Scintillating crystals in a radiation environment

UCLEAR PHYSIC5

PROCEEDINGS SUPPLEMENTS ELSEVIER

Nuclear Physics B (Proc. Suppl.) 44 (1995) 547-556

Scintillating Crystals in a R a d i a t i o n E n v i r o n m e n t * Ren-yuan Zhu, Da-an Ma, Harvey Newman a aPhysics Department, California Institute of Technology Pasadena, CA 91125, USA The unique physics capability of a crystal calorimeter is the result of their superp energy resolution, hermetic coverage and fine granularity. However, its energy resolution can only be maintained in situ if the crystal is sufficiently radiation hard. This report summarizes the performance of large size crystals (CsI, BaF~, CeF3, BGO and PbWO,) in a radiation environment. Technical approaches to solve radiation damage problem are discussed, with particular emphasis on crystal quality control in the growing process. An approach to solve the radiation damage problem by implementing optical bleaching in situ is elaborated.

1. I N T R O D U C T I O N Total absorption shower counters made of inorganic scintillating crystals have been pursued for the next generation of high energy physics experiments: undoped CsI crystals are proposed by the KTeV experiement at FNAL [1], Tl doped CsI crystals are proposed by the BaBar at SLAC [2] and by BELLE at KEK [3], and a PbWO4 crystal calorimeter is the baseline electromagnetic calorimeter choice for the CMS experiment at the Large Hadronic Collider (LHC) at CERN [4]. In the last decade, a large sector of the highenergy physics community has designed and studied crystal calorimeters for multi-TeV hadron colliders, including the late Superconducting SuperCollider (SSC) in the U.S. [5,6] and the LHC at CERN in Europe [4,7]. During this period much have been learned on the physics potential of such detector, as shown in Figure 1 [8]. The unique physics capability of crystal calorimeters is the result of their superp energy resolution, hermetic coverage and fine granularity [9]. In order to maintain the high resolution in situ, however, a principal requirement to the production size crystals is the radiation hardness. Typical dose rate varies from up to a few hundreds rad/h for endcap PbWO4 crystals at LHC [4] to less than 1 rad/h for endcap CsI(T1) crys*The work presented in this report is supported in p a r t by U.S. D e p a r t m e n t of Energy G r a n t No. DE-FG03-92ER40701.

.0920-5632/95/$09.50 © 1995 Elsevier Science B.V. SSDI 0920-5632(95)00584-6

All rights reserved.

tals in the PEP-II Assymetric Collider [2]. All known large size crystal scintillators suffer from radiation damage, as was discussed in the Crystal 2000 International Workshop in 1992 at Europe [10] and in recent Material Research Society Symposium in 1994 at U.S. [11]. The investigations on the problem, however, have provided approaches to improve the crystal quality so that a crystal with satisfactory quality could be developed. This report summarizes the performance of large size crystals (CsI, BaF2, CeF3, BGO and PbWO4) in a radiation environment. Technical approaches to solve radiation damage problem are discussed, with particular emphasis on crystal quality control in the growing process. An approach to solve the radiation damage problem by implementing optical bleaching in situ is elaborated.

2. C R Y S T A L L I G H T R E S P O N S E FORMITY

UNI-

As discussed in reference [12], the light response uniformity of a crystal is very important to keep the systematic constant term down to the design value. Figure 2 shows a GEANT prediction of the energy fraction (top figure) and the intrinsic energy resolution (bottom figure) for a BaF2 crystal calorimeter, as a function of the light response uniformity. In this simulation, the light response (Y) of the crystal was parametrized

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R.-Y. Zhu et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 547-556

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Figure 1. Invariant 77 mass spectra predicted for the production of Higgs particles of 80, 100, 120, 140 and 160 GeV, accumulated with 10 fb -1 at SSC. The spectra are shown with background subtracted statistically. The mass spectra are given for three energy resolutions corresponding to a BaF~ crystal calorimeter (2/v/-E ~ 0.5)%, a liquid argon (LAr) calorimeter (7.5/vFE ~ 0.5)% and a sampling calorimeter (15/v/-E ~ 1.0)%.

as a normalized linear function: Y = Y25 [1 + ~(z/25 - 1)]

(1)

where Y25 represents the light response at the middle (25 cm) of a 50 cm BaF2 crystal, ~ represents the deviation of the light response uniformity, and z is the distance from the small (front) end of a tapered crystal. To maintain a systematic limitation to the intrinsic energy resolution to less than 0.5%, the ~ value is required to be less than 5%. A detailed study using many different functional forms of the light response nonuniformity, in addition to a linear dependence, confirmed this conclusion quantitatively [14].

Figure 2. The relative mean energy fraction and the energy resolution for electromagnetic clusters of 3 x 3 crystals as a function of light response uniformity, derived by using a GEANT simulation [13]. See Equation 1 in the text for the definition of parameter ~.

To achieve this level of uniformity, a rule of thumb is to maintain a light attenuation length ( L A L ) of twice the length of the crystal. In case of BaF2 calorimeter design, an L A L of longer than 95 cm is specified [13]. With a long enough L A L , the light response uniformity can be achieved by treating at least part of the crystal surface to avoid total internal reflection that continues over many bounces and to instead scatter the light diffusely, randomizing the direction of light rays within the crystal, as discussed in details in reference [12]. One principle aspect of the radiation damage study is to address the change of the L A L in a radiation environment. 3. C R Y S T A L LENGTH

LIGHT

ATTENUATION

The L A L can be approximately calculated by using the measured transmittance (T) and the theoretical upper limit of the transmittances (T,),

549

R.-}C Zhu et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 547-556

assuming no light loss in the bulk material :

1000

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. . . .

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. . . .

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'[j

/i

t (2)

LAL = ln(T,/T)

where e is the path length, i.e. the crystal length. By using Equation 2, one can obtain the L A L of a crystal by measuring twice the transmittance (T1 and 7"2) for two different path lengths (~1 and ~2) for the same crystal:

500

For 25 c m long BaF e Crystal ]1 Solid: T , = 9 0 . 0 6 ~ , ]/ :

~---

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LAL = ln(Tl/T2)

This can easily be done, e.g. for a rectangular crystal with well polished surfaces. However, the above equations are approximate, since they do not take into account multiple bounces between two end surfaces of the crystal. Taking this into account, we have T,

=

(l-R) ~+R2(1-R)

=

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(5)

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2+...

where R is the light loss in each surface calculable by using Fresnel's law: R = (n

50

where n and nai r are the refractive indices of the crystal and the air respectively. If the refractive index of the crystal is known, one can calculate R and Ts by using equations 5 and 4. An alternative is to measure the 71, by using a thin crystal with well polished parallel end surfaces. Taking into account multiple bounces, an accurate L A L expression can be written as: g ln{[T(1 - T , ) 2 ] / h / 4 T s 4 + T2(1 - 7",2)2 - 2T}]} (6) Figure 3 shows L A L at 220 nm, as a function of the transmittance, calculated by using Equation 6 for a 25 cm BaF2 crystal. The solid and dashed curves correspond to two ways to extract the 7",. It is interesting to note that the difference beweeu two T, values used in calculating the L A L is very small, except in the region where the measured transmittance is close to the theoretical upper limit of T,.

Figure 3. Light attenuation length at 220 nm as a function of transmittance measured for a 25 cm BaF2 crystal [15]. The solid line is for Ts = 90.06% from a thin sample measurement, while the dashed line is for T, = 91.40% from an extrapolation of Malitson's measured refractive index [16].

4. R A D I A T I O N ENA

DAMAGE

PHENOM-

A principal damage phenomenon, observed in all mass-produced crystals, is the appearance of absorption bands, caused by color center formation. The absorption bands reduce the transmission of scintillation light through the crystals to the photosensors, and hence the apparent light output following irradiation. Additional effects observed in some crystals include reduced intrinsic yield of scintillation light, fluorescence (afterglow), and increased phosphorescence (spontaneous light emission over a long period). It is important that a crystal's scintillation mechanism not be damaged and that the radiation-induced phosphorescence does not affect the readout sig-

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R.-Y. Zhu et al. /Nuclear Physics B (Proc. Suppl.) 44 (1995) 547-556

Table 1 Summary of Radiation Phenomena for Heavy Scintillators

Item

CsI(TI)

Scintillation Damage No Fluorescence Yes Color Centers Yes Damage Recover @RT Yes Thermall Annealing Yes Optical Bleaching No a No recovery for the fast component.

Csl

BaF2

CeF3

No Yes Yes Yes Yes No

No Yes Yes Noa/Yes Yes Yes

No No Yes Yes Yes Yes

nal. Many crystals satisfy these criteria. However, the radiation-induced absorption bands reduce crystal's L A L , so would change the light response uniformity, and degrade the energy resolution, as discussed in Section 2. Table 1 summarizes the performance in a radiation environment for various heavy scintillating crystals: CsI [17], BaF2 [18,13], CeF3 [19], BGO [20-23] and PbWO4 [24-26]. In general, the scintillation mechanism of all these crystals are not damaged, but most crystals, except CeF3, suffer from radiation-induced fluorescence. All crystals also suffer from radiation-induced absorption bands which can be attributed to color centers related to the impurities or intrinsic defects present in the crystal. While small samples are usually viewed as radiation hard even to few Mrad, all large samples show considerable loss in light output. This difference between small and large crystals is common for all crystals, and is caused by two reasons. The first is that the absorption band is less effective in light output or transmittance measurements for a small sample. The second is the difficulty to grow large crystals with high enough purity, while a small sample can be cut from part of a large boule where the concentration of impurities is much smaller. Most damages recover under room temperature, and can be fully cured by thermal annealing although at different temperature, depending on the depth of color center traps. Optical bleaching which may be implemented in situ, is also effective for many crystals. The radiation-induced color center is usually related to impurities and/or structural defects in the crystals. The impurities may be present as

BGO No Yes Yes Yes Yes No

PbWO4 No Yes Yes Yes Yes Yes

substitutional or interstitial trace element atoms in the lattice, or they may occur as molecular ions, or microscopic color center complexes containing many atoms. An in depth study of (1) the trace element content and distribution in the crystals, (2) the quality of the crystal structure, and (3) the density and structure of the inclusions in a series of crystal samples was carried out for BGO in the last decade [20-23], and for BaF2 in this decade [18,13]. From this study, it is known that the impurities may lead to macroscopic structures which have a high density of trace-element rich color center complexes clustered together. These structures form inclusions which are visible under a low power microscopic, and in some cases to the naked eye. This data was cross-correlated with the degree of radiation damage in a series of doped and undoped crystal samples. This led to greatly improved processing technology for both types of crystals. As a result of the improved control of raw materials, crystal growth and annealing methods, crystals with greater radiation resistance were produced. At this writing, studies of the radiation damage problem for pure and doped CsI are now underway.

5. Eu D O P I N G F O R B G O Studies of BGO indicate that key impurities at the sub-ppm level may cause severe damage [21,23]. Figure 4 [21] shows the relative light output as a function of time after irradiation with a 2.5 krad dose, for BGO samples doped with different dopants.This data led to the conclusion that impurities in the BGO crystal can be cat-

R.-}~ Zhu et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 547-556

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egorized in three classes: (1) "harmful" impurities which cause permanent or severe damage (Cr, Mn, Fe and Pb), (2) "less harmful" impurities which cause some damage (Co, Ga, Mg and Ni) (3) "harmless" impurities which cause no discernible damage (Al, Ca, Cu and Si) at the typical trace impurity levels found in standardquality crystals. The figure also illustrates that different doped and undoped BGO crystal sampies recover at different rates, with characteristic recovery times ranging from hours to weeks. For BGO crystals, it was discovered that europium doping improved the radiation resistance by accelerating the recovery from the damage at room temperature. Figure 5 [22] shows the relative pulse height as a function of time after a radiation dose of 2.5 krad from a 137Cs 7-ray source for four BGO crystals doped with different levels of europium: 0, 5, 10 and 100 ppm by weight for the samples labelled BGO0, BGO1, BGO2 and BGO3 in the figure. The damage level is shown in terms of the parameters A r and As from a fit to a function of 1 - AF e -tIts - A s e - q ' , . It is clear that the damage level ( A t + As), especially the slow recovery component decreases with increased europium doping. The Eu-doped BGO

.... 0

i .... 50

I .... 100

I .... 150

I 200

Time (hr) Figure 5. Relative pulse height as a function of the time after a 2.5 krad dose is shown for for four BGO crystals doped with different levels of europium: 0, 5, 10 and 100 ppm by weight for BGO0, BGO1, BGO2, and BGO3 respectively

[22]. crystals are used in the rings of crystals in L3's BGO endcaps closest to the beam line, where the dose is higher than the barrel. 6. O P T I C A L B L E A C H I N G F O R BaF2 The main conclusions from the investigations of radiation damage in BaF2 crystals are: As in BGO, the damage of BaF~ is caused by the formation of color centers, which introduce a self-absorption of the scintillation light. There is no damage to the scintillation mechanism itself. There is no permanent damage in BaF2 caused by doses from photons, neutrons or other hadrons (such as protons, pions or kaons). At room temperature, the recovery of the damage is extremely slow (characteristic times of many months to years). However, all damage recovers fully after thermal

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R.-Y. Zhu et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 54~556 SI02, $302 a n d $402 (25

annealing at 500°C in an inert dry atmosphere for three hours. UV light also has been found to be effective in removing the radiation damage. The radiation damage of BaF~. shows clear saturation, in both transmittance and light yield measured by the photosensor, after an initial dose of 100 krad or less. This means that additional doses of Mrads result in no further change in transmittance, once saturation has been reached. The saturation phenomenon indicates that the number of color centers is relatively few, as expected for damage controlled by trace impurities. • The damage has no dependence on the radiation dose rate. The basic radiation damage mechanism is understood. Impurities (such as rare earths) [27,28], defects (inclusions) [29,28], oxygen [29] and OH- (U and O - substitutional centers) [30,31] are responsible. Figure 6 [13] shows (a) the transmittance before and after 1 Mrad 7-ray irradiation and (b) the relative light output measured for three 25 cm BaF2 crystal produced at the Shaghai Institute of Ceramics (SIC) in early 1991 (SIC102), early 1992 (SIC302) and July 1992 (SIC402). The progressive improvement of the quality of production BaF2 crystals is clearly seen from the figure. The improvement of the intrinsic radiation resistance, by purification of the raw materials and the use of optimized growth and annealing cycles, is a very difficult, time-consuming and expensive process. As shown in Figure 6, the LAL of 25 cm BaF~ crystals currently produced at SIC and Beijing Glass Research Institute (BGRI), after 1 Mrad of saturated irradiation is 42 cm [15]. This did not meet the GEM specifications (95 cm LAL). A technique for annealing BaF2 crystals in situ, i.e. optical bleaching, was developed [3235,31]. It was found that the optical bleaching with visible light was effective in removing the radiation damage. Studies also were performed to characterize the spectral behavior, required light

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Figure 6. (a) Transmittance and (b) relative light output are shown for three 25 cm BaF2 crystal produced at SIC in early 1991 (SIC102), early 1992 (SIC302) and July 1992 (SIC402), after a 1 Mrad 7-ray dose [13].

intensity, and rate of bleaching. The damage effect was found to be annealable with very low light intensities carried over silica fibers [32], and at wavelengths as long as 700 am. The left side of Figure 7 shows the recovery of transmittance of a 25 cm crystal after 1 Mrad irradiation by optical bleaching. The transmittance (T) at 220 nm (BaF2 fast component) and corresponding LAL and color center density (1/LAL) are shown in the right side of Figure 7. It was determined that the light intensity required to restore the crystals to a stable LAL of at least 150 cm to be in a range of m W / c m 2 [33]. Figure 8 shows the transmittance at 220 nm (T), corresponding LAL and bleachable color center density (D) for a test sample (SIC302) and a reference sample (SIC301) as a function of integrated dosage at a rate of 130 rad/h. While $302 was illuminated through an optical fiber, as shown in Figure 9, crystal $301 was placed adja-

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R.-E Zhu et all~Nuclear Physics B (Proc. Suppl.) 44 (1995) 547-556

S I C 3 0 2 : 3 z x 25 x 4 z ( c m )

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Figure 7. Transmittance as a function of wavelength (left) and transmittance at 220 nm, corresponding L A L and color center density as a function of time (right) are shown for a 25 cm BaF2 crystal under optical bleaching with light of different wavelengths [33]. cent to $302, but with no bleaching light. The data measured previously under 123 rad/h without bleaching light is also plotted for a comparison. The result of this test clearly show that with a bleaching light power of 1.1 m W from the xenon lamp, the light attenuation length of a production size BaF~ crystal can be set to 170 cm under 130 rad/h irradiation. Without the bleaching light, the LAL would be reduced to about 75 cm. Figure 9 shows the implementation of optical bleaching through a 2 m long ¢0.6 mm fiber. The large opening angle of the light cone from the fiber (60°), and the reflection of the bleaching light at crystal surface, ensure that the entire crystal is uniformly illuminated. For a large crystal calorimeter, the fiber system could be very

similar to the xenon flasher system used to monitor the L3 BGO crystals [37]. It also has been determined that the color center density in BaF2 crystals follows a simple dynamical model of color center creation and annihilation [33]. If both the creation and annihilation processes exist at the same time for one kind of color center, the density obeys the equation dD = -aIDdt

+ (Dan - D ) b n d t

(7)

where D is the optically bleachable color center density, a is a constant in units of cm 2 m W -z hr -1, I is the light intensity in m W cm -2, Dan is the total density of traps related to the optically bleachable color centers in the crystal, b is a constant in units of krad-x, R is the radiation dose

R.-Y. Zhu et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 54~556

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Figure 8. The transmittance at 220 nm and corresponding light attenuation length and bleachable color center density for test sample SIC302 and reference sample SIC301 under 130 r a d / h irradiation. Both crystals were wrapped with aluminum foil. The test sample was also illuminated by xenon bleaching light through a ~ 0.6 m m optical fiber. The power of the bleaching is 1.9 and 1.1. m W respectively for the first and last two runs. Previously measured data under 123 r a d / h without bleaching light is also shown for a comparison [36]. rate in units of krad hr-1, and t is the time in hours. The solution of Equation 7 is

bRDatt D = Doe -(at+bn)t + aI + b-------R [1 - e -(aI+bR)t]

(8)

where Do is the initial value of the bleachable color center density. For each value of I and of R, and for one kind of color center, an equilibrium between annihilation and creation will be established at an optical bleachable color center density (D~) of

bRDau

+ bn

(9)

Figure 10 [13] shows measured transmittance and corresponding LAL for a series of test runs. The LAL, calculated according to Equation 8 for each specific conditions, is shown as the solid line. The parameters a, b and Dau were determined from previous measurements to be: a = 0.68 and 0.95 cm2j -1 for D < 0.08 and > 0.08 respectively, b = 0.65 krad -1 for accumulated dose < 5 krad, and Dau = 0.73 m -1. The agreement between the data and the model is very good.

R.-Y. Zhu et al./Nuclear Physics B (Proc. Suppl.) 44 (1995) 54~556

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Figure 9. Schematic view of bleaching light propagation in a BaF2 crystal in the test setup used at Caltech, where light directly from fiber (primary light) has an open angle of 600 , while the reflected light (secondary light) covers the whole volume of the crystal.

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7. C O N C L U S I O N The high resolution and uniform hermetic coverage of homogeneous crystal calorimeters have given past and present experiments unique physics discovery potential. To reach and maintain this resolution, radiation hardness is the primary requirement. Recent extensive research and development has demonstrated that massproduced crystals of sufficient quality could be obtained, and stable uniform response and the intrinsic resolution could be achieved (through optical bleaching). This lead us to believe that a precision crystal calorimeter could have a key role to play in a wide range of science program, including, but not restricted by, the next generation of hadron colliders. REFERENCES

1. K. Arisaka et al, KTeV Design Report, FN580, January (1992). 2. BaBar Letter of Intent, SLAC-443, June (1994). 3. BELLE Letter of Intent, K E K R e p o r t 942, April (1994). 4. CMS Technical Report, CERN, December, 1994. 5. L* Letter of Intent to the SSC Laboratory, November (1990).

I

0

. . . .

I

. . . .

I

5 $0 Run Number

. . . .

I

15

Figure 10. Measured (points) and calculated (solid line) (a) transmittance and (b) light attenuation length (LAL) for a 25 cm BaF~ crystal in a series of tests with simultaneous 6°Co irradiation and 450 nm light illumination [13].

6.

7. 8. 9. 10.

11.

12. 13.

GEM Letter of Intent, SSCL SR-1184, November (1991). L3P Letter of Intent, CERN/LHCC 92-5, LHCC/I3 (1992). R.Y. Zhu and H. Yamammoto, G E M TN92-126 and CALT 68-1802 (1992). G. Gratta, H. Newman and R.Y. Zhu, Annu. Rev. Nuci. Part. Sci. 44 453 (1994). Heavy Scintillators for Scientific and Industrial Applications, Editions Frontieres Volume C58, ed. F. Nataristefani et ai, September 1992. Scintillator and Phosphor Materials, Material Research Society Symposium Proceedings 348, ed. M Weber et al, April (1994). R.Y. Zhu et al, Crystal Calorimeters for Particle Physics, in these proceedings. R.Y. Zhu, Nnci. Instr. and Meth. A340 442 (1994).

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