Assessing the performance of SICCAS-type lead tungstate...

Assessing the performance of SICCAS-type lead tungstate scintillators for EM

calorimetry in the CLAS12 Forward Tagger at Jefferson Lab

S.Fegan a,∗, E.Auffrayb, M.Battaglieria, E.Buchananc, B.Caiffia, A.Celentanoa, L.Colanerid, A.D’Angelod, R.De Vitaa,

V.Dormeneve, E.Fanchinia, L.Lanzad, R.W.Novotnye, F.Parodia, A.Rizzod, D.Sokhanc, I.Tarasovf, I.Zontad

aIstituto Nazionale di Fisica Nucleare, Sezione di Genova and Dipartimento di Fisica dell’Universitá, Via Dodecaneso 33, 16146 Genova, ItalybCERN, European Organisation for Nuclear Research, Geneva, Switzerland

cUniversity of Glasgow, Glasgow, G12 8QQ, United KingdomdIstituto Nazionale di Fisica Nucleare, Sezione Roma2 Tor Vergata and Università degli studi di Roma Tor Vergata, Via Scientifica 1, 00133, Roma, Italy

eII. Physikalisches Institut, Universität Gießen, 35392, Gießen, GermanyfGSI - Helmholtzzentrum fur Schwerionenforschung GmbH, Germany

Abstract

In this paper we describe a program of assessment carried out to determine the performance under irradiation of Lead Tungstate

crystals acquired from the Shanghai Institute of Ceramics, Chinese Academy of Sciences for use in the Forward Tagger device,

part of the CLAS12 detector in Hall B at Jefferson Lab. Obtaining crystals matching the demanding specifications of the Forward

Tagger has been a significant challenge, and the tests performed are intended to maximise the performance of the detector within

the practicalities of the crystal manufacturing process.

Results of light transmission, before and after gamma ray irradiation, are presented, and used to calculate dk, the induced radiation

absorption coefficient, at 420 nm, the peak of the Lead Tungstate emission spectrum. These results are compared with identical

measurements carried out on BTCP (Bogoriditsk Technical Chemical Plant) crystals, previously used in several calorimeter devices

but no longer commercially available.

Also presented are a series of tests performed to determine the feasibility of recovering radiation damage to the crystals using

illumination from an LED, with such illumination available in the Forward Tagger from a light monitoring system integral to the

detector.

Key words: Electromagnetic calorimetry, Lead tungstate scintillators, Radiation hardness, Optical transmission, Stimulated

recovery of radiation damage

PACS: 29.40.Vj, 61.80.Ed

1. Introduction1

Following the closure of the Bogoriditsk Technical Chemi-2

cal Plant (BTCP) in Russia, the Shanghai Institute of Ceramics,3

Chinese Academy of Sciences (SICCAS) is one of the most4

practical remaining facilities for the large-scale production of5

Lead Tungstate (PbWO4) crystals. This type of scintillator has6

been used for EM calorimetry in a variety of experimental fa-7

cilities [1, 2, 3, 4], including the Forward Tagger Calorimeter8

(FT-Cal), a subsystem of the Forward Tagger device, part of the9

CLAS12 facility being constructed in Hall B at Jefferson Lab10

[5].11

The Forward Tagger has been developed for meson spec-12

troscopy experiments in CLAS12 using the technique of low Q213

electron scattering. These electrons give rise to quasi-real pho-14

tons, which are reconstructed by detecting the scattered elec-15

tron in the Forward Tagger between polar angles of 2.5◦ and16

4.5◦. At such close proximity to the beamline, the FT-Cal scin-17

tillators will be subjected to radiation rates as high as 5 rad/h,18

∗Corresponding author

Email addresses: [email protected] (S.Fegan )

requiring a sufficiently radiation-hard material to be used. Lead19

Tungstate was chosen as it has been shown in many studies to20

be a material very resistant to radiation damage [6]. Combined21

with its fast decay time and small radiation length, PbWO4 is22

considered to be a good match to the demanding specifications23

of the FT-Cal [5].24

The FT-Cal comprises 332 PbWO4 crystals, each measuring25

200 × 15 × 15 mm, produced by SICCAS using the modified26

Bridgman method [7]. These crystals are read out with in-27

dividual Avalanche Photo Diodes (APD), whose gains can be28

matched, and monitored during run periods, via a light mon-29

itoring system, which can provide LED illumination tuned to30

the luminescence spectrum of the scintillator. The initial speci-31

fications demanded of the FT-Cal crystals are outlined in Table32

1.33

Radiation hardness of the crystals is quantified by the radia-34

tion induced absorption coefficient, dk, given in equation 1,35

dk =1

lengthln

(

Tbe f

Tirr

)

(1)

where Tbe f is the light transmission at 420 nm, the peak of the36

PbWO4 emission spectrum, measured before irradiation, and37

Preprint submitted to Elsevier October 1, 2014

Property Value

Length (mm) 200.00 ±0.15

Width (mm) 15.00 ±0.15

Height (mm) 15.00 ±0.15

Longitudinal Transmission (360 nm) ≥25%



Light Yield (T = 18◦C) ≥13 phe/MeV

Radiation Hardness

(Induced absorption coefficient at

420 nm, dk)

≤1.0m−1

Table 1: Initial specifications requested from SICCAS for the PbWO4 crystals

of the FT-Cal

Tirr the light transmission at 420 nm after irradiation. Crystals38

exhibiting greater levels of radiation damage to light transmis-39

sion have higher values of dk.40

As a consequence of the practicalities of the manufacturing41

process, and the need to produce crystals in a timely and cost-42

effective manner, the crystal properties are often found to be43

outside the ranges specified in Table 1, and a further programme44

of quality control is required to assess whether their character-45

istics are within acceptable values.46

This paper describes the programme of evaluation under-47

taken to assess the SICCAS crystals acquired for the FT-Cal in48

terms of the light transmission and radiation hardness require-49

ments of operations within CLAS12. The program consists of50

three parts. The first is a study of the light transmission, verify-51

ing that, before irradiation, crystals possess light transmission52

values consistent with those specified in Table 1. The second53

part assesses the radiation hardness of the crystals by determin-54

ing the induced radiation absorption coefficient, dk, measuring55

the light transmission before and after irradiation and calculat-56

ing dk using equation 1. The third part is the assessment of the57

ability to recover radiation damage to the crystals by means of58

optical annealing using visible light illumination from an LED,59

exploiting the availability of such illumination from the light60

monitoring system of the FT-Cal.61

A total of 370 crystals, including spares, were initially ac-62

quired from SICCAS, and for each crystal the induced absorp-63

tion coefficient was measured. The results were used to reject64

51 crystals not suitable for deployment in the FT-Cal. These re-65

jected crystals were replaced by SICCAS, and subjected to the66

same irradiation procedure to find dk. The 332 most radiation-67

resistant crystals were then selected for installation, with the68

remainder to be held as spares. LED recovery measurements69

were performed for a smaller subset of these crystals, verifying70

the suitability of the light monitoring system for use in crystal71

annealing.72

The SICCAS crystals were also compared with sample73

BTCP crystals; a set of 8 crystals used for prototyping of the74

FT-Cal [5] and a single crystal of a slightly different geometry75

(160 mm long, tapering from 16× 16 mm at one end to 13× 1376

mm at the other), deployed in several previous calorimeter de-77

vices at Jefferson Lab, including the ECal subsystem of the78

Heavy Photon Search (HPS) experiment [8]. The BTCP exam-79

ples chosen represent a typical range of properties and response80

to irradiation of this type of crystal.81

2. Crystal irradiation tests82

The use of PbWO4 crystals as scintillators in EM calorime-83

ters has been extensively studied for a variety of experiments,84

including CMS and ALICE at CERN [1, 2], PANDA at FAIR85

[3], and the CLAS Inner Calorimeter (CLAS-IC) at Jefferson86

Lab [4]. As a result of these studies, a good understanding ex-87

ists of the mechanisms of radiation damage in PbWO4 crystals,88

and how this damage manifests in terms of crystal light yield89

[9]. Under irradiation, impurities and defects in the crystal90

structure, and traps for electrons and holes lead to the forma-91

tion of colour centres, the overall effect of which is a decrease92

in light transmission of the crystal at optical wavelengths. Af-93

ter exposure, these colour centres spontaneously relax, with a94

fast thermal component acting over a timescale of around 3095

minutes [11]. This fast recovery is demonstrated in figure 1, in96

the form of dk measurements at 420 nm performed on a sample97

SICCAS crystal immediately after irradiation. Following this98

fast recovery, crystals continue to recover on a slower timescale,99

and their light transmission is fully restored after several weeks100

at room temperature.101

Time since irradiation (minutes)0 5 10 15 20 25 30

)-1

dk (

m

1

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

1.09

1.1

Figure 1: Radiation induced absorption coefficient, dk, at 420 nm for a sample

SICCAS-type PbWO4 crystal, measured at three-minute intervals following ir-

radiation with 30 Gy. After around 30 minutes, the fast recovery component is

complete.

Although the processes of radiation damage in PbWO4 are102

well-known, the applications cited above used BTCP-type103

crystals, which are no-longer commercially available. The104

SICCAS-type crystals which will be used for the FT-Cal exhibit105

similar behaviour under irradiation to BTCP crystals, but there106

are appreciable differences with respect to the BTCP-type.107

To understand these differences, and any potential effects on108

the deployment of these crystals in the FT-Cal, a preliminary109

programme of irradiation studies was performed at CERN, us-110

ing the Automatic Crystal quality Control System (ACCOS)111

2

[12] on a subset of the crystals acquired from SICCAS. These112

studies showed significant variations of the crystal parameters,113

with several crystals failing to meet the radiadion hardness114

specifications shown in Table 1. For these reasons, a detailed115

assay of the performance under irradiation of all the crystals116

acquired for the FT-Cal was undertaken.117

To observe the effects of normal operation in CLAS12 on118

the FT-Cal crystals, a 30 Gy dose was chosen, equivalent to119

the dose expected in the region of the FT over one month of120

continuous operation in CLAS12 [5].121

2.1. The irradiation facility at Gießen University122

The biotechnical operating unit of the University of Gießen123

has an irradiation facility, the Strahlenzentrum, used by re-124

searchers in a variety of disciplines. The facility has 60Co125

sources, which can be used in gamma ray irradiation studies126

and have been utilised by the II Physikalisches Institut of the127

university in studies of PbWO4 crystals for the PANDA EM128

calorimeter.129

The irradiation station at the Strahlenzentrum used for crystal130

studies is equipped with a Cary 4000 spectrophotometer, featur-131

ing a customised enclosure into which crystals are loaded for132

measurements of the light transmission along their longitudi-133

nal axis. The spectrophotometer is controlled by a PC running134

Cary WinUV software that allows the viewing and saving of135

spectra and provides options for the export of data to various136

formats for offline analysis. This is located close to the irradia-137

tion chamber, into which the crystals are placed for irradiation.138

The irradiation chamber contains 6 volume sources, with super-139

position between the sources enabling irradiation of the entire140

crystal sample. Any position effects regarding dose imparted141

within the chamber are considered to be negligible, this was142

seen in multiple irradiation cycles of FT crystals, and in previ-143

ous work by the PANDA collaboration at the Strahlenzentrum.144

A 30 Gy dose is imparted to the crystals in approximately 15145

minutes, representing a rate of 120 Gy/hr.146

2.2. Crystal testing procedure147

Before irradiation, measurements were made of the longi-148

tudinal light transmission of the crystals. Crystals were then149

irradiated with a 30 Gy dose from the 60Co sources and stored150

for 30 minutes in a dark environment to allow the fast recov-151

ery component of the crystal to take effect without interference152

from optically-stimulated recovery effects. They are then mea-153

sured a second time, with the same alignment and orientation154

in the spectrophotometer as the initial measurement in order to155

minimise changes in the light transmission due to crystal posi-156

tioning and the presence of dust, dirt, and imperfections on the157

crystal surface. All 370 SICCAS crystals, the 51 replacement158

crystals, and the BTCP crystals from both the HPS ECal and159

the FT-Cal prototype, were measured in this way at least once.160

2.3. Further studies of crystal properties161

In addition to the “once-through” irradiation procedure de-162

scribed above, several crystals were used for more detailed163

studies. This involved exposure to multiples of the 30 Gy dose,164

as well as repeat measurements of the “once-through” irradia-165

tion procedure after thermal annealing of the crystals, the latter166

being performed as a consistency check of the reproducibility of167

measurements. A separate facility at the University of Gießen168

was also used to study crystal light yield, in order to verify AC-169

COS results obtained during the preliminary crystal assessment170

procedure referred to earlier in this section. No significant vari-171

ation in the light yield measurements was seen in comparison172

to the ACCOS studies, or the quoted values from the manu-173

facturer, and all crystals tested in this work for radiation hard-174

ness are considered to have met the light yield requirements of175

the FT-Cal. Also performed were tests of recovery of radiation176

damage via optical annealing, discussed in the next section.177

3. Recovery of radiation damage by LED illumination178

Except in cases of prolonged high doses, for example in high-179

energy collider experiments such as CMS, radiation damage180

to PbWO4 crystals will typically recover on its own at room181

temperature over the characteristic timescale of several weeks.182

The colour centres formed around defects in the crystal release183

their captured electrons and/or holes, and again the crystal ex-184

hibits the same light transmission properties it did before irradi-185

ation. Crystals can also be recovered by thermal annealing, as186

the probability of releasing charge carriers from colour centres187

depends exponentially on temperature, allowing the recovery188

process to be performed in a period of several hours at temper-189

atures of around 200◦C.190

In addition to these processes, illumination from visible light191

is able to induce crystal recovery, by optically-stimulating the192

restoration of the colour centres in the crystals [10, 11]. Dur-193

ing the program of crystal tests performed in Gießen, irradiated194

examples of PbWO4 crystals of the SICCAS and BTCP types195

were exposed to varying degrees of illumination, both from196

LEDs planned for use in the gain monitoring system of the FT-197

Cal, and from readily-available sources of light in the Strahlen-198

zentrum, including fluorescent and incandescent lamps. The199

results of these studies using the LEDs from the FT-Cal mon-200

itoring system on the SICCAS and BTCP PbWO4 crystals is201

shown in section 4.2.202

4. Analysis and Results203

Light transmission measurements before and after irradi-204

ation, as well as all multiply-irradiated, annealed and re-205

irradiated crystals, and LED illumination studies were saved206

as .csv text files from the spectrophotometer control software207

and subjected to offline analysis. This analysis collated light208

transmission measurements at three wavelengths, 360 nm, 420209

nm and 620 nm, computed dk at 420 nm for all irradiated crys-210

tals, and examined the LED-stimulated recovery properties of211

the SICCAS and BTCP crystals.212

Also considered were possible systematic effects, and data213

normalisation issues. The spectrophotometer is automatically214

set up to subtract baseline measurements, a reference measure-215

ment taken with no crystal sample installed and no illumination216

3

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

20

40

60

80

After

Before

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

20

40

60

80Before

After

Figure 2: Longitudinal Light Transmission for SICCAS-type PbWO4 crystals,

before and after 30 Gy gamma irradiation. The irradiated spectra exhibits lower

light transmission, particularly around the 400 - 600 nm region. The plot on the

right represents good response of the SICCAS crystal to irradiation, while the

plot on the left is typical of a very poorly performing crystal.

of the photodetector within the device. Changes in this baseline217

are primarily due to thermal fluctuations, and are mitigated by218

maintaining a stable ambient temperature in the Strahlenzen-219

trum and regularly updating the baseline spectrum used, par-220

ticularly after longer periods where the spectrophotometer was221

not in use. The reproducibility of crystal positioning is the other222

major systematic effect in these measurements, with even small223

misalignments of the crystal having an appreciable effect on the224

measured values of light transmission. All crystals had a small225

identifying mark on one surface of the crystal, and this was used226

to ensure that every crystal had the same orientation each time it227

was measured in the spectrophotometer. Several crystals were228

measured multiple times and their spectra compared, in order229

to ensure that crystal positioning was sufficiently reproducible230

and would have minimal effect on the spectra measured. The231

total systematic error in these measurements is estimated to be232

1-2%, resulting in an error in dk between 0.1 and 0.2 m−1. For233

this reason, the requirement for dk was increased to accept crys-234

tals with dk ≤ 1.3 m−1.235

4.1. SICCAS PbWO4 crystal results236

In figure 2, the light transmission spectra before and after ir-237

radiation is shown for two examples of the SICCAS-type crys-238

tal, demonstrating the different behaviours under irradiation.239

Damage due to the irradiation is seen across the visible spec-240

trum for both crystals, with the right hand plot representing a241

fairly resistant example, accepted for use in the FT-Cal and the242

left hand plot typical of a badly-damaged example, which was243

rejected.244

Figures 3-5 show the light transmission measurements at 360245

nm, 420 nm, and 720 nm, before and after irradiation for all the246

SICCAS crystals tested, demonstrating the spread in physical247

properties of the crystals supplied.248

Crystal ID0 100 200 300 400 500

360n

mLT

5

10

15

20

25

30

35

40

Figure 3: Light Transmission measurements at 360 nm for all SICCAS-type

PbWO4 crystals tested in Gießen. Filled squares are measurements before ir-

radiation and open squares from measurements made after irradiation with 30

Gy. “Crystal ID” is an internal numbering scheme used to track the crystals.

Crystal ID0 100 200 300 400 500

420n

mLT

10

20

30

40

50

60

70





The collected statistics for all crystals with respect to dk at249

420 nm are shown in figure 6. Crystals with a dk higher than250

1.3 m−1 were rejected.251

Two crystals, referred to here as A and B, were subjected252

to multiple exposures to the 30 Gy irradiation, totalling 1, 2,253

4, and 51 times a single dose, allowed 30 minutes in a dark254

environment for fast recovery, then re-measured in the spec-255

trophotometer. The results of these studies are shown in figure256

7. The two crystals show different responses to the irradiation,257

with crystal B experiencing less reduction in light transmission258

as a result of radiation damage. However, after the 51× dose,259

the degradation in light transmission is severe for both crystals,260

indicating that under sufficient gamma irradiation, all SICCAS-261

type crystals will suffer appreciable levels of light transmission262

damage.263

4

Crystal ID0 100 200 300 400 500

620n

mLT

40

45

50

55

60

65

70

75

80

85

90





Property Mean rms

LT Before Irradiation (360 nm, %) 27.05 27.40

LT After Irradiation (360 nm, %) 23.65 24.06





Table 2: Average and rms light transmission values before and after irradiation

of the SICCAS PbWO4 crystals acquired for the FT-Cal

4.1.1. Comparison with BTCP crystal from HPS calorimeter264

Spectra for the BTCP crystal from the HPS ECal and an ex-265

ample FT-Cal SICCAS crystal, both before and after irradia-266

tion, are compared in figure 8. The BTCP crystal is noteable for267

having a greater overall light transmission at all wavelengths,268

and a less pronounced “wiggle” feature which is clearly seen269

around 600 nm in the SICCAS spectra. The BTCP crystal also270

displays a far lower degree of radiation damage across the vis-271

ible spectrum, resulting in a lower value of dk = 0.6 m−1 being272

computed from the light transmission measurements at 420 nm.273

4.1.2. Comparison with BTCP crystals from FT-Cal prototype274

The BTCP crystals show a narrower range of values of dk275

than is seen in the SICCAS crystals, and all have dk less than276

1.0 m−1, the originally-specified value for the FT-Cal. The in-277

duced radiation absorption coefficients for the 8 BTCP sample278

crystals are shown in figure 9, alongside the SICCAS crystals279

which have dk less than 1.0 m−1. Together with the greater light280

transmission at visible wavelengths when compared to the SIC-281

CAS crystals, the superior performance of the BTCP crystals282

has resulted in the available samples being considered for use283

in the regions of the FT-Cal expected to receive the highest rates284

of radiation.285

)-1dk (m0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

No.

of C

ryst

als

0

5

10

15

20

25

30

35

40

Figure 6: Histogram of the radiation induced absorption coefficient, dk, for

all SICCAS-type PbWO4 crystals tested in Gießen. Crystals with a radiation

induced absorption coefficient greater than 1.3 m−1 were rejected.

4.2. LED recovery results286

As shown in the previous section, and in figure 7, even the287

most resistant of the SICCAS-type crystals will suffer degrada-288

tion to their light transmission under sufficient irradiation. For289

the deployment of these crystals in the FT-Cal, thermal anneal-290

ing will not be a practical option for recovering this damage,291

as it would require either the dismounting of all 332 crystals,292

or the capability to heat the calorimeter to a high enough tem-293

perature to significantly speed up the recovery process without294

causing damage to the surrounding components of the FT, or295

indeed to CLAS12.296

One alternative is to use LED illumination, available in the297

FT-Cal from the light monitoring system, to mitigate the effects298

of radiation damage to the crystals. This use of LED illumina-299

tion to recover radiation damage was studied for both the SIC-300

CAS and BTCP type crystals. Irradiated samples of the two301

crystal types were illuminated with blue light from a single-302

colour NICHIA NSBP500AS LED [13], and the light transmis-303

sion recovery effect observed.304

4.2.1. Results for SICCAS crystals305

Three SICCAS crystals, A and B, from section 4.1, and a306

third crystal, referred to here as crystal C, were all used for307

LED recovery studies. All three crystals were subjected to the308

“once-through” irradiation procedure described in section 2.2.309

Additionally, crystals A and B were subjected to multiple irra-310

diations, already discussed in section 4.1. Following their re-311

spective maximum irradiations, 1× 30 Gy dose for crystal C,312

51× 30 Gy dose for crystals A and B, the crystals were then ex-313

posed to LED illumination, and the recovery effects observed.314

For the single 30 Gy dose irradiation, just 30 seconds of LED315

illumination is sufficient to begin to observe recovery in light316

transmission, with significant recovery observed after several317

minutes of exposure to illumination from the LED. Figure 10318

shows the light transmission curves for SICCAS crystal C after319

a series of 30 second illuminations from LED light, demonstrat-320

ing the fast rate of recovery possible via LED illumination.321

5

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

10

20

30

40

50

60

70

80

Before

1530 Gy

60 Gy30 Gy

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

10

20

30

40

50

60

70

80

Before

30 Gy60 Gy

1530 Gy

Figure 7: Light Transmission measurements for multiply irradiated SICCAS

crystals, referred to as A (left) and B (right). Each crystal was subjected to

sequential doses of 30 Gy totalling 1 (30 Gy), 2 (60 Gy), and 51 (1530 Gy)

times the 30 Gy dose representing one month of expected FT-Cal operational

dose, with greater reductions in light transmission seen as the dose received

increases. Labels show intersections of these curves with a line at 440 nm.

For the crystals receiving the higher doses, much longer illu-322

mination times, of order several hours, are required to see any323

significant recovery effect, demonstrated in figure 11, which324

shows the light transmission recovery after 4 hours illumina-325

tion.326

It is expected that the light monitoring system of the FT-Cal327

will be used to regularly optically anneal the crystals, usually328

between experiments. The doses accumulated over the course329

of an experiment are not expected to cause significant damage330

to the crystals, and a few minutes illumination should be suffi-331

cient for recovery, although the possibility exists to illuminate332

crystals between runs during an experiment if necessary, pro-333

vided the opportunity to do so arises through beam operations334

or configuration changes in the experimental hall.335

4.2.2. Results for HPS BTCP crystal336

In a similar programme of tests to the SICCAS crystals, the337

BTCP crystal from the HPS calorimeter was also subjected to338

LED illumination. Like SICCAS crystal C, a “once-through”339

irradiation procedure was performed on the crystal, and expo-340

sure to varying degrees of LED illumination used to realise light341

transmission recovery. After 35 minutes of illumination, almost342

complete recovery of the light transmission is observed, shown343

in figure 12.344

5. Conclusions and Outlook345

The demanding specifications of the Forward Tagger346

calorimeter has proven a challenge with respect to acquiring347

lead tungstate crystals with sufficient performance for deploy-348

ment in the device, of particular concern was the degradation349

in light transmission under irradiation. The program of crys-350

tal assay undertaken at the University of Gießen has enabled351

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

20

40

60

80

Before

After

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

20

40

60

80

After

Before

Figure 8: Light Transmission measurements for a single SICCAS PbWO4crystal, before and after irradiation (left), alongside similar measurements per-

formed for a BTCP crystal (right). The irradiated spectra exhibit lower light

transmission, particularly around the 400 - 600 nm region, however the BTCP-

type crystal exhibits a greater resistance to light transmission damage under

irradiation.

an informed selection to be made of those crystals requiring352

replacement, and the positioning of the selected crystals within353

the calorimeter according to their radiation hardness. Studies of354

possible recovery of radiation damage to the crystals by means355

of LED illumination were also performed, leading to greater356

confidence that the LED-based light monitoring system of the357

FT-Cal, designed for preliminary gain equalisation, can also be358

applied to the optically-stimulated recovery of radiation dam-359

age to the crystals, rather than the time-consuming alternative360

of dismounting the calorimeter for thermal annealing.361

Acknowledgements362

The authors would like to acknowledge the assistance and363

support provided in this work by the members of the PANDA364

group at the University of Gießen and the staff of the Strahlen-365

zentrum, in addition to that of the technical staff and electron-366

ics workshop of the INFN Genova section, in particular Fabio367

Barisone and Paolo Pozzo.368

The proposed application of the Forward Tagger light moni-369

toring system to perform stimulated recovery of radiation dam-370

age via external light on a quasi-online basis (i.e. without dis-371

mantling the calorimeter for annealing between experiments)372

was discovered and developed as a result of research and de-373

velopment on PbWO4 crystals for PANDA. This application is374

described in reference [11], and can be performed with infrared375

or visible light. A European patent for a device providing ex-376

ternal light for this purpose is held by the University of Gießen.377

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6

Crystal ID0 100 200 300 400 500

Gie

ssen

)-1

dK(m

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 9: Induced absorption coefficient, dk, for the BTCP sample crystals from

FT-Cal prototypes (squares), shown alongside all FT-Cal SICCAS crystals with

dk less than 1.0 m−1 (crosses), the originally-specified value for the FT-Cal.

Both crystal types shown have the same 200 × 15 × 15 mm geometry, allowing

these BTCP samples to be deployed alongside SICCAS crystals in the Forward

Tagger.

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[9] P. Lecoq, et. al., Nucl. Instr. and Meth. A 365 (1995) 291394

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[13] NICHIA Corportation, Technical specifications for NICHIA blue LED398

NSPB500AS399

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

10

20

30

40

50

60

70

80 Before

After

30 Seconds60 Seconds

90 Seconds

Figure 10: Light Transmission measurements for SICCAS-type PbWO4 crystal

C, irradiated with 30 Gy (‘After’ curve) and recovered using LED illumination.

Initial recovery of light transmission can be seen after just 30 seconds illumi-

nation (green curve). Successive 30-second illuminations show that significant

recovery of light transmission is possible after a few minutes (further curves

shown representing 60 seconds accumulated illumination, and 90 seconds). La-

bels show intersections of these curves with a line at 440 nm.

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

10

20

30

40

50

60

70

80

Before

After

4 Hours LED

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

10

20

30

40

50

60

70

80

Before

4 Hours LED

After

Figure 11: Light Transmission measurements for multiply irradiated SICCAS

crystals A (left) and B (right), showing pre-irradiated measurements, irradiated

spectra for a 51 times 30 Gy dose, and the effect of 4 hours LED illumination on

the recovery of light transmission. After such heavy doses, where light trans-

mission has been severely decreased, crystal recovery via LED illumination is

more difficult.

7

Wavelength (nm)300 400 500 600 700 800 900

LT (

%)

0

10

20

30

40

50

60

70

80

90

Before

30 Gy

35 Min LED

Figure 12: Light Transmission measurements for BTCP-type PbWO4 crystal,

before and after irradiation with 30 Gy, and recovered using LED illumination

totalling 35 minutes. After this illumination, almost complete recovery of the

light transmission is observed.

8

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