Significant improvement of PbWO4 scintillating crystals by doping with trivalent ions

NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH

. __4 __ l!i!i!

Nuclear Instruments and Methods in Physics Research A 434 (1999) 412-433

ELSWIER

SeclionA

~~~~.rlsr~icl..nl;locate/nima

Significant improvement of PbW04 scintillating crystals by doping with trivalent ions M. Kobayashi”**, Y. Usukib, M. Ishii”, N. Senguttuvan”, K. Tanji”, M. Chibad, K. Hara”, H. Takanoe, M. Niklf, P. Bohacekf, S. Baccarog, A. Ceciliag, M. Diemozh ’ KEK, High Enery

Accelerator Research Organixtion. 1-I Oho, Tsukuhnshi. Iharaki-ken 305-0801. Japan b Fwukawn Co., Kamiyoshima. Yoshima, IH,aki 970-I 153. Japan ‘SIT. Shonarr Itrstitute of’ Technolos. Fujisalra 251-0046. Japan ’ Departttrerlt of Physics. Toh~o Metieti’opolilanUniveni@, Hachiqji 192-0364. Japan ‘Institute of Plzmic.r, Universi(v oj Tsukuba, Tsukuba 305-8571, Japan ’ Institute ofPhyics, Academy ofSciences oj‘C:ech Republic, Ctrkrovarnicka IO. 16200 Prague. C:ech Rep&/ii g ENEh, INNITEC. Via hnguillarese 301. S. Maria di Galerin, 00060 Rowa, Italy h INFN, Se-_. di Rorna & Unicersita di Roma “La Sapienza”. Roma. Itall,

Received 4 May 1999; accepted 4 May 1999

Abstract Doping by different trivalent ions (La3+, Lu3+, Gd3+, Y3+, Sc3+) was systematically studied in PbWO, crystals in the same way. using raw material from the same source. The result was compared with an undoped sample (actually a Pb-rich one) and samples doped with divalent (Cd”‘) and pentavalent (Nb5’) ions. All the trivalent ions but Sc3’ gave a significant improvement in transmittance in the short wavelength region (330-450 nm) and in radiation hardness. Among all the tested dopants, Y3+ and Gd3+ showed the best result; these dopants may be worth further studies, since they have another advantage of the segregation coefficient close to unity. ~!c 1999 Elsevier Science B.V. All rights reserved.

1. Introduction In our previous papers [l-4], we reported that La3+ doping dramatically improves the characteristics of lead tungstate (PbWO, or PWO) scintillating crystals including optical transmittance, decay time and radiation hardness. The obtained improvements have been successfully explained in

*Corresponding 298-64-7831.

author. Tel.: + 81-298-641171: fax: + 81-

E-mail address: [email protected]

(M. Kobayashi)

terms of the following hypothesis [S]. The two absorption bands at 350 and 420 nm can be due to the defects related to Pb3+ and O- colour centres, respectively, which should be created as a result of Pb” + deficiency in the lattice during the growth and/or annealing process in order to recover the total charge balance in the entire crystal. Introduction of La3+ could compensate the Pb”+ delireducing the densities of the ciency, thereby Pb3+/O--related defects. Doping by the other ions of different charges (2 + . 3 + , 5 + , etc) has also been studied in several institutes (for example, Refs. [6-83). Gd3 + doping [6] was found to give

0168-9002/99/$-see front matter !r’ 1999 Elsevier Science B.V. All rights reserved. PII: SO168-9002(99)00550-l

M. Kobayashi et al. 1 Nuclear Instrument.s and Methods in Ph>ks

a similar or even more promising improvement compared with La3 ’ doping. Doping by Nb5 ’ was intensively studied in the CMS group with an aim to cause oxygen leakage thereby reducing the density of the Pb3+/0 -related defects, and was found [S] to give an apparently similar improvement to that of trivalent ions. However, the magnitude of the assumed Pb’+ deficiency seems to be as small as or even smaller than 1000 at ppm level. In order to estimate the magnitude, we changed the composition of the melt by introducing a small excess of PbO or W03, and measured the resultant effects in the lattice constant, the density of the grown crystals, and the melting point. The result [9] showed that there exist no or almost no (within 0.5 at%) solid solutions at the stoichiometry composition. The quality of the grown crystals became much worse if the PbO or W03 level in the melt exceeded 1%. This indicates that the Pb’+ deficiency cannot be as large as or larger than 1 at% level, if we assume that the Pb’ + deficiency should be caused by the lack of PbO in the melt and/or the evaporation of PbO (or Pb) or Pb2WOs [lo] during crystal growth. Congruent composition of PbW04 melt was found at 49.92% of PbO (i.e. Pb-deficient) [11] as well, which further supports the hypothesis about the existence of Pb-deficiency in the grown crystals at the levels of several hundreds at ppm. Although the direct evidence of the Pb’+ deficiency has not yet been obtained from the study of the microstructure of the grown crystals. a rather clear evidence has recently been obtained by Han et al. [12] from the observation of dielectric relaxation in PWO : La3+ crystals. They confirmed that the density of the mobile defects, which exist in undoped PWO, is reduced as the concentration of La3+ is increased. The observed polarization is most probably due to the creation of [2(Lasl)’ - V&J dipole complexes. and indicates the existence of Pb2+ deficiency (as mobile defects) in undoped PWO. La3+ ions occupy the Pb’+ sites and form the [2(La&+)’ - V%,] dipole complexes, thereby changing the mobile Pb2+ vacancies to immobile ones. The improvement brought about by La3 + doping can be understood well if the Pb2+ vacancies cause degradation in transmission and radiation hardness. However. the conjectured Pb3 + defects was not observed in

Research A 434 (1999) 412-423

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ESR measurement L-131;this may indicate that the defects related to the conjectured Pb2’ deficiency may not be of a simple (i.e. point defect-like) nature. In order to understand the significant improvement by La3 + doping and thereby understand PbWO, crystals more clearly with an aim to achieve further improvement in this scintillator, we have studied the effects of different stable trivalent ions (La3+, Y3+, Gd3’, Lu3+, Sc3+) doped into PWO crystals with respect to transmittance, light yield, decay time, radiation hardness. etc. We compared the obtained results with those obtained for undoped PWO and PWO doped with divalent (Cd”) and pentavalent (Nb”) ions. We will report the obtained results mainly with respect to the radiation hardness and optical transmission, since the other properties (light yield, decay time, etc.) did not change much.

2. Test samples All the test samples were grown by Furukawa Co in the same way (3-times crystallization [14] in air by the Czochralski method using a platinum crucible and annealing at 600°C in the air for 6 h) by using raw material from the same source. The test samples are listed in Table 1. All the samples except two La 3‘-doped samp les (PWO25U and PWO25L) were cut from an ingot approximately 35 mm in diameter and SOmm in length. PWO25U and PWO25L were cut from a much larger ingot (PW025, see Ref. [4]) of 6.5 cm in diameter and 30 cm in length near the seed and tail ends, respectively. The nominal purities of the starting raw materials (PbO and W03) were 99.99%. Dopant ions was added into the melt in the third recrystallization process [143 with a concentration of 135 at ppm for all the dopants but Nb5 ’ which was added with 1350 at ppm. The large concentration of Nb5+ was chosen because 135 at ppm did not give sizable effects [15]. True concentration of the doped ions in the grown crystals is given in Table 1. It was either measured by the Inductively Coupled Plasma (ICP) analysis, or calculated using the known segregation coefficient. The uniformity of the dopant concentration along the length will be discussed in the final section in relation to the

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Instrumer~ts arid Methods itI Physics Research A 434 (1999) 41_7-423

Table 1 List of the doped PbWO, samples studied in the present paper. The thickness italics in the “size” column Sample name

Dopant

PWO : Lab PWO25U PWOXL PWO : Lu PWO : Cd PWO:Y PWO : SC PWO : Pb PWO : Cd PWO : Nb PWO : La,Nb

La3+(135/202(b)) La3’(135/220(b)) La3+(135/140(b)) Lu3+(135/45(a)) Gd3+(135/175(a)) Y3+(135/l10(a)) Sc”+(135/51(b)) Pb’+(l35) Cd’+(135/58(b)) Nb” +( 1350/670(b)) La3+(135/220(a)) Nb’+( 1350/650(a))

ions (atppm)

‘See our papers [1,2]. h See our paper [4]. (a) Calculated for the segregation (b) Measured by ICP analysis

(in melt/in

coefficient

crystal)

3. Measurement Transmission spectra were measured with a spectrophotometer (Hitachi 220). Photoluminescence (excitation and emission) spectra from the crystal surface were measured by using a fluorescence spectrophotometer (Hitachi F4500). The spectra and decay time of phosphorescence were also measured with the spectrophotometer F4500, by start2 ms after the excitation was ing measurement

was measured

is given in

Size (mm)

Notes

10x 10x-70 10x10x20 10x lOx,?O 10 x 10 x 45.2 10x 10x175.5 10x 10x-70.5 10x lOx’0.2 10x10x10 10x10x-70.3 10x 10x.?/ 10 x 10 x 30

From the same ingot as PWO : La. La2” Seed end of the ingot PW025 ’ Tail end of the ingot PW025 #

k = 2.5 for La 3f, 0.3 for Lu”.

segregation coefficient. The impurities remaining in the grown crystals were analyzed by Glow Discharge Mass Spectroscopy (GDMS). Although not all the samples were measured, we can expect from the results obtained for more than 10 present and past samples measured so far that the impurities were well controlled to be at a similar level in all the studied samples. Typical results of GDMS given in Ref. [4] for La3 +-doped samples, PWO25U and PWO25L, show that most of the impurities are below 0.1 at ppm level with no impurity above 1 at ppm level. MO impurity, which induces very slow components in the luminescence and scintillation decays [16-191, was less than 0.02 at ppm level.

across which the transmittance

Clouded

in a small portion

(about

5%)

1.4 for Gd3+. 0.8 for Y’+. and 0.4 for NbSf.

stopped. We measured the scintillation intensity for 6oCo y-rays by comparing the pulse height of the 1.25 MeV peak (the average of the 1.17 and 1.33 MeV photoelectron peaks) with the single photoelectron (p.e.) peak, which was created by an LED pulser mounted at a given distance from the photomultiplier. The sample was mounted on a 2-in photomultiplier tube (PMT, Hamamatsu R2259, with a bialkali photocathode and a silica window). The pulse height was analyzed with a PHA (LeCroy qVt) in the charge mode within a 1 us gate. We measured the decay time spectra within 1 us with a Lecroy qVt using a conventional single photoelectron technique. The start-pulse for this measurement was taken from a small GSO:Ce scintillator placed close to a 6oCo y-ray source, to which a PbW04 sample was coupled. The stoppulse was the single p.e. signal of the PbW04 sample detected with a fast 2-in. PMT (Hamamatsu H3177, with a bialkali photocathode and a silica window) which was placed at a given distance from the sample. The obtained spectrum was fitted with two (three if necessary) exponentials superimposed on a constant background. Radiation damage was measured by irradiating the samples with 6oCo y-rays at Japan Atomic

M. Kobawshi

et al. 1 Nuclear Instmn~ents and Methods in Phvsics Research A 434 (1999) 412-423

Energy Research Institute (JAERI) for the accumulated dose from lo4 to lo8 rad (lo’-lo6 Gy). The irradiation period was 1 h, 1 h, 2 h 45 min, 18 h 30 min, and 70 h for 104, 105, 1.2 x 106, 107, and lo8 rad, respectively. The measurement of the damages was carried out in 56 h. 31 h, 42 h, 31 h, and 31 h after irradiation by 104, 10’. 1.2 x 106, lo’, and 10’ rad, respectively. Recovery of the radiation damages was also measured in 2.3 d, 9.3 d, 24.3 d and 205 d after the 10’ rad irradiation. We calculated the induced absorption coefficient /lir defined as Pir

=

(WMT,lT)

(1)

where d is the thickness across which the transmittance T, (before irradiation) and T (after irradiation) were measured.

4. Results 4.1. Trivalent-ion

doping

The transmission spectra measured before irradiation

of different samples are given in Fig. 1.

415

Comparing the trivalent-ion-doped samples with the undoped (actually PWO:Pb-rich) and divalent-ion-doped (PWO:Cd’+) samples, we see that all the tested trivalent ions (except Sc3+) significantly improved the transmittance in the short wavelength region (330-450 nm). The wavelengths of the excitation and emission peaks were almost the same for all the tested samples (see Table 2). Small differences were, however, seen between the trivalent-ion-doped samples and the undoped or divalent-ion-doped samples. Besides the main blue emission peak at 420 nm for the excitation at 310-320 nm, a second broad peak was seen at around 600 nm in the undoped or divalention-doped samples. However, this second peak almost vanished by the doping of trivalent-ions (except Sc3+). An example is given in Fig. 2 for PWO:Y3+ and PWO:Pb-rich. At the same time, weak phosphorescence (very slow emission) was observed at around 600 nm in three samples, PWO:Pb-rich, PWO:Cd’+ and PWO:Sc3+. The phosphorescence was significantly reduced by doping of trivalent ions (except Sc3+) as typically seen again in Fig. 2. The decay constant of the phosphorescence was roughly 4-9 ms for all the

80

300

350

400

450

500

WAVELENGTH Fig. 1. Comparison of the transmission spectra before irradiation. 20 mm except for PWO:Lu3+ (45 mm): see Table 1.

The thickness

550

600

650

700

(nm) across which the transmittance

was measured

is about

416

M. Kobayashi

Table 2 Measured result. j.,, and i,, for “Co y-rays (see text) Sample

denote the excitation

i,l& (nm)

PWO: La.#4 PWO25U PWO25L PWO : Lu PWO :Gd PWO:Y PWO : SC PWO : Pb-rich PWO : Cd PWO : Nb PWO:La .Nb

308/410 315/415 315/415 3 I51422 315/418 315/418 312/410 315/418 315/418 3181422 3141405

The ith decay constant constant

et al. 1 Nuclear It~struments and Methods rn Physics Research A 434 (1999) 412-423

(T),,

and its intensity

is given by the weighted

and emission

LY (p.e./MeV)

31 31 30 37 33 40 37 37 41 26 27 (in percentage sum of

peak wavelengths,

respectively.

Decay constants

(ns)

r,U,) (%)

Tz(f:) (%)

2.5(79) 2.7(86) 2.4( 74) 4.1(75) 3.3(81) 3.3(75) 1.4(54) 1.5(61) 1.6(60) 2.1(80) 2.2(72)

14(21) 14( 14) 15126) 9.7(25) X(19) 1 l(25) 5.5(24) 8.9(26) 8.2(26) 8.3(14) 16(28)

of the total intensity)

are denoted

and LY the scintillation

r,(I,)

light intensity

(r),,

(5’0)

20(E) 29(13) 2604) 42(6)

as

ti

and I,, respectively.

4.9 4.3 5.7 5.5 6.9 5.2 6.5 7.0 6.7 5.4 6.1 The average

decay

~~1,.

above-mentioned three samples. It is interesting to note that the red emission centres are taking part in the afterglow processes as well. because the decay time of about 10 min was determined for the red emission centres related to 50°C TSL peak [S]. The phosphorescence could not be measured for the other samples because the intensity was too small. The above-mentioned effect of trivalention doping on scintillation and phosphorescence characteristics was already observed in the past [l] for La3 ’ doping, and was confirmed this time for all the tested trivalent ions (except scJ+). As for the decay time, the main components were as short as or shorter than a few to ten ns for all the samples. While the decay time of undoped PWO usually consisted of three components, the third slowest component vanished when trivalent ions (except SC3 +) were doped; see Table 2. This small but clear change was also observed for La” + doping in the past [l], and was confirmed this time for the other trivalent ions. The light yield of 2.5-40 p.e./MeV (see Table 2) was obtained for the samples studied. The samples with higher dopant concentration (around or above 200 at ppm in the crystal) show systematically lower values of light yield.

The sir spectra are given in Fig. 3 for all the tested samples. The sir at 420 nm, the emission peak wavelength, is compared in Fig. 4 (for trivalent-ion-doped samples except Sc3+) and in Fig. 5 (for undoped and divalent-ion-doped samples, and PWO:Sc3+). The results of measurement carried out in Italy below 3 x 10’ rad are also shown in Fig. 4 for PWO doped with Lu3+, Y3+, Gd3+, and Nb’+. For the trivalent-ion dopings, both results obtained in Italy and Japan are consistent with each other. From these figures, we see that the doping with trivalent ions (except Sc3+) significantly improves the radiation hardness. This significant improvement was already reported in the past [2] for La3+ doping, and was confirmed this time for all the tested trivalent ions (except Sc3+). The recovery after lO*rad irradiation, also shown in Figs. 4 and 5. seems to continue for a period as long as 200 days. The recovery does not seem to be complete even in 200 days in some samples. PWO:Y3+ shows “recovery” although it did not show any large damage upon irradiation. This apparent “recovery” needs more study with more samples. This may be related to the initial transmittance (before irradiation) which was a little low among the present test samples.

h4.Kohawshi

Fluorescence

et al./Nuclear ImirumerrtsandMethodsin PhysicsResearch A 434 (1999) 412-423

Phosphorescence

340 320 300 280 260

F

320

c c

300

x

280 260

Aem (nm)

Xem (nm)

Fig. 2. Excitation and emission spectra for UV excitation in three typical samples of PWO:La3+. PWO:Nb’+ and PWO:Pb-rich. The scintillation is shown on the left. while the phosphorescence on the right. Measurement of the phosphorescence was started 2 ms after the excitation was stopped. The contour of the emission intensity is plotted in the (A,,. I,,) plane. The numbers attached to the contours give rough relative intensity, which can be compared within the luminescence and phosphorescence. separately. Comparison between luminescence and phosphorescence may not be correct.

4.2. Doping with Nb5+ and codoping with La-” and Nb”

Addition of Nb5+ (both in Nb’+-doped only and (La3+, Nb’+) co-doped samples) made the short wavelength cutoff a little sharper in the transmission spectra (see Fig. 6) compared with the trivalent-ion dopings, although the underlying mechanism is not yet clear. The excitation-emission spectra for both scintillation and phosphorescence

417

are similar to those of trivalent-ion-doped samples as shown in Fig. 2. The decay time has a third slowest component of about 40 ns (for several % intensity) for single doping of Nb5+ but not for codoping of La3 ’ and Nb’+. The remaining third component is different from the case of all the tested trivalent-ion dopings (except Sc3+). The light intensity was lower than that of the other (trivalent-ion-doped, divalent-ion-doped and undoped) samples; see Table 2. The radiation hardness was significantly improved by Nb5 + doping compared with the undoped samples (see Figs. 4 and 5). As seen in Fig. 4, the result was similar for both single doping of Nb’ + and codoping of La3+ and Nb’+. But the improvement induced by Nb’ + doping (and co-doping of Nb” and La3+) seems to be a little smaller than that achieved by trivalent-ion doping. Measurement of the Nb’+-doped sample carried out in Italy at lower dose gave larger damage than that carried out in Japan (see Fig. 4). One of the possible reasons for this difference may be the case of fast recovery in PWO:Nb5’: measurement was carried out in Italy in a few tens minutes after irradiation, but in a few days in Japan. 4.3. Longitudinal ?fpwo:Lu’+

uniformity

in a large ingot

We compared the top with the bottom of a large ingot of PWO:La (65 mm in diameter and 300 mm in length). After a long rectangular block of 23 cm length was taken from the ingot, the two test samples. PWO25U and PWO25L. were cut from the rest of the top and bottom ends, respectively. The transmittance spectrum and radiation-induced absorption coefficient are compared in Figs. 7(a) and (b), respectively, between the two samples. Although the transmittance seems to be slightly better for PWO25U (the top end with La3’ - 330 ppm) than for PWO25L (the bottom end with La3 + - 150 ppm), such effect might be related to varying quality of optical polishing of both samples. However, the smaller values of /lir in PWO25U than in PWO25L should be true, indicating the better crystal quality in the top end of the ingot. Such nonuniformity was also reported by Lei et al. [20] in Bridgman-grown PWO crystals.

M. Kohqvashi

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20 15 YE 10 25 0 -5

6

10 8 7

6

;: q

4 2 0 -2

10 ^

6

5.

6

i

4 2 0 300

I

3

I

I /

500

600

WAVELENGTH

(nm)

300

400

Fig. 3. Spectra (b) PWO25U, (j) PWO:Nb5+.

700

500

WAVELENGTH

Cd)

PWO:Lu I_’

400

300

400

500

WAVELENGTH

of the radiation-induced absorption coefficient (c) PWO25L, (d) PWO:Lu3+. (e) PWOGd”, and (k) PWO:La3+. Nb5+.

According to Ref. [20], annealing at high temperature (- 960°C) may improve the La3+ distribution so that it is more uniform by diffusion, as seen in the improvement of radiation hardness. But annealing at such high temperature may not be appropriate from the viewpoint of possible Pb evaporation to create intense Pb3+ and/or O- defects. The result of PWO:La, #4, which was cut from a smaller ingot of about 35 mm in diameter and 80 mm in length, is also shown in Figs. 6(a) and (b). This sample seems to be better than or as good

600

600

700

(nm)

700

(nm)

pi, versus wavelength are given at different doses: (a) PWO:La3+. #4, (h) PWO:Pb-rich, (i) PWO:Cd”. (f) PWO:Y3+, (g) PWO:S?+.

as PWO25U, showing that the crystal quality might be slightly degraded when large ingots are grown.

5. Summary and discussions The summary of the obtained results together with some discussions are as follows: (1) All the trivalent ions (La”+. Lu3’, Gd3+, Y3 +) except Sc3 + gave similar and significant

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and Methods it1 Physics Research A 434 (1999) 412-423

ACCUMULATED

DOSE

419

(rad)

Fig. 4. Radiation-induced absorption coefficient pir at 420 nm (the emission peak wavelength) versus accumulated dose is compared among different samples. The results measured in Italy below about 3 x 10’ rad are also shown by connecting the data points with dotted lines (see text). The recovery in 2.3.9.3, 24.3 and 205 d after 10’ rad irradiation is also given.

30

25 /-x-

PWO:Nb

l-e- PWO:La.#4 Pb-rich

0 l.E 04

1.%,5

l.E)06

l.E 1 07

l.EfOB

-5

ACCUMULATED

DOSE

(rad)

Fig. 5. Radiation-induced absorption coefficient pir at 420 nm (the emission peak wavelength) versus accumulated dose for the samples which showed significant degradation at lo8 rad. The results of PWO:La3+ and PWO:Pb-rich, PWO:Cd’+. and PWO:Sc3+ PWO:Nb’+, given already in Fig. 3. are reproduced for comparison. The recovery in 2.3,9.3,24.3 and 205 d after 10’ rad irradiation is also given.

420

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and Methods in Physics Resea,rh

350

450

400

WAVELENGTH

A 434 (1999/ 41,‘-423

500

(nm)

spectra as seen in both cases of Nb5 + doping Fig. 6. Effect of Nb 5+ doping in sharpening the short wavelength cutoff in the transmission are also shown for comparison. and (La3+, Nb5+) co-doping. The transmission spectra of PWO:La3+, PWO:Y3+ and PWO:Pb-rich The thickness across which the transmittance was measured is about 20 mm except for PWO:La3+,Nb5’ (10 mm); see Table 1.

improvement in transmittance in the short wavelength region (330-450 nm), and in radiation hardness. One of the reasons why Sc3+ does not give any sizable improvement may be related to its ion radius (0.81 A) which is much smaller than that of Pb”(1.20 A). Although the concentration of Sc3+ in the crystal (51 ppm by ICP) is not small. the Sc3+ ions may have not entered into Pb2+ sites but as interstitial atoms. In this case, such mechanism of charge compensation as expected for La3 ’ doping does not work any more. Absence of the improvement for the doping of A13+ ions (ion _ 0.51 A) [S] may be explained in the radius same way. (2) Addition of Nb ‘+, in both Nb’+ doping and (La3+. Nb5’) co-doping, made a little sharper the short wavelength cutoff in the transmission spectra compared with the trivalent-ion dopings, although the underlying mechanism is not yet clear. (3) Among the trivalent ions (La3+, Lu3+, Gd3+ and Y3 ‘), Y 3+ showed the best radiation hardness, although the measurement should be confirmed for more ingots. The present result is consistent with the previous one [7] that the sir at 420 nm in

a Czech PWO:Y3+ crystal is much smaller than in PWO:La3 + or PWO:Lu3 +. (4) From the viewpoint of uniform dopant concentration along the crystal length, the segregation coefficient (k) close to unity should be ideal. From this viewpoint, Y3+(with li w 0.8). and than Gd3+(k - 1.4) may be more appropriate La3+(li _ 2.5) and Lu3+(k _ 0.3). If the crystal is grown by the Czochralski method with the crystallization ratio (g) from 0 up to 0.5, the maximum/minimum ratio of dopant concentration in the crystal becomes 1.15 for Y3’. 1.41 for Gd3+. 1.62 for Lu3+ and 2.83 for La3+ (see Fig. 8). (5) The thermo-stimulated luminescence (TSL) results obtained in Italy after irradiation at room temperature are in good consistency with the results of radiation hardness. A large TSL peak is seen at 50°C for all the tested divalent-ion-doped samples and PWO:Sc 3’ . It was absent in all the other trivalent-ion (except Sc3+) doped samples, leading to low intensity and structureless TSL glow curves spectra [21]. (6) The third (slowest) component in the scintillation decay time vanishes by the trivalent-ion

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Instruntents and Methods in Physics Research A 434 (1999) 412-423

421

80 70 g

60

s z

50

2 k E

40

z

30

d I-

20 10 0 300

(a)

350

400

450

550

500

WAVELENGTH

600

650

700

(nm)

5

E c

4 3

‘E x =L

(h)

1

ACCUMULATED

DOSE bad)

Fig. 7. Difference between the top and bottom ends of a large ingot transmission and (b) radiation-induced absorption coefficient spectra.

doping (except Sc3+). As a result, the average decay became a little shorter except constant (t),, t h e reason why the Gd3+for Gd 3 + doping; doped sample gave a little long decay time ((z),, = 6.9 ns) is not clear. (7) For the La3+-doped PWO, the \tir in PWO25L (the bottom end of ingot, La3+ - 150 ppm) is sizably larger than that in PWO25U (the seed side end of the same ingot, La3+ - 330 ppm). This nonuniformity must be partially due to the different concentration of La3+

of PWO:La 3+ (65 mm diameter

and 300 mm length)

in (a)

ions since the segregation coefficient of La3+ is much larger than unity. (8) All the undoped (PWO:Pb-rich), divalention-doped (PWO:Cd’+), and PWO:Sc3+ samples got a tint of black after 10’ rad irradiation. Compared with the trivalent-ion-doped samples PWO:(La3+. Lu3+, Y3+, Gd3+), the radiation damages of these three samples, as seen in pir at 420 nm, are not much larger at the dose level as low as lo6 rad, but much larger above lo7 rad. However, even below lo6 rad, PWO undoped or doped

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0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.1

0.8

0.9

1

CRYSTALLIZATION RATIO: g Fig. 8. Dopant concentration C according to the equation of C = C&l ratio and CO = initial concentration in the melt at g = 0.

with Cd2+or Sc3+ may not be good candidates for calorimeter materials, since the transmittance is poor in the 330-450 nm region. (9) AS seen in Fig. 3, the ~lir(~) spectrum has two maxima at around 400 and 550 nm in undoped PWO (PWO:Pb-rich). PWO:Cd’+, PWO:Sc3+ and some of PWO:La3+ samples (PWO25U, PWO25L). In the other samples, however, the pir(jV) spectrum is almost flat, indicating a good compensation of well-defined colour centres by trivalentor pentavalent-ion dopings. (10) As for the improvement in radiation hardness, Nb5+ doping was found to be as good as La3 ’ doping, but a little poorer than Y3 ‘, Gd3+ and Lu3+ doping. Co-doping of La3+ and Nb5’ did not make any big difference from the single doping of either La3’ or Nb’+. (11) Recovery of the radiation damage seems to continue for a period as long as 200 days. The recovery does not seem to be complete even in 200 days in some of the samples. Obtained experimental data show that even the concentration of several tens at ppm of (La3+, Lu3+. y3+, Gd3’) is enough to give significant improvement by charge compensation of the Pb2+ deficiency. On the contrary, rather higher La

- g)‘-‘,

where II = segregation

coefficient,

g = crystallization

and/or Nb concentration (around or above 200 at ppm in the crystal) resulted in noticeable loss in light yield which was explained by the creation of new kind of nonradiative traps [22]. Optimization of dopant level probably around several tens at ppm in the crystal is thus important. For further improvement of PWO scintillators, it should be worth understanding in which part of the technological process the Pb2+ deficiency is introduced and what is its true Ievel. It is also very demanding to find out the nature of the defects and colour centres induced by such a deficiency and their role in the processes of energy transfer and storage in PWO structure.

Acknowledgements The authors would like to express their deep thanks to H. Ishibashi (Hitachi Chem. Co.) for helpful and exciting discussions. One of the authors (MK) is deeply thankful to H. Sugawara, S. Iwata, S. Yamada and K. Nakamura of KEK for their support. The present work is partly supported by a Grant-in-Aid from the Japanese Ministry of Education, Science and Culture.

M. Kobayashi

et al. 1 Nuclear Instruments

and Methods in Physics Research A 434 (1999) 412-423

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