Nuclear Instruments and Methods in Physics Research A 629 (2011) 192–196

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Continued development of thallium bromide and related compoundsfor gamma-ray spectrometers

H. Kim a,n, A. Churilov a, G. Ciampi a, L. Cirignano a, W. Higgins a, S. Kim a, P. O’Dougherty a,F. Olschner b, Kanai Shah a

a Radiation Monitoring Devices Inc., 44 Hunt Street, Watertown, MA 02472, USAb Cremat Inc., 45 Union Street, Watertown, MA 02472, USA

a r t i c l e i n f o

Article history:

Received 18 June 2010

Received in revised form

21 October 2010

Accepted 22 October 2010Available online 29 October 2010

Keywords:

TlBr

Semiconductor

Gamma-ray spectroscopy

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.10.097

esponding author. Tel.: +1 617 668 6825; fax

ail address: hkim@rmdinc.com (H. Kim).

a b s t r a c t

Thallium bromide (TlBr) and related ternary compounds, TlBrI and TlBrCl, have been under development

for room temperature gamma-ray spectroscopy due to high density, high Z and wide bandgap of the

material. Low melting point and cubic crystal structure of selected compositions of these compounds

facilitate crystal growth by melt techniques. Recent advances in material purification, crystal growth, and

device processing have led to mobility–lifetime products of electrons in the mid 10�3 cm2/V range

enabling working detectors of greater than 15 mm thickness to be fabricated.

In this paper we report on our recent progress on TlBr detector development and first results from

TlBrxCl1�x devices. Pulse height spectra will be presented from TlBr arrays as thick as 18 mm. Depth

corrected spectra will also be presented. For a 5 mm thick TlBr array, energy resolution of less than 1%

(FWHM at 662 keV) was obtained after depth correction.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Gamma-ray spectrometers are important tools to identify andlocate special nuclear material (SNM) [1]. The requirements forgood gamma-ray spectrometers include good energy resolutionwith high detection efficiency and easy portability. Currently,scintillation spectrometers or semiconductor detectors are com-monly used for monitoring of nuclear materials. Scintillationspectrometers typically suffer from poor energy resolution whilesemiconductor detectors are costly and available in limited sizes.Even though lanthanum halide and related scintillators havesuperior energy resolution to sodium iodide, their size is stilllimited and their relatively high cost is also another drawback. Thecharge transport properties of CZT and CdTe have been noticeablyimproved over the last few decades [2]. However the relatively pooruniformity of CZT and CdTe limits detector performance and resultsin the high cost [3].

Radiation Monitoring Devices Inc. (RMD) has been exploringhigh-Z compound semiconductor detectors, which have potentialfor high-energy resolution, high detection efficiency, easy port-ability, and are of low cost. Thallium bromide (TlBr) has promisingmaterial properties for gamma-ray detection at room temperature.The combination of high density, 7.5 g/cm3, and high atomicnumber constituents (ZTl—81 and ZBr—5) provides high detection

ll rights reserved.

: +1 617 926 9980.

efficiency for gamma rays. TlBr has higher stopping power com-pared to common semiconductor materials because it has thehigher photoelectric and total attenuation coefficients over a wideenergy range from 100 keV to Z1 MeV. For example, 50% or 153%additional thickness of CZT or NaI:Tl is, respectively, required tostop equal numbers of 662 keV gamma rays [4]. TlBr is also a widebandgap semiconductor (2.7 eV); hence it can operate at roomtemperature in the same manner as CdTe or CZT. In addition,unipolar charge sensing scheme can also be applied to improveenergy resolution for large detector. At room temperature, itselectrical resistivity is sufficient (41010 O cm) [5] to limit darkcurrent, and therefore electronic noise. Furthermore, the relativelylow melting point, 480 1C, of TlBr and simple cubic crystal structureof CsCl type are other promising features. The cubic structure ofTlBr provides isotropy of thermophysical properties, which isfavorable for crystal growth. It is known that anisotropic crystals(hexagonal, tetragonal, orthorhombic, etc.) present crystal growthchallenges in terms of twinning, cracking, cleavage, and preferredcrystallographic orientations [6]. Based on these promising fea-tures, TlBr has been investigated as a detector material in the pastby several investigators [7,9–12] including us at RMD. Due to thepoor charge transport properties, however, the detector thicknesswas limited to less than 1 mm [8]. Tohoku Institute of Technologygroup showed a strong correlation between TlBr purification andits charge transport parameters [13,14]. Recent efforts at RMD haveshown that the mobility–lifetime product of electrons is approach-ing that of CdTe and CZT on sufficient purification [15]. As a result ofimproved charge transport properties, we were able to make large

H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 629 (2011) 192–196 193

volume TlBr detectors with excellent energy resolution andsensitivity [16]. Furthermore we can improve the energy resolutionby applying the depth correction method developed by Dr. ZhongHe at University of Michigan [17]. Even though good energyresolution was observed with very large volume detectors, detectorinstability was still observed [15,18]. Therefore, we are investigat-ing not only TlBr but also mixed thallium halides, thalliumbromoiodide (TlBrxI1�x) and thallium bromochloride (TlBrxCl1�x),as gamma-ray spectrometers.

Fig. 2. (a) About 0.5 mm thick TlBrI planar detectors for characterization and

(b) TlBr arrays, 5 mm thick (0.9 mm pixels and 1.0 mm pitch), and (c) 18 mm thick

(1.8 mm pixels and 2.0 mm pitch), mounted onto the circuit boards.

2. Crystal growth and device fabrication

Just like other semiconductor detector materials, there is astrong relationship between TlBr purification and its chargetransport parameters as published by the Tohoku Institute ofTechnology group [13,14]. So, rigorous purification is critical toimproving detectors. We purchased commercially available 5 N(99.999%) pure powders as starting material and then purifiedthem by multiple pass zone refining in a horizontal furnace.Purification is achieved in multiple zone refining passes as a resultof the segregation of impurities at a liquid–solid interface.

In general, most impurities have a greater solubility in the liquidphase than the solid phase. Therefore, by moving the small moltenzone from one end to the other along the ingot length, theimpurities were collected at the end of the ampoule, as shown inFig. 1. In order to increase the crystal purity, the passes wererepeated many times in the same direction. The crystal growth isperformed in a very similar manner except that the speed of aheater was reduced to get better crystallization.

In order to make the detectors shown in Fig. 2, the growncrystals were cut with a diamond wire saw and slurry. Slices werethen lapped and polished mechanically with an abrasive slurry ofAl2O3. The slices were then chemically etched with a mixture of Brand methanol solution. Finally, chrome and gold contacts weredeposited on each side by thermal evaporation through a shadowmask. To characterize detector properties, simple planar detectorswith circular shaped contacts were fabricated. Detectors were

Fig. 1. TlBrxI1�x crystals with varied compositions. (a) x¼0.35, (b) x¼0.5, (c) x¼0.65

and (d) x¼1 (TlBr). The accumulated impurities are seen in the right end of crystals.

annealed at 150 1C for 24 h in argon atmosphere in order to reducethe defects and to improve the adhesion of contacts. Fine palladiumwires were then attached to the electrodes with a carbon-basedsuspension and the detectors were mounted on ceramic substrates,as shown in Fig. 2(a). For the array detectors, a pixellated mask wasused to get desired pattern on one side of detector and a full contactwas deposited on the other side. Arrays, shown in Fig. 2(b) and (c),were similarly mounted onto specially designed circuit boards andthen connections were made between the device electrodes andthe pins on the circuit board using fine palladium wires. All outerpixels were connected to each other and grounded (as a guard ring).Energy spectra were collected from the inner pixels using pulseprocessing electronics.

3. Experimental results and discussion

3.1. Electrical characterization

The planar detectors, as shown in Fig. 2(a), were used forcharacterizing the crystal by electrical and radiation testing tomeasure the bulk resistivity and charge transport properties. Highresistivity and high charge transport properties (mt product ofelectrons and holes) are important characteristics for room tem-perature radiation detectors. The bulk resistivity was determinedfrom the detector geometry and plot of the current–voltage curvesnear the origin, as shown in Fig. 3(a) and (b). The estimated bulkresistivities were 9.9�109 and 2�1010 O cm for TlBrI and TlBrCldetectors, respectively. These estimated bulk resistivities for TlBrIand TlBrCl are one order of magnitude lower than the typical bulkresistivity of TlBr detector.

3.2. Charge transport properties

Not many researchers, surprisingly, paid attention to TlBr untilRMD published the mobility–lifetime product of electrons andholes (1.3�10�5 and 1.5�10�6 cm2/V) [8] after Hofstadterstarted to investigate TlBr as a radiation detector material in1947 [7]. By horizontal zone refining method, Shah et al. improvedthe crystal purification, and as a result the charge transportproperties. From that time, many other groups have investigatedthe improvement in crystal quality with purification [9,11],annealing [19], and surface processing effects [20,21]. With ourrecent efforts, for TlBr we obtained (mt)e of 6.5�10�3 cm2/V,

-0.2

-0.1

0

0.1

0.2

Bias (V)

Leak

age

Cur

rent

s (n

A) Data

Linear (Data)

Resistivity

-6

-4

-2

0

2

4

6

-6 -3 0 3 6

-5 0 5 10 15

Bias (V)

Leak

age

Cur

rent

s (n

A)

DataLinear (Data)

Resistivity9.9X109 �-cm

-15 -10

2X1010 �-cm

Fig. 3. I–V curves: (a) TlBrI detector (0.17 mm thick and 3 diameter electrode

contacts) and (b) TlBrCl detector (0.52 thick and 2 mm diameter electrode contacts).

Estimated bulk resistivities are 9.9�109 and 2�1010 O cm from the geometry and

I–V curves, respectively.

0

0.2

0.4

0.6

0.8

1

0Bias Voltage (V)

Cha

rge

Col

lect

ion

Effic

ienc

y

Obtained DataFitting Data

200 400

Fig. 4. Charge collection efficiency versus applied bias. The relationship is used to

estimate the mobility–lifetime product of holes in TlBr to be �4�10�4 cm2/V.

0

2000

4000

0 50 100 150 200Channel Numbers

Cou

nts

7.5 V15 V30 V40 V

109Cd spectraTlBrI0.6-mm thick

Fig. 5. Room temperature spectra of gamma-rays from 0.6 mm thick planar TlBrI

semiconductor detector.

H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 629 (2011) 192–196194

which is close to the values for CdTe and CZT [22]. With a recentlygrown crystal, we estimated the mobility–lifetime of holes with22 keV X-rays. Since the mean penetration depth of 22 keV X-raysin TlBr is about 25 mm this mean that a free path length is muchshorter than the thickness of the detectors (520 mm). Fig. 4 showsthe measurement of charge collection efficiency of holes at roomtemperature. The peak positions of 22 keV X-rays were recordedwith varying applied biases. The single-carrier Hecht equation [23]was then fit to the obtained data using mt as a fitting parameter. Asshown in the figure, the mobility–lifetime product (mt) of holes wasestimated to be approximately 4�10�4 cm2/V. This relatively highmt of holes is higher than that for CZT and very close to (mt)h

for CdTe.

3.3. Radiation test

The spectroscopic performances of detectors were evaluated byirradiating them with common g-ray sources using standard NIMpulse processing electronics: a charge sensitive preamplifier, aspectroscopy amplifier, and a pulse height analyzer.

3.3.1. TlBrI planar detector

As shown in Fig. 5, 109Cd (�22 keV primary emission) pulseheight spectra were recorded as a function of bias voltage (7.5, 15,30, and 40 V) for planar device TlBr0.35I0.65 (�0.6 mm thick, 2 mmdiameter Cr/Au contacts). Gamma source irradiated the top contact(cathode) while bias was applied to the bottom contact. Thedetector was operated at room temperature with 12 ms shapingtime and standard readout electronics were used for thismeasurement.

3.3.2. 4.3 mm thick TlBrCl array

A 4.3 mm thick, 3�3 array with (0.9 mm)2 pixels on 1 mm pitchwas mounted on a circuit board and spectra were obtained at roomtemperature with a 137Cs source from all 9 pixels as shown inFig. 6(a). The amplifier shaping time was 10 ms for all the pulseheight measurements recorded from each pixel while 200 or 400 Vwere applied to the full contact (cathode) of the pixellated detector.The leakage currents measured from the whole detector wereapproximately 20 and 40 nA at 200 (dotted line) and 400 V (solidline), respectively. The 662 keV full energy peak and the Tl escapepeak are clearly observed for each pixel at a bias of 400 V.

One pixel of the 4.3 mm thick TlBrCl detector exhibited anenergy resolution of 2.3% FWHM for 662 keV gamma rays. InFig. 6(b), pulse height spectra are shown from one pixel as afunction of bias voltage at room temperature.

A 662 keV photopeak is clearly resolved at bias voltage above400 V and 3.4%, 2.9%, and 2.8% energy resolutions were measured at400, 600, and 800 V, respectively.

3.3.3. 18 mm thick TlBr array

Due to improved purification and fabrication techniques, we areable to fabricate a working 10 mm�8 mm�18 mm TlBr arraydetector (see Fig. 2(c)). An 18 mm thick, 3�4 array with (1.8 mm)2

pixels on 2.0 mm pitch was fabricated and mounted on a circuitboard. Spectroscopic performance with gamma rays was evaluatedat room temperature without any further correction. The cathodeside was irradiated with two gamma sources, 241Am and 137Cs,while 1950 and 2200 V were applied to the full contact. Amplifiershaping times of 10 and 12 ms were selected for 241Am and 137Cs,respectively.

0

100

200

300

0 300 600 900 1200Channel Number

Cou

nts

200V400V600V800V

#TlBrCl (3x3 array)#6 Pixel

200V400V

#1 Pixel

200V400V

#2 Pixel200V400V

#3 Pixel

200V400V

#4 Pixel 200V400V

#5 Pixel200V400V

#6 Pixel

200V400V

#7 Pixel

200V400V

#8 Pixel200V400V

#9 Pixel

Fig. 6. (a) Room temperature spectra of 662 keV gamma rays from all 9 pixels

4.3 mm thick TlBrCl semiconductor detector. The energy spectra were recorded

when 200 (dotted line) and 400 V (solid line) were applied to the full contact.

(b) 137Cs pulse height spectra as a function of bias voltage recorded at room

temperature with one pixel of a 4.3 mm thick TlBrCl array.

0

300

600

1000Energy (keV)

Cou

nts

TlBr 3x4 array241

Am spectrum

0

300

600

10005000Energy (keV)

Cou

nts

TlBr 3x4 array137

Cs spectrum

200

Fig. 7. 241Am (a) and 137Cs (b) spectra obtained from one pixel of 18 mm thick TlBr

semiconductor detector at room temperature.

0

500

1000

0 300 900Energy (keV)

Cou

nts

CorrectedUncorrected

TlBr 3x3 Array#1 Pixel

Kα and Kβ escape peaks

600

Fig. 8. 137Cs spectra before and after depth correction for one pixel of a 5.14 mm

thick TlBr array.

H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 629 (2011) 192–196 195

Fig. 7(a) shows an 241Am spectrum obtained from a pixel of the18 mm thick TlBr array. The device was biased to 1950 V andoperated at room temperature. Presence of the 60 keV photopeakindicates that the full thickness of the device is active since mostsignal induction occurs when electrons reach the anode. Note thatthe mean penetration depth of 60 keV photons is approximately300 mm in TlBr; thus most charge generation occurs near thecathode and electrons must traverse nearly the full device thick-ness to generate a photopeak signal. For the gamma energy of662 keV, the peak is clearly seen, as shown in Fig. 7(b).

3.4. Depth correction

A depth correction technique valid for single interaction events[17] was applied to 137Cs spectra to compensate for depthdependence of signal induction. The ratio of cathode signal toanode signal is used as a depth parameter. Photopeaks for eachdepth parameter are aligned and the aligned spectra are summedup to give a corrected spectrum.

In Fig. 8, a depth corrected 137Cs spectrum is shown for one pixel ofa 5.14 mm thick TlBr array with 1 mm pitch. The array was cooled to –18 1C. A bias voltage of 700 V was applied and the amplifier shapingtime was 16 ms for cathode and anode. As shown in Fig. 8, Ka and Kb

escape peaks are separated in the corrected spectrum. The energyresolution of the 662 keV peak is o1% (FWHM).

4. Conclusion

Thallium bromide (TlBr) and related ternary compounds, TlBrIand TlBrCl, were successfully purified by multi-pass zone refiningand good quality crystals were grown by zone melting andrecrystallization at a slower rate as a last step in the zone refinerto reduce handling and possible contamination.

Planar detectors were used for characterizing the bulk resistiv-ity and charge transport properties of the crystals. The estimatedbulk resistivities were 9.9�109 and 2�1010 O cm for TlBrI andTlBrCl detectors, respectively, and these estimated bulk resistiv-ities are one order of magnitude lower than the typical bulkresistivity of TlBr. With 0.6 mm thick planar TlBr0.35I0.65, 22 keVpulse height spectra from 109Cd source were recorded at room

H. Kim et al. / Nuclear Instruments and Methods in Physics Research A 629 (2011) 192–196196

temperature as a function of bias voltage. With 4.3 mm thick TlBrClarray (with 0.9 mm�0.9 mm pixels, 1 mm pitch), 662 keV energyspectra were successfully obtained from all 9 pixels at roomtemperature. The 662 keV gamma-ray peaks with the escape peaksfrom Tl atom were clearly observed at 400 V. One pixel of TlBrClarray exhibited an energy resolution of 2.3% FWHM at 662 keV.Recent improvements in material purification and detector proces-sing allowed the fabrication of 18 mm thick TlBr array thatexhibited a photopeak when irradiated with 60 keV photons from241Am. The 60 keV photopeak indicates that the full device thick-ness is active. Also, mobility–lifetime product (mt) of holes as highas 4�10�4 cm2/V has been obtained from highly purified TlBr. Thisapproaches the (mt)h value for cadmium telluride (CdTe).

By applying the depth correction method, much improvedenergy spectrum (with the energy resolution less than 1%) froma 5.14 mm thick TlBr array was obtained at 662 keV. In addition,two clearly separated escape peaks from thallium Ka and thalliumKb were shown.

Acknowledgment

This work was supported by the Department of HomelandSecurity/Domestic Nuclear Detection Office (DHS/DNDO) and theDefense Threat Reduction Agency (DTRA).

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