1152 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 3, JUNE 2008
Li-Based Thermal Neutron Scintillator Research;
Rb
2
LiYBr
6
: Ce
3+
and Other Elpasolites
M. Danang Birowosuto, Pieter Dorenbos, Johan T. M. de Haas, Carel W. E. van Eijk, Member, IEEE,
Karl W. Krämer, and Hans U. Güdel
Abstract—We investigated - and neutron-scintillation prop-erties of A2LiLnX6 : Ce3+(A = Cs, Rb, K, Na, Li; Ln = La, Y, Lu, Sc;X = Br, I) with the aim to develop new scintillators with high neutron detection efficiencies. Among the investigated thermal neutron scintillators, Rb2LiYBr6 : Ce3+ shows an excellent neutron peak resolution of 3.6%. This is the best neutron peak resolution ever reported. Together with the large ratio of 0.74, Rb2LiYBr6 : Ce3+ offers the possibility of excellent neutron/ discrimination. The highest thermal neutron scintil-lation light yield of 83,000 photons/neutron is also reported for Rb2LiYBr6 : Ce3+.
Index Terms—Discrimination, peak resolution, scintillator, thermal neutron.
I. INTRODUCTION
I
NCREASING interest in thermal-neutron scintillator re-search is due to the increasing need for neutron detectors for spallation neutron facilities, inspection and security systems [1]. Research and development of thermal-neutron scintillators is focused on Li-based compounds. Li isotopes may capture thermal neutrons and convert them into ionizing particles according to the reaction(1) The two charged particles produced in the reaction have a total kinetic energy of 4.8 MeV and scintillation light is produced along their ionization tracks.
Thallium and tin activated lithium iodide crystals were the first investigated thermal-neutron inorganic scintillators [2], [3]. Later, other activators were introduced in lithium iodide [4]. In [4], it was shown that a good neutron peak resolution can be used to discriminate between neutrons and -background detections. Ce has also been introduced as an activator for lithium based compounds [5]–[7]. Ce shortens the response time of the scintillator due to its dipole allowed transition.
Manuscript received July 16, 2007; revised November 20, 2007. This work was supported in part by the Netherlands Technology Foundation (STW), in part by the Swiss National Science Foundation, and in part by the Saint Gobain, division crystals and detectors, Nemours, France.
M. D. Birowosuto, P. Dorenbos, J. T. M. de Haas, and C. W. E. van Eijk are with Radiation Detection and Matter, Faculty of Applied Sciences, Delft University of Technology, Delft 2629 JB, The Netherlands (e-mail: dbirowo@iri.tudelft.nl; p.dorenbos@tudelft.nl; j.t.m.dehaas@tudelft.nl; c.w.e.vaneijk@tudelft.nl).
K. W. Krämer and H. U. Güdel are with the Department of Chemistry and Biochemistry, University of Bern, Bern 3012, Switzerland (e-mail: karl.kraemer@iac.unibe.ch; hans-ulrich.guedel@iac.unibe.ch).
Digital Object Identifier 10.1109/TNS.2008.922826
The efficiency for thermal-neutron detection is an important parameter for the search of new thermal-neutron scintillators. A high efficient thermal-neutron detection can be achieved by Li enrichment, a minimal detector volume, and small thermal-neu-tron absorption cross section by the elements or isotopes other than Li in the scintillation crystal, which in general do not con-tribute to signal formation [7]. In this work, - and neutron-scin-tillation properties of Rb LiYBr Ce and other new crystals are presented. More details on Rb LiYBr Ce can be found elsewhere [8].
II. THERMAL-NEUTRONDETECTIONEFFICIENCIES OFNEWSCINTILLATORS
Table I shows the cross section (barns) for thermal-neutron capture in the elements of the compounds, and the fractions of the neutrons absorbed in Li in the case of natural abundance or in the case of 95% Li enrichment. A large fraction is important for thermal-neutron detection efficiency.
From Table I, we see that Li enrichment is a first requirement for efficient thermal-neutron detection. Among all compounds, Li NaYBr shows the highest fraction. Of the captured neu-trons, 77% are captured by Li and for the enriched compound, the fraction increases to 98%. This high detection efficiency is caused by the two times higher Li concentration as compared to the other compounds. An increase of the fraction leads to short-ening of the absorption length. The absorption length of 0.18 nm neutrons in a 95% Li enriched Li NaYBr is calculated to be 1.7 mm. This is much shorter than 3.5–4.6 mm of other com-pounds in Table I.
III. EXPERIMENTALMETHODS
The single crystals were grown by the vertical Bridgmann technique by using natural abundance of Li [9]. The compounds are hygroscopic, and all experiments were performed on sam-ples of approximately 15 mm , sealed in a quartz ampoule.
X-ray excited luminescence spectra were recorded using an X-ray tube with Cu-anode (XR). The anode was operated at 35 kV and 25 mA. The sample under study was mounted in the sample holder located in front of the X-ray tube. The sample chamber and the monochromator were operated under vacuum. The emission of the sample was dispersed using an Acton Research Cooperation (ARC) VM502 monochromator and detected by a Hamamatsu R934-04 Photomultiplier tube (PMT). The monochromator contains an abberation-corrected concave holographic grating (1200 groves/mm and blazed at 300 nm) with a 0.39 meters focal length. The spectra in this study were corrected for the wavelength dependence of
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BIROWOSUTO et al.: LI-BASED THERMAL NEUTRON SCINTILLATOR RESEARCH; Rb LiYBr Ce AND OTHER ELPASOLITES 1153
TABLE I
CROSSSECTION ANDNEUTRONABSORPTION INNEWTHERMAL-NEUTRONSCINTILLATORS.%6IS AFRACTION OF
ALLABSORBEDNEUTRONS IN Li OBTAINEDAFTERDIVIDING Li CROSSSECTION BY THETOTALCROSSSECTION
photodetector quantum efficiency as well as monochromator transmission.
For -ray and thermal-neutron pulse height measurements, the crystals, that are inside a sealed quartz ampoule, were mounted on a Philips XP2020Q PMT. Coupling fluid and Teflon tape were used to optimize light collection. The crystals were irradiated with 662 keV -rays from a Cs source and with thermal-neutrons from a beam line of an open pool-type research reactor at Delft University of Technology, using Ma-terials Test Reactor (MTR) fuel assemblies and low-enriched U as fuel. Standard spectroscopic techniques with 10 s shaping time and employing the single photoelectron spectrum as a reference were used [10].
Scintillation decay curves under Cs 662 keV -ray exci-tation were recorded by the single photon counting technique described by Bollinger and Thomas [11]. For this method, scintillation decay curves were recorded at time scales up to 200 s with Philips XP2020Q PMTs, Ortec 934 Constant Fraction Discriminators (CFDs), an Ortec 567 Time to Analog Converter (TAC) and an AD413A CAMAC Analog to Digital Converter (ADC). The neutron-excited scintillation decay curves were recorded by means of Tektronix TDS 3032B digital oscilloscope, averaging over in the order of 100 pulses.
IV. RESULTS ANDDISCUSSION
Fig. 1 shows the X-ray excited emission spectra of Ce activated Cs LiLuI , Rb LiLaBr , Rb LiYBr , Rb LiYI , K LiYBr , and Li NaYBr recorded at room temperature (RT). The emission spectra of Ce activated Rb LiLaBr , Rb LiYBr , K LiYBr , and Li NaYBr show two bands that peak at 363 and 387 nm, 381 and 414 nm, 385 and 420 nm, and 383 and 413 nm, respectively. These bands are attributed to the transitions from the lowest 5d excited state of Ce to the two spin orbit split F and F ground state levels. Other bands in the emission spectra of bromides, that fall between 250 and 350 nm, are attributed to the host lattice emission. The shoulder band that peaks at 450 nm in the emission spec-trum of Rb LiLaBr Ce is also attributed to host lattice emission. The emission spectra of Ce activated Cs LiLuI and Rb LiYI show more complicated structures. The char-acteristic Ce F F emission is tentatively attributed to the peaks at 435 and 478 nm, and 435 and 480 nm for Ce activated Cs LiLuI and Rb LiYI , respectively. The origin of other bands at 295, 388, 505, 530, 554, 580, and 670
Fig. 1. X-ray excited emission spectra at RT of various thermal-neutron scin-tillators.
nm in the emission spectra of Ce activated Cs LiLuI and Rb LiYI is not clear.
Fig. 2 shows thermal-neutron excited scintillation pulse height spectra of Ce activated Cs LiLuI , Rb LiLaBr , Rb LiYBr , Rb LiYI , K LiYBr , and Li NaYBr . Pulse height spectra for other Ce concentration of Rb LiYBr Ce can be found in Birowosuto et al. [8]. The x-axes in -equivalent energy was calibrated using the 662 keV peak in the -ray excited pulse height spectra of the corresponding compounds, see a dotted line and lines with empty circles in Fig. 2. It is assumed that non-proportionality effects can be neglected at high energy.
Pulse height spectra of Rb LiYBr : 0.1% Ce and Li NaYBr : 1% Ce show clear neutron capture peak, respectively, at 3.5 and 3.0 MeV, see Fig. 2. The best neutron capture peak resolution of 3.6% is observed for Rb LiYBr :
1154 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 3, JUNE 2008
TABLE II
-RAYLIGHTYIELD, 662 keV ENERGYRESOLUTION, NEUTRONCAPTUREYIELD,AND = RATIO OFTHERMAL-NEUTRONSCINTILLATORS ATRT
Fig. 2. Scintillation pulse height spectra of Ce activated Cs LiLuI , Rb LiLaBr , Rb LiYBr , Rb LiYI , K LiYBr , and Li NaYBr at RT irradiated with thermal-neutrons from a reactor beam line (solid lines). Pulse height spectra at RT irradiated with 662 keV -rays from a Cs source (lines with empty circles) were added for calibration. The asterisks(3) and the dotted line mark the position of the corresponding thermal-neutron and 662 keV peaks, respectively.
0.1% Ce . Other pulse height spectra show deformed struc-tures. We do not have an explanation for this. On one hand, we may argue that some parts of a crystal produce more photons, and/or photons are collected more efficiently from one part than from another part due to inhomogeneity in crystal properties.
Table II shows -ray and thermal-neutron scintillation prop-erties derived from pulse height spectra. ratio in column 6 is the ratio of the number of photons/MeV produced by the
heavy charged reaction products, H and , and the number of photons/MeV produced by electrons. Since pulse height spectra of Ce activated Cs LiLuI , Rb LiLaBr , Rb LiYI and K LiYBr in Fig. 2 show broad deformed structures, the neutron capture yield and the ratio found for those compounds have larger errors than those of Ce activated Rb LiYBr and Li NaYBr . The highest neutron capture yield of 83,000 photons/neutron is found for Rb LiYBr : 0.5% Ce whereas the highest -ray light yield of 33,000 photons/MeV is recorded for Rb LiLaBr : 1% Ce . The ratio in Rb LiLaBr : 1% Ce is only 0.34 and therefore the neutron capture yield of this compound is 29,000 photons/neu-tron smaller than that of Rb LiYBr : 0.5% Ce . For other compounds, the neutron capture yield and the ratio are of the same order: 35,000–42,000 photons/neutron and 0.63–0.71, respectively.
Fig. 3 shows the -excited scintillation decay curves of Ce activated Cs LiLuI , Rb LiLaBr , Rb LiYBr , Rb LiYI , K LiYBr , and Li NaYBr recorded using the single photon counting technique at RT [11]. The neutron-excited scintillation decay curves recorded using the digital oscilloscope are not presented. The neutron-excited decay curves are identical to those excited by -rays recorded with the oscilloscope. In order to analyze the data, the decay curves were fitted with three exponential decay components. The decay times of the three components with their contributions to the total light yield are presented in Table III.
The contribution of the fast component to the total light yield in the two iodide compounds is about 30%, see column 3 in Table III. This contribution is significantly larger than the 1 to 5% of observed bromide compounds. However, the fast com-ponents in iodides are slower compared to those in bromides, see column 3 in Table III. In general, the intermediate and the slow components of iodides are faster than those of bromides, see columns 4 and 5 in Table III. This can be attributed either to the faster energy transfer from host lattice to Ce or the faster intrinsic decay time of host lattice emission in iodides than in bromides [12].
V. CONCLUSION
Rb LiYBr Ce has a neutron yield of 83,000 pho-tons/neutron and a neutron peak resolution of 3.6%. This reso-lution is the best ever reported for a thermal-neutron scintillator.
BIROWOSUTO et al.: LI-BASED THERMAL NEUTRON SCINTILLATOR RESEARCH; Rb LiYBr Ce AND OTHER ELPASOLITES 1155
Fig. 3. Scintillation decay curves of Ce activated Cs LiLuI , Rb LiLaBr , Rb LiYBr , Rb LiYI , K LiYBr , and Li NaYBr under 662 keV -ray ex-citation.
TABLE III
CHARACTERISTICCOMPONENTS OF THEDECAYCURVES OFCe ACTIVATEDCs LiLuI , Rb LiLaBr , Rb LiYBr , Rb LiYI ,
K LiYBr ,ANDLi NaYBr
Together with the large ratio of 0.75, Rb LiYBr Ce offers the possibility of excellent neutron/ discrimination. Li NaYBr Ce has also some potentials. The highest detection efficiency among other investigated compounds is the most important one, see Table I. Additionally, the pulse height spectrum of Li NaYBr Ce shows a clear neutron line with a neutron yield of 37,000 photons/neutron.
Disadvantages of these thermal-neutron scintillators are the relatively low Li concentration and high hygroscopicity. The shortest absorption length for 0.18 nm neutrons of 1.7 mm is calculated for a 95% Li-enriched Li NaYBr Ce crystal. Compared to 0.54 mm of the traditional scintillator LiI Eu , the absorption lengths of 0.18 nm neutrons in Li NaYBr Ce is large [7]. However, this length is twice shorter than 3.5 mm of Rb LiYBr Ce . Therefore, replacing Cs, Rb, K and Na by Li provides a higher detection efficiency. Generally, relatively thick layers are needed for efficient neutron detection if these new scintillators will replace the old traditional scintil-lators for future applications.
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