics yielding a coincidence timing resolution of 550 ps FWHM for a 22Na source and an energy resolution of 2.8% at 662 keV. As the experimental goal was the measurement of the reduction of Doppler broadening utilizing the position sensitivity of the GRETINA detector, the uncertainty in b was kept small. This was achieved by using a primary beam of 36Ar at 86 MeV/nucleon with a momentum spread of 0.1% as the incoming beam on target. Furthermore the target itself was beryllium with a thickness of 100 mg=cm2 , in which the 36Ar beam of b ¼ 0:403 suffered a deacceleration of db ¼ 0:013. The change in velocity db contributed to the energy resolution with less than dE=db ¼ 1:4% for the detector at 143 and dE=dbo1% at 37 . The various fragments of elements ranging from carbon to argon produced in this reaction were identified event-by-event with the S800 spectrograph [23]. Gamma-ray events in coincidence with specific isotopes were selected to identify the excitation spectrum for each isotope. The direction and magnitude of the velocity vectors of the outgoing fragments were also measured in the S800 spectrograph and used to improve the Doppler correction of the gamma-ray energies. An event-by-event reconstruction of y using the S800 tracking capability results in a significant improvement of the energy resolution. Fig. 3 shows the
ARTICLE IN PRESS D. Weisshaar et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 56–60
In−beam energy resolution FWHM [%]
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Fig. 4. Measured in-beam gamma-ray energy resolutions of the LaBr3:Ce scintillators and the Segmented Germanium Array (SeGA) for various gammaray energies and a beam energy of 86 MeV/nucleon impinging on a 100 mg=cm2 thick beryllium target. SeGA was two orders of magnitude more efficient than the LaBr3:Ce detectors, so more gamma-ray lines could be investigated with better accuracy. Note that the spatial resolution of SeGA was two times better than the spatial resolution for the LaBr3:Ce scintillators in this experiment. Fig. 2. Photograph of the experimental configuration. The 11 detectors with cylindrical endcaps are the HPGe detectors of SeGA at 37 and 90 with respect to the beam axis. On the right, the GRETINA gamma-ray tracking detector consisting of three encapsulated, 36-fold segmented germanium detectors in a common cryostat is mounted. The two LaBr3:Ce scintillators are indicated by arrows. The target is located inside the beam pipe in the center of the array, the beam enters from the left.
velocity after the target and does not provide any information on where within the target and at which velocity the nucleus emitted the gamma ray.
4. Results
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Energy [keV] Fig. 3. The Doppler corrected energy spectrum of the 1634 keV gamma-ray transition in 20Ne measured with the LaBr3:Ce detector at 37 . For the gray spectrum a fixed angle between detector and beam line was assumed for the Doppler reconstruction while in the black spectrum the emission angle was calculated event-by-event between the detector position and the trajectory of the particle measured with the S800 spectrograph.
Doppler corrected gamma-ray spectrum measured with the LaBr3:Ce detector at 37 in coincidence with 20Ne with and without the event-by-event correction applied. The velocity of the fragment measured in the S800 does not help to reduce the uncertainty in b because the measured value corresponds to the
The gamma-ray spectra seen in coincidence with the different fragments were analyzed in terms of energy resolutions. As seen in Fig. 3, the low statistics in the spectra limited the precision of the measured resolutions. As expected, the spectra do not show any contribution of the intrinsic contamination to the background. The measured energy resolutions of the detected gamma-ray transitions were very similar for the two scintillation detectors. This can be explained by the considerable shift of the gamma-ray energy in the laboratory frame. While the uncertainty in y favors the detector at 143 , the gamma-ray energy in the 37 detector is shifted to a 30% higher value and is thus measured with a better detector resolution than at 143 where the energy is Doppler shifted to lower values. Fig. 4 shows the in-beam resolutions obtained from the Doppler-corrected sum spectra of both detectors and we conclude that gamma-ray transitions above 1 MeV are measured with a resolution better than 3%. At lower energies the detector resolution starts to dominate and energies below 400 keV are measured at the detector resolution of 4–5%. For comparison, the gamma-ray spectra measured with the segmented germanium detectors of SeGA were also analyzed. Applying the standard procedure for the Doppler-shift correction by assuming y as the angle between beamline and detector segment, a resolution of 2.6% was obtained. This result is improved to 1.6% resolution if the additional correction utilizing the particles’ tracking data measured by the S800 spectrograph was done. The energy resolutions measured with SeGA for various gamma-ray transitions are also shown in Fig. 4. As the efficiency of SeGA is two orders of magnitudes higher than for the two 25 mm 25 mm LaBr3:Ce detectors, many more gamma-ray lines could be analyzed at much better accuracy. Detectors in SeGA are arranged in two rings at 90 and at 37 , where one of the LaBr3:Ce detectors was also positioned. While the impact of the uncertainty
ARTICLE IN PRESS D. Weisshaar et al. / Nuclear Instruments and Methods in Physics Research A 594 (2008) 56–60
in b on the resolution is slightly lower at 90 compared to 143 , the overall Doppler broadening due to db is less for SeGA. The spatial resolution of SeGA is twice as good as for the LaBr3:Ce detectors. Together with the good intrinsic energy resolution of germanium this explains the in-beam resolution being almost twice as good as for the scintillators in this experiment. It further confirms that the measured in-beam resolution of the LaBr3:Ce detectors is not only limited by the intrinsic detector resolution but also by the Doppler broadening due to their larger opening angles. The limitation of the energy resolution by the Doppler broadening is especially true for the LaBr3:Ce scintillator at forward angle where the gamma-ray energies are shifted to energies above 2 MeV and measured at an intrinsic detector resolution better than 2%. The fast timing properties of the LaBr3:Ce detectors were investigated in the time difference spectra between them and the
timing between LaBr3 :Ce and TOF detector
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time-of-flight diamond detector used for particle identification. The high-rate capable diamond detector [24] for the time-of-flight measurement was located at a distance of 22.4 m before the target. The timing resolution for prompt gamma-ray events against the diamond detector was 700 ps FWHM, more than 10 times better than for germanium detectors. This fast timing can be used to remove background from the gamma-ray spectra as shown in Fig. 5. The inset in the top panel shows the difference timing of all coincidences of the LaBr3:Ce at 37 and identified 21 Ne fragments. The top spectrum itself shows the gamma-ray energies measured within a 10 ns wide time gate while the bottom spectrum is gated on the prompt timing peak. In the bottom spectrum a considerable amount of background is removed, and the 7=2þ -state with a gamma-ray transition at 1395 keV can be clearly identified. In fact the tight prompt time gate reduces the background counts under the gamma-ray line by a factor of three. The inset in the bottom spectrum shows the gamma-ray spectrum gated on a 400 ps wide time window next to the prompt time peak in which the prompt 351 keV gamma-ray transition is gone. This shows that all prompt gamma-ray events are collected in the tight 1.8 ns wide time window. On the other hand further analysis shows that the 511 keV annihilation line can be seen even within time gates delayed by 10 ns. In addition, within time gates starting approximately 8 ns after the prompt peak, the 844 keV gamma-ray transition of 27Al appears in the energy spectrum. This suggests that one origin of background events might be particles originating from the reaction in the target, traveling towards the aluminum beam pipe of 102 mm diameter and reacting there to create gamma rays.
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Fig. 5. The inset in the upper panel shows the time difference spectrum between the LaBr3:Ce scintillator and the time-of-flight diamond detector. The area highlighted in gray indicates a 1.8 ns wide prompt time gate. The energy spectrum in the upper panel is obtained with a 10 ns wide coincidence window, which corresponds to a time resolution of HPGe detectors. The spectrum in the lower panel is gated on the prompt 1.8 ns wide timing peak and allows to identify the 1395 keV transition in 21Ne. The inset shows the energy spectrum obtained with a 400 ps wide time window just next to the prompt peak. The prompt 351 keV transition has disappeared in this off-prompt gate.
Our results show that in-beam gamma-ray spectroscopy using fast beam is feasible with LaBr3:Ce scintillation detectors and that an in-beam energy resolution of 3% can be achieved. This result can be improved in the future by increasing the spatial resolution of the detectors as the measured in-beam energy resolution was not limited by the intrinsic detector resolution. This applies in particular to detectors placed at forward angles, where the difference in intrinsic resolution between LaBr3:Ce and germanium becomes less important given that the gamma rays are shifted to higher energies and that the line shapes in the spectra are dominated by the Doppler broadening. Especially for detectors located at forward directions the fast timing property of LaBr3:Ce scintillators is a valuable advantage for background reduction. In the forward section, the increase in detection efficiency due to the Lorentz boost is always accompanied by an increase of background events in the gamma-ray spectra. Those background events are correlated within a 10 ns wide time window after prompt gamma rays being emitted from the target. They can be suppressed very efficiently utilizing the fast timing of LaBr3:Ce, while this distinction is impossible with the timing properties of germanium detectors. The higher stopping power of LaBr3:Ce is an additional benefit particularly for gamma rays being Doppler shifted to higher energies. The results of the measured in-beam performance demonstrate that LaBr3:Ce scintillation material is well-suited for highresolution gamma-ray spectroscopy with fast beams and should be considered in future designs of devices for such experiments.
Acknowledgments We thank Mohini Rawool-Sullivan from Los Alamos National Laboratory for loaning us the LaBr3:Ce detectors used in the
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