Nuclear Instruments and Methods in Physics Research B61 (1991) 491-496 North-Holland

491

Excellent radiation-resistivi~ of ccriu~-doped gadoliniu~ silicate scintillators

Masaaki Kobayashi KEK National Laboratory for High Energy Physics, Oho, Tsukuba, 305 Japan

Mitsuru Ishii Shonan Institute of Technology, Tsujido Nishikaigan, Fujisawa, 251 Japan

Received 1 May 1991

Radiation-resistivity of a fast and high-intensity scintillator GSO(Ce) was measured for 6oCo y-rays of 104-IO9 rad. No sizable radiation-damage was observed up to 10’ rad irrespective of the amount of cerium between 0.5% and 2.5% in mol.

1. Introduction

Cerium-doped gadolinium silicate (Gd,SiO,; GSO(Ce)) is known [l-4] to be one of the excellent materiafs for total-absorption electromagnetic (EM) calorimeters; it is dense with the unit radiation length of 1.38 cm, has no hygroscopicity and gives large light- output - as much as 20% of that in NaI(TI). The decay time of scintillation, TV, which is about 60 ns for a Ce admixture of 0.5% in mol, is decreased down to 30 ns for Ce amount of 2.5%. The performance of a large GSO single crystal has recently been examined with an e-/Y beam giving a satisfactory result [S]. The above situation indicates that if the radiation hardness should also be excellent, GSO could be useful in physics experiments even at very high energies and/or at very high beam intensities, which may be obtained in the near-future accelerators such as the SSC, the LHC, the UNK, the kaon factories, etc.

Though no degradation [2] has been observed in a GSO crystal doped with 0.5% Ce against low energy y-rays from 6oCo up to lo4 rad, we need to examine higher-level irradiation. Since the GSO crystals get, visually, a brownish tint as the amount of Ce is in- creased from 0.5% to 2.5%, it is also necessary to check whether the increased Ce might result in more significant radiation damage. We have measured the radiation damage of GSO(Ce) crystals doped with vari- ous amount of Ce from 0.5% to 2.5% against the accumulated dose of lo4 to lo9 rad of 6oCo y-rays.

2. Measurement and results

Ail the GSO crystals [6] used in the present test is rectangular in shape with a size of 1 X 1 X 3 cm3. Four surfaces out of six in total were polished to optical grade, including a pair of opposite 1 X 3 cm2 surfaces through which the transmission measurement was done. The transmission spectra were measured across the l-cm thickness with a spectrophotometer (Hitachi 330).

Irradiation with “Co y-rays was carried out at Japan Atomic Energy Research Institute (JAERI). The irradiation period was kept typically at one hour for lo4 rad, 42 hours for 10’ rad and 330 hours for 10’ rad by mounting the GSO crystals at different distances from the 6oCo source. The exposure rate [7] was lo4 R/h for an accumulated dose of lo4 rad, lo5 R/h for lo5 rad, 5 x lo5 R/h for both lo6 and 10’ rad, 2.3 x lo6 R/h for lo8 rad and 4.3 X lo6 R/h for lo9 rad. The irradiated samples were sent to KEK and the transmission measurement was carried out a few days after irradiation. One cycle of irradiation and transmis- sion measurement took about a week, including the mailing of the samples between JAERI and KEK, except about 20 days for the highest dose of 10’ rad. Five cycles were repeated for all the five samples so that the dose could be increased from lo4 to lo8 rad by a factor of 10 per cycle. Three samples out of five were further irradiated up to lo9 rad. Since the radia- tion damage in some other crystals such as CsI(T1) IS], PbF, [9], etc. is known to recover upon annealing with

0168-583X/91/$03.50 0 1991 - Elsevier Science Publishers B.V. All rights reserved

492 MI Kobuyushi, M Ishii / resistiviry of GSOfCel

0 107

_...._ . -‘.

lo8 rad i ” i

GSO(Ce:0.5 %I “Oco -Y No. 2

q~ 107 O.%i-

, i

fa) 4 ibf

01, ,A / 2 / 1 ia I -_I 400 500 600 400 500 ---?

WAVELENGTH Cm-n ) WAVELENGTH (nmf

Fig. 1. Transmission spectra across l-cm thickness of two GSO crystals doped with 0..5% Ce (in mol) for different accumulated doses. Solid c~urve: before irradiation. All the curves are normalized at 600 nm (see the text).

ultraviolet (?..JV) light, we paid attention lest the crys- tals should be exposed to strong UV-light, including sunshine, during any course of the irradiation-measure- ment cycles. Each GSO crystal was usualty wrapped with paper or aluminium foil except during the trans- mission measurement which took about 15 min per each measurement per sample.

The transmission spectra of the five GSO crystals doped with various amount of Ce are given in figs. 1 and 2 for different accumulated doses, i.e. 0 (before irradiations, lo7 and 10” rad, and also for 10’ rad for three samples. Since the spectra for 104, 10’ and 10’ rad have fallen very close to those before irradiation within I%, for simplicity they are not shown. The spectra for different doses were normalized at 600 nm, since the transmittance was the same at this wave- length to within the reproducibility of i: 1% which is due to calibration error of the s~ectrophotometer and co small positioning and tilting errors of the crystal from measurement to measurement. The transmission spectra of two CSO crystals doped with 0.5% Ce are given in fig. 1. Both the crystals have shown similar behaviour against irradiation as can be seen in fig. la and lb. We do not see any sizable degradation in transmission up to 10’ rad. Though degradation within l%/cm at 10s rad and lS%/cm at 10” rad may not be excluded, it wiil not be difficult to monitor the

degradation and correct it in actual physics experi- ments if the degradation occurs slowly. The results for the crystals doped with 1.5%, 2% and 2.5% Ce are given in fig. 2a, 2b and 2c, respectively. We hardly see either any sizable degradation.

The excitation and emission spectra [IO] have been measured with a spectra-fluoro-photometer (Shimadzu RF-5101 before any irradiation and after irradiation by 10s rad. The result is given in fig. 3 for a crystal doped with 0.5% Ce and in figs. 4a and 4b for crystals with 1.5% and 2,.S% Ce, respectively. We do not see any significant effect of irradiation on the excitation-emis- sion characteristics within a normalization ambiguity as large as 10% which comes from reproducibility error in positioning the crystal on the spectra-~Lloro-photome- ter.

We have compared the background y-rays in the GSO crystals before and after irradiation. Measure- ment was done for the five irradiated GSO samples as well as for the other virgin GSO samples before irradi- ation. Each crystal was mounted on a 2-in. photomu~ti- plier (Hamamatsu, R329) as sketched in the inset of fig. 5a. Since no special shielding was made, the ob- tained spectra may include intrinsic y-rays from the

M. Kobayash~, M. I&ii / Rff~i~tio~ res~ti~~~ of GSOfCe) 493

GSO, cosmic rays, and y-rays as well as neutrons in the MeV and 200 events/h above 0.5 MeV, they are simi- room background. Fig. 5b gives a background y-ray lar to the room background level. They were also the spectrum in the GSO crystal doped with 0.5% Ce (no. same within +20% for all the five irradiated GSO 2 sample shown in fig. lb) after irradiation by lo9 rad. samples irrespective of the Ce amount and of the The specrum was similar for virgin GSO crystals doped accumulated dose of 10’ or lo9 rad. with 0.5% Ce. The obtained background rate was about In the above measurement, the energy scale was set 1000 events/h above 0.2 MeV, 600 events/h above 0.3 by measuring y-rays of known energies. Fig 5a gives

0.81

0.2 -

0.. 400 500 600

WAVELENGTH (nm)

‘~~oIo;.;I

0 ’ /--

\ lo8 rad

~ i 0.2 /

(b) -

0 $4 I / I I I 400 500 600

WAVELENGTH (nm)

0.8

0 400 500 600

WAVELENGTH (nm)

Fig. 2. Transmission spectra across l-cm thickness of GSO crystals doped with (a) 1.5%, (b) 2% and (cl 2.5% Ce for different accumulated doses. Solid curve: before irradiation. All the curves are normalized at 600 nm (see the text).

494 M. Kohuyashi, M. Ishii / Radiation resistivity of GSO(Ce)

WAVELENGTH (nm)

Fig. 3. Excitation-emission spectra of a GSOfCel sample II x 1 x 3 cm31 doped with 0.5% Ce are compared between the accumulated dose af zero (solid and chained curves) and IO” rad <solid points). A, and Xem are excitation and emission

wavelengths? respectively. The measuring setup is schematically sketched in the inset.

200 300 400 500 600

WAVELENGTH (nm)

0 200

WAVELENGTH (nm)

Fig. 4. Excitati~~-~~~ssj~~ spectra of a GSO(Ce) sample (1 x 1 x 3 cm33 doped with fa) 1.5% or fl$ LW% Ce are compared ktwem

the accumulated dose of zero (s&d and chained curvesf and IO8 rad fsdid pointsf.

M. Kobayashi, M. Ishii / Radiation resistiuity of GSO(Ce) 495

an energy spectrum of y-rays from 13’Cs obtained in the GSO doped with 0.5% Ce (no. 2 sample) after irradiation by 10” rad. The typical energy resolution of 14% in the fullwidth at half-maximum (fwhm) was similar as in the samples before irradiation.

The above result, i.e. the absence of irradiation effect on the background y-ray rate within the fluctua- tion of the room background level, is consistent with an intuitive consideration that irradiation with low-energy y-rays should not cause any significant radioactivities due to the lack of sufficient energy transfer.

3. Summary and discussions

Single crystals of GSO doped with Ce have shown excellent radiation-resistivity against low energy y-rays. No sizable degradation in transmission has been ob-

served up to 10” rad, the maximum dose tested, irre- spective of the Ce amount between 0.5% and 2.5%. No sizable degradation has been observed either in the excitation-emission characteristics up to 10s rad, nor in the background y-ray rate up to 10’ rad. The above result indicates that, though the fast GSO crystals doped with 2.5% Ce (T,, _ 30 ns) has a brownish tint, they are as strong as the normal crystals doped with 0.5% Ce (rd _ 60 ns) which is visually cleaner. Conse- quently, GSO(Ce1 should be one of the best candidates of high-speed (rd N 30 ns), dense, large light-output, and highly radiation-resistant materials for EM calorimeters at high energy and/or intense beam envi- ronments. It is to be noted that, since the transmission spectra were measured a few days after irradiation, we cannot exclude a possibility of quick recovery (much faster than hours) of radiation-damage.

GSO seems to be more radiation-resistant than

A (a) GSO with ‘37Cs t h ti

FWHMzO.14

‘37cs Isotope /

GSO

‘,“rs3

> inch PMT

(b) GSO only

Ce : 0.5% .

5 80-. - ?

t 60- Z

p 40- = Z

20-

10' radf’Co- Y)

Average over 110 hr?

z 1.4MeV

1 .

0.4 0.6 Q8 1 1.2 1.4

Y-RAY ENERGY (MeV)

Fig. 5. (a) Gamma-ray spectrum from 13’Cs and (b) the background y-ray spectrum in the GSO crystal doped with 0.5% Ce (no. 2 , sample shown in fig. lb) after irradiation by lo9 rad. The bin width is 0.0075 MeV in (a) and twice that in (b). An ADC (Lecroy

QVT) was used in the charge mode. The pedestal is already subtracted.

496 M. Kohaynshi, M. Ishii / Radiation resistivity of GSO(Ce)

BaF,[XI], which is resistant up to lO”-IQ7 rad and has been known to be one of the most radiation-resistant scintiilators for EM calorimeters. It is not yet clear whether the excellent rad~atjon-resj~t~v~ty of GSO should come from the purity of the crystals or from any other effects. In this respect, it may be worth noting that admixure of a smalf amount of CeG, (of the order of 0.1% in mol) has been ~xpcrimentall~ known flZ] to prevent the y-ray induced colouring in ordinary glasses consisting of K,O, BaO and SiO,. According to Soga and Tashiro [12], Ce +s ions should play the role of catalyst to help recombination of the positive holes and the ionized electrons which may be produced by irradi- ation and become the colour centres; Ce”’ ions give their weak-bonded 4f-electrons to the oxygen ions, and the Ce” .7 ions, now holding positive holes, catch the

ionized electrons trapped by oxygen-ion vacancies. It may be interesting to compare the Ce-doped GSU with undoped GSU with respect to the radiation damage.

CeF,, which has been developed recently [13.14], has a similar decay time frd - 30 ns) of scintillation as GSO and is not hygroscopic similarly as GSO. CeF:, should also be promising since it is radiation-resistant up to 10” rad. Though a slight damage is observed at 10’ rad? it recovers with an exponential time constant of about 40 days. Consequently, CeFj could also be used safely up to 10’ rad, even up to 10” rad if the radiation damage occurs slowly. When WC compare GSG with CeF,, however, GSG has advantages of a shorter radiation-length (I.38 cm in GSO, 1.7 cm in CeF& a longer wavelength at the emission maximum (440 nm in GSU, 375 nm in CeFJ) and a larger light- output of about four times that in CeF;. Disadvantages will be the higher melting point (1900°C in GSO, 1443” C in CeF, [14]) and the higher cost of the raw material.

Acknowledgements

The authors would like to express their deep thanks to H. Nagayarna of JAERI for many precise and quick irradiations, to T. Utsu, S. Akiyama, and H. Ishibashi of Hitachi Chem. Co. for supptying excellent crystals, and to A. Suzuki of KEK for hetpfui in~oTmation on the room background. One of the authors (M.K.1 is deeply thankful to all colleague of the CAMS group,

especially to Yu.D. Prokoshkin of IHEP and K. Taka- matsu of KEK for their continuous helps and cncour- agement, and to V.I. Rykahn, V.G. Vasil’chenko of IHEP and V. Vyrodov of Kurchatov Institute for stimu- lating discussions.

References

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[2] M. Ishii, 211. lshibashi, S. Akiyama, G. Miura, K. Takayi and T. Fukazawa, Development of new scintillation crys- tal GSfXCel for y-ray detection, 4th Experts Meeting on Positron CT between AfST Japan and STU Sweden, Stockholm, March 1986.

[3] H. Ishibashi, K. Shim& K. Susa and S. Kubota, IEEE Trans. Nucl. Sci. NS-36 (19891 170.

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IEEE Trans. Nucl. Sci. NS-37 11990f 161. M. Kobayashi, K. Takamatsu, S, Ide, K. Mori, S. Suyi- mote, I-l. Takaki, M. Yuasa and M. Ishii, A beam test on a fast EM-calorimeter of GSOfCe), KEK Preprint 90-133 (1990). Grown by Hitachi Chem. Co., Japan in the Czochralski method. We use an approximate conversion from 1 R (rncntgen) of ‘“Co y-ray exposure to 0.88 rad ( = 0.0088 Gy) of the absorptian dose in GSO. M. Kobayashi and S. Sakuragi, Nucl. Instr. and Meth. A254 (1987) 275. D.F. Anderson, M. Kobayashi, CL. Woody and Y, Yoshimura, Nucl. Ins&. and Meth. A290 fl990) 385. Though two large peaks are seen around 355 and 4#5 nm in the excitation spectra of figs. 3 and 4, the 355 nm peak should be more dominant than the 405 nm one in actual calorimeters where the detection efficiency of the scintil- lation should be approximately the same for both peaks: see ref. [5]. S. Majewski and K. Bentley, Nucl. Instr. and Meth. A2ht”l t 1987) 373; A.J. Caffrey, R.L. Heath, P.D. Riffer, CD. Van Siclen, D.F. Anderson and S. Majewski, IEEE Trans. Nucl. Sci. NS-33 (1986) 230. N. Saga and M. Tashiro, J. Ceram. Assoc. Japan 70 (19621 $43. D.F. Anderson, Nucl, Ins&. and Meth. A287 fl99f.l) 606. M. Kobayashi, M. i&ii, E.A. Krivandina, M.M. Litvinov, Al, Peresypkin, Yu.D. Prokoshkin, V.I. Rykalin, BP. Sobotev, K. Takamatsu and V.G. Vasil’chenko, Nucl. Instr. and Meth. A302 (1991) 443.