Neutron Response of 1.5” LaBr3:Ce CrystalScintillators for PARIS

Oliver Roberts, Pankaj Joshi, David Jenkins, Bob Wadsworth, Adam Tuff

University of York, Department of Physics, Heslington, YO10 5DD, UK.

ojr500@york.ac.uk

November 2008

Abstract

LaBr3:Ce crystals are high resolution scintillators that hold a lot of potential in gamma-rayspectroscopy. The crystals have a self-activity due to the 138La isotope in the crystal (0.09%abundance). This was observed and used to better understand the inner dynamics of the de-tector. Little is known about what happens to the crystals due to neutron activation and thiswas investigated by studying the pulse shapes from neutron and gamma sources as well asdetermining its activated spectrum from a 241Am/9Be source. It was determined that (n,γ)discrimination was not possible, and that the majority of gamma emission in the spectra afterneutron activation were due to excited states of La and Br (140La, 80Br and 82Br).

1 Motivation

PARIS or the Photon Array for the studieswith Radioactive Ion and Stable Beams, is anewly formed collaboration, the aim of whichis to design and build a high efficiency de-tector consisting of two shells of crystals formedium resolution spectroscopy and to de-tect gamma-rays over a large range of ener-gies.

The inner shell of crystals will be highlygranular, and made from LaBr3:Ce, with thereadout done in either a Phoswich arrange-

ment with another crystal, APDs or withanother configuration involving digital elec-tronics and possible light-guide to limit deadspace and allow for pulse shape analysis.

The outer hemisphere of crystals will havea lower granularity, larger volume, and largestopping power (e.g. CsI:Na). This outer shellof inorganic alkali halide crystals will be usedto measure high energy photons, and act as aCompton suppressor.

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1.1 Lanthanum Halide Scintillators

LaBr3:Ce is a relatively new inorganic scin-tillator that has been gaining popularity foruse in gamma-spectroscopy due to its excel-lent timing resolution ( 200 ps depending onthe size of the detector), energy resolution( 3% at 662keV), high efficiency and light out-put (60,700 photons/MeV with Ce3+ concen-tration of 0.2%). The decay times are gen-erally quick with the fast components hav-ing a value of between 15-23ns depending onthe Ce3+ concentration[1]. The rise time istypically between 3.5 and 0.7ns. The peakemission wavelength of the LaBr3:Ce scintil-lator is in the blue-UV part of the electromag-netic spectrum, with λmax=350nm (character-istic of Ce3+ luminescence). A shortcomingof these detectors is that they exhibit ”self-activity” which can be seen as deceptive linesin experimental data. These spurious linescould limit the practicality of the detector andits uses, especially with regards to low levelcounting.

Figure 1: A picture showing the detector set-upwith the 10.5Gbq 241Am/9Be neutron source

1.2 Self-activity of LaBr3:Ce

This ”self-activity” is due to Lanthanum-138,a naturally occurring radioisotope of Lan-

thanum with an abundance of 0.09% and alarge half life of 1.05x1011 years. In 66.4% ofits decays, 138La undergoes electron captureto an excited state of 138Ba via photon emis-sion of a 1436keV gamma ray. The refilling ofthe electron shell results in a coincident 35keVBarium X-ray. The remainder of 138La decays33.6% of the time via beta emission to 138Ce,emitting a 789keV gamma ray in the 2+ state.A spectrum of these features in LaBr3:Ce isshown below outlining its decay processes[2].

Figure 2: An Energy level diagram showing thedecay branches of the metastable state 138La [2]

A Background spectrum was then obtained toshow the effects of the decay process of the138La metastable state in the crystal. In thespectrum, there is an off-set of 45keV as thenumber of counts from the 138Ba 32keV X-raywere large compared to the rest of the spec-trum. These X-rays occur due to reoccupa-tion of K-shell electrons. The spectrum is alinear and not logarithimic representation ofthe background, as the many interesting fea-tures that are attributed to this structure canbe more easily seen in this way.

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Figure 3: The background spectrum of theLaBr3:Ce detector

1.3 Detector Setup

A 1.5”x1.5” LaBr3:Ce scintillation detectorcoupled with a Photonis 2” XP20D0 PMTwas obtained from Saint-Gobain. Operationalvoltage recommended by Saint Gobain was-1200V, however this resulting in a highlynon-linear spectrum distribution as the Pho-ton yield produced by the crystal is extremelyhigh. Therefore the working voltage was re-duced to a rather low -900V. The dynamicrange was rather inconsistent and is possiblydue to saturation phenomena inside the PMT,thus effecting the overall response in energy.A pre-amplifier with a shaping time of 500nsand low coarse gain was used to shape the an-ode signal.

The neutron response of these detectorswere then tested with the neutron source241Am/9Be, where a study of how neutronactivation affected the crystals was investi-gated.

2 Testing for Neutron Re-sponse

After measuring the background spectrum ofthe LaBr3:Ce detector, the detector was then

placed roughly 20cm from the neutron source241Am/9Be. As well as detecting neutrons,there will be some coincidence with a 4.4MeVγ-ray due to the reaction 9Be and an alphaparticle to produce the excited 2+ state ofCarbon-12. Due to the size of the crystal,the 4.4MeV gamma ray was hard to locate,and the FWHM of the photo-peak was ratherpoor. However, it was used to calibrate thegraph, and identify the excited states of bothLanthanum(140La) and Bromine (80Br,82Br).There are two stable states in naturally occur-ing Bromine that contribute significantly tothe activated spectrum of LaBr3:Ce; 79Br and81Br with abundances of 50.69 and 49.31% re-spectively. The (n,γ) cross-sections for theformation of metastable 79Br and 81Br statesare 2.5 and 2.4b respectively (where 1b =10−28m). For the formation of both 79Br and81Br ground-states, the (n,γ) cross-sectionsare 8.3 and 0.24b respectively[3]. The result-ing calibrated spectrum is shown at the endof the article in figure 7.

Alpha contamination is revealed by 4broad peaks in the 1750-3000keV energyrange (the actual alpha energies range from5000-7500keV). This 235U decay chain has along lived 227Ac member (τ1/2 = 21.2yrs).227Ac is also in the same periodic group(group IIIB) as Lanthanum. Heavy chargedparitcles such as alphas, exhibit less light thangamma rays of a similar energy. A spectrumwith 4096 channels is enlarged to show both227Ac contamination and the 4.4MeV gammaray with escape peaks in figure 8 at the end ofthis article.

2.1 Energy Resolution and FWHMCalculations

The energy resolutions were calculated forthe 137Cs, 241Am and 60Co sources that werethen used to calibrate the resulting activatedspectra. Below is a small table of some of the

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peaks in the calibrated spectrum.

Table 1: Energy Resolutions from LaBr3:Ce spectra

Element γ ray (keV) FWHM (keV)ERes(%)241Am 59.54 9.23 15.50137Cs 661.73 16.89 2.5560Co 1173.21 22.18 1.9660Co 1332.54 24.14 1.8112C* 4438.90 8.46 0.19140La 1596.11 24.33 1.5282Br 1317.47 23.65 1.8082La 827.83 19.34 2.3482Br 554.35 14.56 2.63208Tl 511 14.05 2.75140La 487.02 13.52 2.78140La 328.76 10.23 3.1180Br 11.89 2.22 18.67

The above FWHM were calculated usingRadware, and have an error of ±1.15keV.These values of resolution were then takenand plotted against energy to get the graphbelow.

Figure 4: A figure showing the results of theFWHM measurements made, with energy resolu-tion plotted against increasing energy

As we can see, the energy resolution of theLaBr3:Ce becomes worse at lower energies,but is still better than standard NaI detectors(7.0% at 662 and 8.9% at 122keV)[2]. Nowthat the resolution of the crystal has beenthoroughly analysed, the next thing that wasstudied were the pulse shapes of the crystal

when subjected to both gamma and neutronsources.

3 Pulse Shape Discrimina-tion

Scintillator material exhibits a phenomenaknown as luminescence. If remission of thisenergy is immediately after the absorption(within 10−8s) then this process is known asfluorescence. However, if the remission is de-layed due to an excited state being metastableat the atomic level, another process known asphosphorescence occurs. The time evolutionof this remission process can be accuratelymodeled on the equation:

N = Aexp−t

τf+ Bexp

−t

τs[4] (1)

The finite rise time from zero to the max-imum amplitude is usually much smaller orshorter, than the decay time. For most scin-tillators, one component of the decay con-stant is much faster than the other, and are re-ferred to as fast and slow components of thepulse shape. The existence of these two com-ponents forms the technique for pulse shapediscrimination. A diagram of the two pulsesis shown below with the gamma pulse hav-ing a different shaping time to better illustrateboth curves individually, in reality these twocurves had a similar pulse height.

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Figure 5: A comparison neutron and gamma pulseshapes

Pulse shape analysis was carried out to seeif one could discriminate between gammaand neutron pulse shapes. Generally, or-ganic scintillators provide such discrimina-tion due to delayed remission of energy.Several batches were recorded using 2 wellknown gamma-ray emitters; 137Cs (662keV)and 60Co (1173 & 1332keV). The files werecompiled into a single file and then the slowand fast components of their pulse shapeswere obtained by using a code to extract theparameters needed to make the discrimina-tion. At first the discrimination was fairlyscattered, dividing the slow component bythe sum of both components making the fitbetter. An example of this with 60Co is shownin the figure below.

Figure 6: Fast and slow components from gammaand neutron anode pulse shapes to try to observeany discrimination

The gamma points in red show twothresholds that correspond to the gammapeaks at 1132 and 1332keV. As one can see,there is no discrimination between both setsof data unlike what would be expected fororganic crystals. The magnitude of the slowcomponent is small, and the discrimination,if any, too small to be useful in experimen-tal conditions. However, there is modestdiscrimination of both neutrons and chargedparticles with LaCl3:Ce [5]. More coincidencemeasurements with a beamline would helpverify these results.

4 Conclusions and FutureWork

Tests done on the inorganic LanthanumHalide crystal LaBr3:Ce with the neutronsource 241Am/9Be has shown the back-ground spectrum before and after activationdue to the natural occurring metastable inLanthanum, 138La. Calibrated spectra for thiscrystal were presented, and revealed gammalines from 140La, 80Br and 82Br after neu-tron activation. The self-activity present inLaBr3:Ce, does not make the detector a suit-able candidate for low-level counts. Pulseshape analysis also revealed that very littleor no discrimination between both gammaand neutron slow and fast components waspossible, which agrees with previous workdone[5].

Tests with larger LaBr3:Ce crystals, suchas 3”x3” or even 2”x2” crystals should be con-ducted to better verify the results, and also al-low for better discrimination of gamma raysat higher energies. For now, the group in Yorkwill focus next on tests with CsI(Tl) crystalswith a High gain Silicon APD from Sensl. Theaim will be to determine whether or not thisarrangement can be used in a phoswich ar-rangement with the tested LaBr3:Ce crystalswhen construction of the PARIS calorimeterstarts in 2009/2010.

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5 Acknowledgements

I would like to thank Dr. Pankaj Joshi and Mr.Adam Tuff for their help regarding the experi-ment, especially Dr. Pankaj Joshi, whose code

for subtracting parameters from the pulsesto determine discrimination was a valuabletime saver! I would also like to thank Dr. BobWadsworth and Dr. David Jenkins for valu-able discussions regarding the resulting dataand how best to intepret them.

References

[1] W.J.Kerman. Self Activity in Lanthanum halides. IEEE Trans. Nucl. Sci., 53:395–400, 2006.

[2] C.M.Rozsa, P.R.Menge, and M.R.Mayhugh. BrilLance Scintillators Performance Summary.Saint-Gobain Crystals, Scintillation Products, April 2008. www.detectors.saint-gobain.com.

[3] G.Pfennig, H.Klewe-Nebenius, et al. Karlsruher nuklidkarte, 1995.

[4] W.R.Leo. Techniques for Nuclear and Particle Physics Experiments. Springer-Verlag, New YorkLLC, Second Revised Edition edition, 1994. Chapter 7, pp158-159,[7.2].

[5] C.Hoel et al. Pulse Shape Discrimination of La Halides. Elsevier, NIM, A540:205–208, April2005.

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Figure 7: The calibrated spectrum showing both the background spectrum of the lab as well as lines from140La, 80Br and 82Br

Figure 8: Neutron activation within the LaBr3:Ce detector at high energies

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