Journal of the Korean Physical Society, Vol. 63, No. 7, October 2013, pp. 1466∼1472

Scintillation Properties and Luminescence Response of the ZnWO4 Crystalto γ and Proton Beams

Sang Jun Kang∗

School of Liberal Arts, Semyung University, Jechon 390-711, Korea

Y. S. Hwang, J. M. Park and G. H. Chae

Department of Physics, Kyungpook National University, Daegu 702-701, Korea

Sunghwan Kim

Department of Radiological Science, Cheongju University, Cheongju 360-764, Korea

J. K. Cheon

Department of Radiation, Sorabol College, Gyungju 780-711, Korea

(Received 26 December 2012, in final form 25 June 2013)

We investigated the scintillation properties, including the energy resolution, the pulse height spec-trum, and the decay time, of ZnWO4 (ZWO) crystals by using a 661-keV γ-ray source and protonbeams with various energies. The luminescence response of the ZWO crystal to the energy of theproton beam was measured with proton beam energies in the range from 20 MeV to 35 MeV byusing the MC-50 cyclotron at the Korea Institute of Radiological and Medical Sciences (KIRAMS)with aluminum degraders of different thicknesses. We also measured the proton luminescence spec-trum of the ZWO crystal. For the determination of the proton-beam energy incident on the crystalafter it had passed through the degrader and 270 cm of air, we used a Monte Carlo simulation basedon the Geant4 code. We calculated the differential stopping power, dE/dr, of the ZWO crystal forthe available proton beam energy by using a simulation with the SRIM code. Using these simulatedand the experimental results, we calculated the Birks parameter, kB, of the semi-empirical Birks

law, dLdr

=A dE

dr

1+kB dEdr

.

PACS numbers: 29.27.Fh, 29.40.McKeywords: ZWO crystal, Birks law, Scintillation detector, Light response, Proton beamDOI: 10.3938/jkps.63.1466

I. INTRODUCTION

Scintillation detectors are being used in many fields,such as high-energy physics, nuclear instrumentation, ra-diation measurements, medical imaging, etc. For mostapplications, good-performing scintillation crystals aredemanded and should have good scintillation propertiessuch as fast decay time, high light output, high density,and high Z-number [1].

Many studies have been performed to develop an ef-fective scanning system for X-ray imaging, such as X-raytomography [2]. Among the several candidate scintilla-tors, the CdWO4 (CWO) crystal has many advantagesover other crystals for effective X-ray scanning. It has aneffective Z number of � 61 because of the two materials

∗E-mail: sjkang@semyung.ac.kr

Cd and W and a high density (7.9 g/cm3). The strongradiation hardness (103 Gy) of the CWO crystal makesit a widely-used crystal in X-ray detection, such as incomputer tomography (CT) and airport security [3].

The CWO crystal has other advantages also, such aschemical stability and non-hygroscopicity, because it isan oxide crystal. Nevertheless, it has a disadvantage be-cause of it’s poisonous chemical component, cadmiumCd. Thus, we studied ZnWO4 (ZWO) crystals as analternative to CWO. The ZWO crystal has propertiesrather similar to those of CdWO4, CaWO4, and someother oxide crystals [4].

The amount of light produced in a scintillation crystalby highly-ionizing particles is known to be lower thanthat produced by electrons of the same energy [5]. Thus,if the light signals from a scintillation crystal producedby ions are calibrated with that of an electron or a γsource, they show lower energy than that of an ion re-

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Scintillation Properties and Luminescence Response of· · · – Sang Jun Kang et al. -1467-

ally produced in the crystal. The transformation factorbetween the energies really produced by ions and theenergies of the light output from the scintillation crys-tals is known as the quenching factor. Knowledge of thequenching factors of the scintillation crystals those arebeing used or going to be used for some experiments orapplication is very important.

Among a few approaches to calculating quenching fac-tors, the Birks approach (Birks semi-empirical law) isknown to be simple. Because this law contains only oneparameter (the Birks factor kB), which might be fittedto calculate the quenching factors, once the Birks param-eter can be found by fitting experimental data measuredfor particles of one kind in some energy range, then itcan be used to calculate the quenching factors for otherparticles and in other energy region. In this paper, wecalculated the Birks parameter of the ZWO crystal for aproton beam in energy range from 20 MeV to 35 MeV.To fit the Birks parameter, we measured the scintilla-tion properties, including the energy resolution, the pulseheight spectrum, and the decay time, of ZWO crystal byusing a 662-keV γ-ray source and proton beams withvarious energies from 20 MeV to 35 MeV from the MC-50 cyclotron at the Korea Institute of Radiological andMedical Sciences (KIRAMS) with an aluminum degraderof variable thickness.

We also measured the proton luminescence spectrumof the ZWO crystal. The proton-beam energy incidenton the crystal after it had passed through the degraderand 270 cm of air was calculated by using a Monte Carlosimulation based on the Geant 4 code. We calculated thedifferential stopping power, dE

dr , of the ZWO crystal forthe available proton beam energy by using a SRIM codesimulation. By using these results from the simulationand those from experiments, we calculated the Birks pa-rameter kB of the semi-empirical Birks law by using anumerical method.

II. QUENCHING FACTOR AND BIRKSPARAMETER

The light yield of a scintillation crystal depends notonly on the energy of particle E but also on its stoppingpower dE/dr in the crystal. If the excitation centerscreated in a crystal are spaced at large distances and theinteractions between them can be neglected, for particleswith low stopping power, the light yield dL is known tobe proportional to the released energy dE. This can beexpressed in differential form as

dL

dr= A

dE

dr, (1)

where A is the absolute scintillation efficiency. If thelowering of the light yield for highly-ionizing particles(protons, α particles and heavy nuclei, which are calledheavy ions) is to be accounted for, Eq. (1) might contain

some terms associated with the stopping power, whichwould make the equation non-linear. Birks proposed hissemi-empirical law as

dL

dr=

AdEdr

1 + kB dEdr

, (2)

where B(dE/dr) is the density of ionization, B is a pro-portionality constant, and k is the fraction of ionizingparticles, leading to a quenching and lowering of the scin-tillation effect [5]. Practically, the product kB is treatedas an adjustable parameter to fit the experimental datafor a specific scintillator [1]. In general, Eq. (2) gives thelight yield as

L(E) =∫ E

0

dL =∫ E

0

AdE

1 + kB dEdr

. (3)

The quenching factor for ions is the ratio of the lightyield of ions, Li(E), to that of electrons of the sameenergy, Le(E):

Qi(E) =Li(E)Le(E)

=

∫ E

0AdE

1+kB( dEdr )i∫ E

0AdE

1+kB( dEdr )e

=

∫ E

0dE

1+kB( dEdr )i∫ E

0dE

1+kB( dEdr )e

.

(4)

In Eq. (4), the factor A disappears in the ratio, andQi(E) depends only on a single parameter, kB, the Birksparameter. Frequently, instead of the quenching factor,a relative light output, the ratio of Li to the energy E,normalized to ratio Le for an electron or a γ, at the someenergy E0 is used:

Ri(E) =Li(E)/E

Le(E0)/E0. (5)

Equation (5) can be written as

Ri(E) =Li(E)Le(E)

Le(E)/E

Le(E0)/E0= Qi(E)

Le(E)/E

Le(E0)/E0. (6)

From Eq. (5), we can find the electron equivalent energyof the light output as

Eeq =Li(E)Le(E0)

E0 = RiE. (7)

Equation (6) shows that when E and E0 are in the energyregion where the electron light yield Le is proportionalto energy, Ri(E) is the same as Qi(E) [4]; thus, Eq. (7)can be written as

Eeq =Li(E)Le(E0)

E0 = RiE = QiE . (8)

If we know the electron equivalent energy of the lightyield and the dE/dr is a known function of E, then wecan calculate the Birks parameter from Eq. (3). The

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Fig. 1. (Color online) Schematic of the experimental setupfor measuring the scintillation properties of a ZWO crystalirradiated with 662-keV γ−rays.

electron equivalent energy of the light yield of a crystalfor some ion can be obtained by using experimental mea-surements. However, unfortunately, the stopping powerof a crystal, dE/dr, is not known in general. Someonewho wants to find the Birks parameter must find thedifferential stopping power dE/dr. We studied the lightresponse of the CsI(CO3) crystal to the proton energy.In that work, we could calculate only approximately thedependency of the electron equivalent energy of the lightoutput on the ion energy in narrow range without thedifferential stopping power dE/dr [6]. In this paper, wecalculate the differential stopping power dE/dr of theZWO crystal for protons with available beam energiesby using SRIM code simulations.

III. EXPERIMENTS

1. Measurements of the Scintillation Propertiesof ZWO with 662-keV γ-rays

We prepared a ZWO crystal sample with dimensionsof 4 × 7 × 30 mm3. Except for one end surface, allthe surfaces of the crystal sample were wrapped with0.4-mm-thick Teflon tape, followed by black tape. Theunwrapped surface of the crystal sample was coupled di-rectly to the surface of a 2-inch high-gain photomultipliertube (PMT, Photonis XP2260) by using optical grease.

We connected the PMT to an ORTEC 570 shapingamplifier; then the shaping amplifier to a 25-MHz FADC(flash analog-to-digital converter). Finally, the 25-MHzFADC was connected to a personal computer throughthe Ethernet. We supplied a high voltage of −1400 V tothe PMT. The analog output signal of the PMT was sentto the ORTEC 570 shaping amplifier. We set the shap-ing time to 10 µs, the coarse gain was set to 200, and thefine gain was adjusted to 0.5. The amplified signals fromthe shaping amplifier were fed to the 25-MHz FADC. Asoftware threshold setting was applied to trigger an event

Fig. 2. (Color online) Schematic of the experimental setupfor measuring the luminescence response of a ZWO crystal toa proton beam.

by using a self-trigger algorithm on the field-programablegate array (FPGA) chip of the FADC board. The FADCdata were recorded by using a personal computer andwere analyzed with a C++ data analysis program basedon the ROOT package [7]. A schematic of the experi-mental setup is shown in Fig. 1.

The ZWO sample attached to the PMT were put intoa stainless-steel tube to shield against background radia-tion. Then, we measured the pulse height spectrum anddetermined the energy resolution of the ZWO crystal byusing 662-keV γ−rays.

2. Measurement of Scintillation Properties ofZWO for the Proton Beam

To measure the pulse height spectrum and the energyresolution of a ZWO crystal for proton beams, we coveredthe Teflon-wrapped ZWO sample coupled to the surfaceof PMT with a 1-mm-thick stainless-steel tube to shieldagainst high-energy protons. We collimated the protonbeam line to a small spot on the ZWO sample by using a1-mm-diameter lead collimator. We connected the PMTto the ORTEC 570 shaping amplifier, 25-MHz FADC,and computer, as mentioned above. We supplied a highvoltage of −1400 V to the PMT and set the ORTEC 570shaping amplifier to a shaping time of 10 µs, a coarsegain of 20, and a fine gain of 0.5. We used the 45-MeVproton-beam test facility at the MC-50 cyclotron of theKIRAMS [8,9].

The 45-MeV proton beam from the MC-50 cyclotronpassed through a 0.2-cm-thick aluminum window cap-ping the beam pipe, a variable thickness of Al degrader,and 270-cm-thick air to arrive at our sample. By chang-ing the thickness of the degrader, we controlled the pro-ton energy at the sample’s position. We used an energyrange from 20 MeV to 35 MeV. A schematic of the ex-perimental setup is shown in Fig. 2.

We also measured the pulse height spectrum with aknown 662-keV 137Cs γ-ray source to get the energy cal-ibration standard. In that case, we set the gain of theshaping amplifier to 200 and the shaping time to 10 µs.We supplied a high voltage of −1400 V, and the otherparameters were set to the same values as those for the

Scintillation Properties and Luminescence Response of· · · – Sang Jun Kang et al. -1469-

Fig. 3. (Color online) Fluorescence decay time spectrumobtained from the ZWO single crystal at room temperature.

proton beams. The proton beam energy at the sample’sposition was calculated by using a Monte Carlo simula-tion based on the Geant4 simulation toolkit [10,11]. Theresults of the calculation are shown in Table1.

IV. EXPERIMENTAL RESULTS

1. Fluorescence Decay Time

The fluorescence decay times of the ZWO crystal sam-ple were measured at room temperature under 137Cs γ-rays by using a single-photon counting technique. Forthis measurement, we used a 400-MHz FADC, instead ofa 25-MHz FADC, and shaping amplifier [12]. The mea-sured decay time curve of our ZWO crystal sample isshown in Fig. 3. The decay time curve was fitted with atime constant of 18 µs.

2. Pulse Height Spectrum and Energy Resolu-tion for 661-keV γ-rays

The energy resolution is the most important factor formeasuring incident radiation precisely. The energy res-olution is specified as R = ∆E/E, where ∆E is the fullwidth at half maximum (FWHM) in the pulse heightspectrum. Figure 4 shows the pulse height spectrumof the ZWO crystal for 662-keV γ-rays from 137Cs .The measured energy resolution is 13.3% FWHM with aGaussian fitting.

3. Pulse Height Spectrum and Energy Resolu-tion of the ZWO Single Crystal for Proton Beams of34.2 MeV

Fig. 4. (Color online) Pulse height spectrum and energyresolution of the ZWO single crystal irradiated with 137Cs662-keV γ-rays.

Fig. 5. (Color online) Pulse height spectrum and energyresolution of the ZWO single crystal for a 34.2-MeV protonbeam.

The measured pulse height spectrum and energy res-olution of the ZWO single crystal irradiated by a 34.2-MeV proton beam is shown as Fig. 5. Figure 5 showsthat the energy resolution of the ZWO single crystal is10.82% FWHM. We set the gain of the shaping amplifierto 20 to avoid signal saturation for this and the followingmeasurements of proton beams. When we measured thepulse height spectrum of the ZWO single crystal with662 keV γ−ray from a 137Cs source to get the calibra-tion constant, we set the gain of the shaping amplifier to200, as mentioned earlier.

We get the energy calibration constant by comparingthe pulse height spectrum of the ZWO single crystal for662-keV γ−rays to that for proton beams:

k =γ energyγ peak

× γ gainproton gain

. (9)

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Table 1. Various values of the proton beam energies at the sample position, as calculated by using a Monte Carlo simulationbased on the Geant4 simulation toolkit.

Thickness of the Degrader (mm) Proton Energy (MeV) Energy Spread FWHM (%)

0.0 34.2 2.13

0.5 32.4 2.76

1.0 30.6 2.84

1.5 28.6 3.20

2.0 26.6 3.80

2.5 24.4 4.53

3.0 22.0 5.45

3.5 19.4 7.03

Fig. 6. (Color online) Electron equivalent energy spectrumof the ZWO single crystal for proton beams in the energyrange of 19.4 MeV to 34.2 MeV.

The pulse height spectrum and the energy resolution ofthe ZWO single crystal for proton beams of various en-ergies were measured in the energy range from 19.4 MeVto 34.2 MeV, which were supplied by using the MC-50cyclotron of the KIRAMS with a variable-thickness alu-minum degrader.

We got the electron equivalent energy spectrum bymultiplying the pulse height spectral peaks by the cali-bration factor k. The results are shown in Fig. 6 andTable2, where the values of the proton energy are meanvalues calculated from the simulation, and the values ofthe luminescence response as electron equivalent energyare the light output of the crystal.

From the values in Table 2, we can see the dependencyof the energy resolution of the ZWO crystals on the pro-ton energy. Figure 7 shows that the energy resolutionof the ZWO single crystal becomes better as the protonenergy increases because the scintillation light yield in-creases. The proton energy spread becomes larger as theproton energy decreases because of the thick degrader.Thus, the energy resolution at low proton energy is dueto a combination of the intrinsic energy resolution causedby photo-electron statistics and the energy spread.

Fig. 7. Dependency of the energy resolution of the ZWOsingle crystal on the proton energy.

V. CALCULATED BIRKS PARAMETER KB

In the previous section, we showed the experimen-tal results for the scintillation properties of the ZWOcrystal, including the luminescence response as electronequivalent energy. We calculated the differential stop-ping power dE/dr of the ZWO crystal for the avail-able proton beam energy by using the SRIM code sim-ulation. By using these results of the simulation andthose of the experiments, we calculated the Birks pa-rameter kB of the ZWO crystal for protons to be0.014 ± 0.003 (g/MeVcm2) by using a numericalmethod. For this work, we developed a calculation pro-gram based on the ROOT package [7]. The result isshown in Fig. 8.

The fitted Birks parameter allows us to calculate thequenching factors of the ZWO crystal for various protonenergies, which are shown in Fig. 9. The blue squarepoints in Fig 9 are experimental data, and the red cir-cular ones are calculated by using Birks semi-empiricallaw with kB = 0.014.

Scintillation Properties and Luminescence Response of· · · – Sang Jun Kang et al. -1471-

Table 2. Electron equivalent energy of the luminescence response of ZWO to proton beams with energies in the range of 19.4MeV ∼ 34.2 MeV.

Proton Energy Proton Energy Spread Light Response Eeq Spread

(MeV) FWHM (%) Electron Equivalent Energy (MeVee) FWHM (%)

34.2 2.13 27.56 7.99

32.4 2.76 25.55 10.22

30.6 2.84 22.76 14.46

28.6 3.20 20.98 16.26

26.6 3.80 18.29 22.19

24.4 4.53 15.97 27.37

22.0 5.45 12.88 36.97

19.4 7.03 9.15 61.05

Fig. 8. (Color online) The Birks parameter of the ZWOcrystal for the proton energy range from 24 MeV to 34 MeVis fitted as 0.014 (g/MeVcm2).

VI. CONCLUSION AND DISCUSSIONS

We investigated the scintillation properties, includingthe energy resolution, pulse height spectrum, and thedecay time, of ZWO crystals by using a 661-keV γ-raysource and proton beams with various energies. The lu-minescence response of the ZWO crystal to the energy ofthe proton beam was measured for proton beam energiesin the range from 19 MeV to 35 MeV. We also measuredthe proton luminescence spectrum of the ZWO crystal.The proton beam energy incident on the crystal after ithad passed through the degrader and 270 cm of air wascalculated by using a Monte Carlo simulation based onthe Geant 4 code. We calculated the differential stop-ping power dE/dx of the ZWO crystal for the availableproton beam energy by using a SRIM code simulation.

We developed a calculation program based on theROOT package to fit the Birks parameter kB. As shownin Fig. 8, we found the Birks parameter of the ZWOcrystal for proton energy range from 19.4-MeV to 34.2-

Fig. 9. (Color online) Quenching factor of the ZWO crystalfor the proton energy range from 20 MeV to 50 MeV. Theblue square points are experimental data, and the red circlesare calculated by using Birks semi-empirical law with kB =0.014.

MeV to bw 0.014 ± 0.003 (g/MeVcm2). The quenchingfactors of ZWO for various proton energies were calcu-lated by using the fitted Birks parameter. The calcu-lated quenching factors and those from experimental re-sults for the ZWO crystal for various proton energies areshown in Fig. 9. Because of the uncertainty of the pro-ton energies, even throgh within error-bars, a small biasin proton energies, as shown in Table 2, may exist be-cause of the Teplon tape and black tape, which are notconsidered for the proton energy incident on the crystal.When the proton energy becomes smaller, the differencebetween the quenching factor obtained by using Birksformula and the experimental values gets larger, and theerror bars on the experimental values for the quenchingfactors become larger as shown in Fig. 9. Thus, the pre-dicted points in Fig. 9, the red circular points, may beverified by later experiments.

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ACKNOWLEDGMENTS

This research was supported by the Basic Sci-ence Research Program through the National Re-search Foundation of Korea (NRF) funded by theMinistry of Education, Science and Technology(No.2012R1A1A4A01009943).

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