Radiation induced coloring of Cherenkov counter glasses

NUCLEAR INSTRUMENTS AND METHODS 108 (I973)

RADIATION INDUCED

COLORING

119-I23;

© NORTH-HOLLAND PUBLISHING CO.

OF CHERENKOV

COUNTER

GLASSES*

M. GOLDBERG *, P. L. MATTERN +, K. LENGWEILER and P.W. LEVY Brookhaven National Laboratory, Upton, New York 11973, U.S.A.

Received 27 November 1972 The radiation induced absorption, i.e, coloring, has been determined for two lead silicate glasses used in Cherenkov counters. The measurements were made on Schott SF5 and Ohara PEMG4 glasses. Both the radiation induced absorption spectra and the coloring, or growth, curves of the two glasses are quite similar. The absorption of both glasses extends from the ultraviolet "edge" to the near infra-red and can be resolved into three broad Gaussian shaped bands. The growth of two of the bands

is accurately described by the expression ~(tb) = A (1 - e -a~) + ctL~ where ~ is the total dose imparted to the glass and A, a and 0~r, are constants which are different for each band. The constants specifying the growth curves and the absorption bands have been determined. Thus, the radiation induced absorption in these glasses can be computed if the dose rate and irradiation time or total dose is known.

1. Introduction

induced a b s o r p t i o n in these two glasses for a variety o f r a d i a t i o n environments.

Glasses and o t h e r t r a n s p a r e n t m a t e r i a l s used in high energy particle detectors become colored when exposed to r a d i a t i o n . In other words, when these materials are exposed to different r a d i a t i o n s , whose energy m a y be between a few keV a n d m a n y GeV, they develop one or m o r e optical a b s o r p t i o n b a n d s which may occur a n y w h e r e in the region o f optical t r a n s p a r e n c y . F o r tunately, in m a n y materials used in particle detectors the rate o f coloring, i.e. the rate o f a b s o r p t i o n b a n d f o r m a t i o n , is quite low and the induced a b s o r p t i o n usually does not i m p a i r the detection function. H o w e v e r , in some applications, e.g. in glass C h e r e n k o v c o u n t e r s where the optical paths are relatively long, even small increases in optical a b s o r p t i o n m a y reduce a p p r e c i a b l y the total light incident on p h o t o m u l t i p l i e r detectors. I n f o r m a t i o n on the r a d i a t i o n - i n d u c e d c o l o r i n g of d e t e c t o r materials is so sparse it is difficult to estimate the light losses which m a y occur d u r i n g the course o f a given experiment. W h e t h e r or not a p a r t i c u l a r substance will develop optical a b s o r p t i o n sufficient to i m p a r e its usefulness is a constantly reoccurring question. M o s t recently it arose during the d e v e l o p m e n t o f a new C h e r e n k o v c o u n t e r configurationl). C o n s e q u e n t l y , the study described below was u n d e r t a k e n . Specifically, this p a p e r will describe the r a d i a t i o n - i n d u c e d c o l o r i n g which occurs in the lead silicate glasses, m a n u f a c t u r e d by Schott and by Ohara, which were tested for the a p p l i c a t i o n described in ref. 1. F u r t h e r m o r e , the results will be presented in a way which should facilitate estimates o f the r a d i a t i o n * Research supported by the U.S. Atomic Energy Commission. t Now at Westchester Community College, Valhalla, New York, U.S,A. + Now at Sandia Laboratories, Livermore, California, U.S.A. 119

2. Characteristics of the radiation-induced coloring The physical processes which are responsible for the coloring are r e a s o n a b l y well u n d e r s t o o d 2 4). W h e n glasses and t r a n s p a r e n t crystalline substances are f o r m e d they contain b o t h lattice defects and impurities. The defect most easy to visualize is a vacancy, i.e. a void in the glass or crystal lattice from which a single a t o m is missing. Usually impurities occur substitutionally, e.g. an AI 3+ ion in a site n o r m a l l y occupied by an Si4+ ion. W h e n exposed to radiation, a few percent of the ionization electrons and (electronic) holes which are f o r m e d are subsequently t r a p p e d by defects a n d / o r impurities. A large fraction o f the centers f o r m e d in this w a y are optically active, i.e. they absorb, and in m a n y cases, emit light (luminescence). The centers t h a t a b s o r b light are the well-known color centers. The most familiar color center, the F-center (F for farbe), is an electron t r a p p e d by a single m o n o valent negative ion vacancy. The stability o f the different centers varies greatly. C e r t a i n centers r e m a i n at a c o n s t a n t c o n c e n t r a t i o n for m a n y years if the material is kept at r o o m t e m p e r a t u r e . A f t e r a given irradiation, the concentration of some centers, in the same or different materials, g r a d u a l l y decreases; ultimately reaching a c o n s t a n t finite level or d i s a p p e a r i n g entirely. M a n y centers are quite unstable at r o o m t e m p e r a t u r e and d i s a p p e a r rapidly. Consequently, m e a s u r e m e n t s on r a d i a t i o n - i n d u c e d c o l o r i n g m a d e by first i r r a d i a t i n g a s a m p l e and then m e a s u r i n g the a b s o r p t i o n at a later time m a y seriously underestimate the a b s o r p t i o n present d u r i n g irradiation. F o r this and a n u m b e r o f a d d i t i o n a l reasons, a facility was

120

M. GOLDBERG et al.

recently constructed to measure the optical absorption, and other optical properties, e.g. luminescence, of crystals and glasses during 6°Co irradiation. Although intended for basic radiation effect studies, this facility is ideal for the measurement of the radiation-induced coloring in materials such as the lead silicate Cherenkov counter glasses which will be described in this paper. The information needed to characterize the radiationinduced coloring of glasses can be divided into two categories. First, the nature of the radiation-induced absorption spectrum, and second, the growth and decay of the absorption during and after irradiation. Before irradiation most glasses which appear colorless are transparent from the near ultraviolet, 0.3 to 0.4 pm, to the near infrared, 1 to 5 pmS). The absorption in the ultraviolet, usually called the " e d g e " absorption, increases very rapidly with decreasing wavelength and can be approximated by the extreme long wavelength tail of a very intense Gaussian-shaped absorption band. The radiation-induced infrared absorption usually is small and/or unimportant for particle detectors and need not be considered. The radiationinduced absorption in the visible usually consists of the superposition of Gaussian-shaped absorption bands and, in some materials, a shift of the absorption edge toward longer wavelengths6). Often this shift can be approximated by an increase in intensity of the original tail or by introducing an additional band whose tail accounts for the induced absorption. When the principal absorption consists of more than one band it is useful to resolve the spectrum into component absorption bands. Usually, this separation is made by computerized best-fit procedures; the one used in this study is described in ref. 7. Each Gaussian band is specified by the expression ~(E) = ~ m e x p (

41n2

- -b- r- (Eo-

)

e) 2 ,

where c~(E) is the absorption coefficient at photon energy E, ~.1 the absorption coefficient at the band maximum, E0 is the photon energy at the maximum, and U is the full width at half maximum. The quantities E, E o and U will be given in eV; c~(E) and c~,, in cm -~ (see ref. 5). To demonstrate the Gaussian shape of the color center absorption bands found in glasses it is essential to plot optical absorption versus photon energy, i.e. a plot of absorption versus wavelength is not Gaussian shaped. Each of the individual absorption bands, contributing to the spectrum, exhibits its own kinetic or growth behavior. In other words, curves of = versus

irradiation time, t, at a given dose rate, qS, or versus total dose, q), can vary greatly from one band to another. However, recent results, obtained with the radiation facility, on alkali halides 8-~) and glasses ~2) show that the most likely growth curves consist of the sum of one linear and one or more saturating exponential components. In this case curves of band maximum versus irradiation time are given by the expression n

0~m(/) ---- ~

Ai(l - e -a't) q- 0~Lt,

(1)

i=1

where n is the number of exponential components, Ai and al are magnitude and growth constants, and 7 L is the slope of the linear component. A variety of different coloring kinetic theories lead to this equationlZ). If different absorption bands contribute to the absorption at a given wavelength this expression is also applicable. In any case, this expression may be considered an accurate description of the growth of each band during irradiation. As mentioned above, after the irradiations have been terminated the coloring present during irradiation may decay. The studies on alkali halides 8-H) and glasses ~z) show that the decay behavior can be accurately described as the sum of exponentially decaying components and possibly one or more stable components. The magnitude of the radiation-induced absorption can be roughly related to the defect and impurity content of the glasses. One may expect defect concentrations in the 10 l 5 to 10 ~6/cm3region. When converted to color centers such concentrations give rise to absorption coefficients in the I to 100cm -~ region. Also, very pure glasses often contain 10 p.p.m, of impurities which can produce color center absorption in 1 to 100cm -1 region. Impurity concentrations of 100 p.p.m, or larger are not unusual and in these cases proportionately greater coloring can develop during irradiation.

3. Experimental details Measurements of the radiation-induced absorption were made on lead silicate glass from two manufacturers, Schott Glaswerk, Mainz, Germany and Ohara of Tokyo, Japan. The principal constituents of the glasses are very nearly the same and are given in table I. All samples were I0 x 20 x 4 m m 3 with their large faces accurately parallel and polished to a window-glass finish. All irradiations and absorption measurements were made at room temperature, i.e. approximately 21 °C. The samples were kept in the dark except when exposed to the spectrophotometer light, which is too weak to effect the absorption.

121

RADIATION INDUCED COLORING TABLE 1

TABLE 2

Glass composition.

Absorption spectra components.

Type Composition

Glass

Schott SF5 Ohara PEMG4 (weight percent)

SiOz NazO KzO PbO Unspecified

39.2 1.8 3.9 55.1 -

Schott Ohara

38.5 1.0 5.0 54.5 1.0

Samples for absorption spectrum measurements were exposed to 6°Co gamma rays until the total dose reached 8. l × 104 R. The spectra were measured with a Cary model 14R spectrometer, approximately 20 min after irradiation. The induced absorption was obtained by subtracting the effective absorption (the volume absorption plus the apparent absorption due to the reflectivity of the surface) before irradiation from the total absorption present after irradiation. A typical spectrum is shown in fig. 1. Also shown are the various bands obtained by resolving the spectrum into Gaussian components using the computerized best-fit procedureT). After the parameters describing the bands contained in each spectra were determined, the growth of the

Band llI E0 U (eV) (eV)

1.61 1.63

2.94 2.79

4.22 4.00

0.46 0.49

1.51 1.29

1.22 1.50

4. Absorption spectra The spectra induced in the two glasses studied are surprisingly similar. The principal absorption consists of a band at about 2.9 eV and low intensity bands occur at 1.6 eV in both materials. Additional radiation induced absorption occurs between the 2.9 eV band and the " e d g e " absorption. This can be regarded either as a shift in the band edge or as an additional absorption band whose peak occurs near 4.0 eV. Fig. 1 shows the absorption induced in the Schott glass, and

7.0

I-Z 143

Band 1I E0 U (eV) (eV)

bands were studied in the facility for making optical absorption measurements during 6°Co irradiation. A dose rate of 9.6 x 103 R/h was used. Also, the absorption measurements were continued after the irradiation was terminated to determine if the induced coloring decayed appreciably.

RADIATION INDUCED ABSORPTIONIN SCHOTT SF5 GLASS

4.0

Band I E0 U (eV) (eV)

GROWTH OF THE 2.94 eV BAND IN SCHOTT SF 5 G L A S ~

~ 6.0 "~

.~U:::>u~r

-

~

u v

ABSORPTION

5.0

3.0

/"

(P

LL LI-

/"

EL 4.0 LI_ hi 0 (D

z 2.0 0

z 0 I-

13_ t~ 0

t23

0 ,1.0

5.0 2.0 PHOTON ENERGY (eV)

1.0

/ / /

/

5.0

~: 2.0

g

/

/

/ I

i

i

/

I

i

I

l.O

2.0

5.0

4.0

5.0

6.0

7.0

TIME (IN UNITS OF 104 SECONDS)

Fig. I. The absorption of the Schott SF5 lead silicate glass before irradiation and after exposure to a total dose of 8.1 × 104 R of 60Co gamma rays. The induced absorption can be accurately resolved into three Gaussian shaped bands. The line through the data points is the sum of the individual bands. The absorption spectrum of the irradiated Ohara glass is quite similar.

Fig. 2. Growth of the 2.94 eV absorption band in the Schott SF5 glass during 6°Co gamma-ray irradiation at a dose rate of 9.6 × 103 R/h. The line through the data was computed from the equation(s) and parameters given in table 4; demonstrating that this expression accurately describes the growth curve.

122

M. G O L D B E R G

the three bands which account for the coloring. The solid line through the data points is the sum of the absorption of the individual bands obtained from the separation procedure. Clearly, the resolution of the observed absorption into individual bands is quite accurate. Very similar results were obtained with the Ohara glass. The band parameters obtained for both glasses are given in table 2. 5. Growth curves The absorption at the peak of the two fully resolved bands in both glasses was measured during irradiation. The absorption at 2.94 eV (Band II) in the Schott glass is shown in fig. 2. This data was fitted to the expression e(t) = A ( l - e - " ' )

+ c~Lt,

(2)

using a least squares procedure. The solid line through the data points was computed using the parameters obtained by the fitting process. The agreement is sufficient to conclude that eq. (2) describes the growth at 2.94 eV very accurately. Similar measurements and analysis were performed for the band at 1.61 eV in the Schott glass and the bands at 1.63 and 2.84 in the Ohara glass. In each case the growth curves could be fitted by eq. (2). The parameters, in eq. (2) obtained for all four bands, are given in table 3. TABLE 3 M e a s u r e d g r o w t h c u r v e p a r a m e t e r s , eq. (2).

Glass

Band (eV)

A ( c m -1 )

a (s - 1 )

(cm -1 s -1)

Schott

2.94 1.6[

4.26 0.87

3.8 x 10 . 5 3.2 x 1 0 - 5

4 . 9 7 x 10 5 0.65 x 10 5

Ohara

2.79 1.63

3.14 0.64

4 . 9 x 10 - 5 5.1 x 10 - a

3.96 x 10 5 0,73 x 10 5

et al.

Cherenkov counters it is satisfactory to use the approximation that the absorption spectrum in this region is independent of wavelength. Consequently, it is not necessary to undertake the extended effort required to determine accurately the growth parameters for the band near the absorption "edge". As mentioned above, the radiation-induced coloring is, to a varying extent, unstable at room temperature. Surprisingly, the coloring in both the Schott and Ohara glasses is relatively stable. The coloring existing at the end of an irradiation can be considered in two categories, either stable or unstable. Of the four bands described in table 3 the largest decrease or decay observed was between 8-9%. Consequently, to a good approximation the coloring in these glasses can be regarded as stable, i.e. non-decaying. 'TABLE 4 Growth curve parameters. D o s e r a t e e q u a t i o n : a(t)=A(l-e-~%ot)+~L¢d~t; t o t a l d o s e e q u a t i o n : :~(~) = A ( l - e - a ¢ ~ ) + c % a , q ) ; t in s,~b = d o s e r a t e in R / s , q5 = t o t a l d o s e in R.

Glass

Band (eV)

A (cm -1)

a, ( R 1)

cq,,~
Schott

2.94 1.61

4.26 0.87

1.43x10 5 1 . 2 0 x 10 . 5

1.86x10-5 0 . 2 4 x 10 . 5

Ohara

2.79 1.63

3.14 0.64

1.84x10 5 1.91 × 10 . 5

1 . 4 9 x 10 . 5 0 . 2 7 × I0 - 5

c%

The absorption at 2.94 in the Schott glass and at 2.84 in the Ohara glass contains a small contribution from the 4.27 eV (and 4.00 eV) band at shorter wavelengths. In principal the growth curve parameters for the bands near 4 eV could have been determined. However, the peaks of these bands occur on the short wavelength side of the absorption edge and the procedure required is laborious. However, the induced absorption between the 2.94eV (and 2.84) peaks and the band edge is nearly constant, i.e. independent of wavelength. Thus, for evaluating coloring effects in devices such as

6. Estimates of the radiation-induced coloring

To facilitate the application of the data to practical problems involving these two glasses, table 4 gives the measured growth curve parameters in terms of the dose rate ~b, or alternatively in terms of the total dose ~( = ~bt). This requires the assumption that the coloring is independent of dose rate; a quite reasonable assumption for the applications described here. However, it is important to include a word of caution regarding the application of this data to the evaluation of practical coloring problems. The data outlined above can be applied with confidence only to the Schott and Ohara glasses which were tested. Although these glasses are, presumably, from different manufacturers and they respond to radiation in a surprisingly similar fashion one cannot assume that all lead silicate glasses having the compositions given in table 1 will respond to radiation in this way. In particular, other glasses could contain sufficient quantities of undetected impurities

RADIATION INDUCED COLORING t o give rise t o d i f f e r e n t a b s o r p t i o n b a n d s , o r a c o m b i nation of one or more of the bands observed here and other bands. Furthermore, the kinetic parameters (the % ' s a n d a ' s ) c o u l d b e q u i t e d i f f e r e n t , etc. T h u s , f o r c r u c i a l a p p l i c a t i o n s it is e s s e n t i a l t o d e t e r m i n e t h a t t h e p a r t i c u l a r b a t c h o f glass a c t u a l l y u s e d is sufficient resitant to radiation induced coloring. Alternatively, the transmission could be monitored during measurement.

6) 7) s)

References 1) B. J. Blumenfeld, L. M. Lederman, R. L. Cool and S. L. Segler, Nucl. Instr. and Meth. 97 (1971) 427. 2) j. H. Schulman and W. D. Compton, Color centers in solids (MacMillan Co., New York, 1962). a) E. Lell, N. J. Kreidl and J. R. Hensler, Progress in ceramic science, vol. 4 (ed. J. Burke; Pergamon, Oxford, 1966) p.l. 4) A. M. Bishay, J. Non-tryst. Solids 3 (1970) 54. 5) Throughout this paper absorption, ~ is defined by the usual

9) 10) 11) lo)

123

equation I = l0 e x p ( - ~ x ) , where 10 is the light intensity at the interior side of the incident surface, I is the intensity of the light at the interior side of the emitting surface, and x is the thickness of the sample in cm. In other words, corrections have been made for surface reflection effects, usually by subtracting the effective absorption present before irradiation. Also, for photons, wavelength (in n m ) = 1239.6/ energy (in eV) and lt, m = 104/~ = 10a nm. p. W. Levy, J. Am. Ceramic Soc. 43 (1960) 389. R. E. Biggers, J. T. Bell, L. C. Long and O. W. Russ, Oak Ridge National Report 3834 (1971). p. W. Levy, P. L. Mattern and K. Lengweiler, Phys. Rev. Letters 24 (1970) 13. p. L. Mattern, K. Lengweiler and P. W. Levy, Solid State Commun. 9 (1971) 935. K. Lengweiler, P. L. Mattern and P. W. Levy, Phys. Rev. Letters 26 (1971) 1375. p. W. Levy, P. L. Mattern, K. Lengweiler and M. Goldberg, Solid State Commun. 9 (1971) 1907. p. W. Levy, P. L. Mattern, K. Lengweiler and A. M. Bishay, Studies on nonmetals during irradiation ; V-growth and decay of color centers in barium aluminoborate glasses containing cerium, submitted to J. Am. Ceramic Soc.