LSO/LYSO Crystals for Future HEP Experiments

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LSO/LYSO Crystals for Future HEP Experiments

Rihua Mao, Liyuan Zhang and Ren-Yuan Zhu

256-48, HEP, Caltech, Pasadena, CA 91125, USA

E-mail: zhu@hep.caltech.edu

Abstract. Because of their high stopping power (X0 = 1.14 cm), fast (t = 40 ns) and bright(4 times of BGO) scintillation and good radiation hardness, cerium doped silicate based heavycrystal scintillators (LSO and LYSO) have attracted a broad interest in the high energy physicscommunity pursuing precision electromagnetic calorimeter in severe radiation environment. Wepresent in this paper current status of large size LSO and LYSO crystals adequate for HEPapplications. The optical and scintillation properties and their radiation hardness are discussed.

1. IntroductionIn the last two decades, cerium doped silicate based heavy crystal scintillators have beendeveloped for the medical industry. As of today, mass production capabilities of lutetiumoxyorthosilicate (Lu2(SiO4)O, LSO) [1] and lutetium-yttrium oxyorthosilicate (Lu2(1−x)Y2xSiO5,

LYSO) [2, 3] are established. Because of their high stopping power (> 7 g/cm3), high light yield(200 times of PWO) and fast decay time (40 ns) this material has also attracted a broad interestin the physics community pursuing precision electromagnetic calorimeter, such as the proposedSuperB forward endcap calorimeter [4], the KLOE experiment [5] and the Mu2e experiment [6].Our initial investigation on large size (2.5 × 2.5 × 20 cm) LSO/LYSO crystals also shows thatthis new generation of heavy crystal scintillators has a superb radiation hardness against γ-rays [7], neutrons [8] and charged hadrons [9]. They are thus an excellent material to be usedin a severe radiation environment, such as the proposed high luminosity large hadron collider(HL-LHC). The main obstacles of using these crystals in the experimental physics are two fold:the availability of high quality crystals in sufficiently large size and the high cost associatedwith their high melting point (∼2,000◦). This report is a part of an on-going R&D program tofurther develop this material to be of practical use at the HL-LHC.

2. Properties of Scintillating CrystalsTable 1 lists basic properties of heavy crystal scintillators: NaI(Tl), CsI(Tl), BaF2, CeF3,bismuth gemanade (Bi4Ge3O12 or BGO), lead tungstate (PbWO4 or PWO) and LSO [10].As shown in the table, all crystals, except CeF3, have either been used in, or actively pursuedfor, high energy and nuclear physics experiments. Figure 1 is a photo showing twelve crystalsamples. In addition to samples listed in Table 1 pure CsI, CsI(Na), LYSO as well as LaCl3 andLaBr3 are also shown in this photo although the last two are not yet in mass production stage.Samples are arranged in an order of their density, or radiation length. All non-hygroscopicsamples are wrapped with white Tyvek paper as reflector. Hygroscopic NaI, CsI, LaBr3 andLaCl3 are sealed in packages with two ends made of quartz plates of 3 or 5 mm thick to avoid

XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) IOP PublishingJournal of Physics: Conference Series 293 (2011) 012004 doi:10.1088/1742-6596/293/1/012004

Published under licence by IOP Publishing Ltd 1

Table 1. Properties of Heavy Crystal Scintillators with Mass Production Capability

Crystal NaI(Tl) CsI(Tl) BaF2 CeF3 BGO PbWO4 LSO(Ce)Density (g/cm3) 3.67 4.51 4.89 6.16 7.13 8.3 7.40

Melting Point (◦C) 651 621 1280 1460 1050 1123 2050Radiation Length (cm) 2.59 1.86 2.03 1.70 1.12 0.89 1.14Moliere Radius (cm) 4.13 3.57 3.10 2.41 2.23 2.00 2.07

Interaction Length (cm) 42.9 39.3 30.7 23.2 22.7 20.7 20.9Refractive Indexa 1.85 1.79 1.50 1.62 2.15 2.20 1.82Hygroscopicity Yes Slight No No No No No

Luminescenceb (nm) 410 560 300 340 480 425 420(at Peak) 220 300 420

Decay Timeb (ns) 245 1220 650 30 300 30 400.9 10

Light Yieldb,c 100 165 36 7.3 21 0.30 854.1 0.077

d(LY)/dTb,d (%/◦C) -0.2 0.4 -1.9 ∼0 -0.9 -2.5 -0.20.1

Experiment Crystal CLEO TAPS - L3 CMS SuperBBall BaBar BELLE ALICE KLOE

BELLE PrimExBES III Panda

a At the wavelength of the emission maximum.b Top line: slow component, bottom line: fast component.c Relative light yield of samples of 1.5 X0 and with the PMT quantum efficiency taken out.d At room temperature.

surface degradation. To minimize the uncertainties caused by the sample size dependence in thelight output measurement all samples have a cubic shape of 1.5× 1.5× 1.5 X3

0, except NaI(Tl)

PWO LSO LYSO BGO BaF2CeF3

CsI CsI(Na) CsI(Tl) LaBr3(Ce) NaI(Tl)

LaCl3(Ce)

Figure 1. A photo shows twelve crystal scintillators with dimension of 1.5 X0.

XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) IOP PublishingJournal of Physics: Conference Series 293 (2011) 012004 doi:10.1088/1742-6596/293/1/012004

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0

20

40

60

80

100

em: 480 nm ex: 304 nm

BGO

em: 402 nm ex: 358 nm

LSO

em: 356 nm ex: 315 nm

LaBr3(Ce)

BaF2

X-ray luminescence

Peaks: 220 nm, 300 nmem: 415 nm ex: 308 nm

NaI(Tl)

20

40

60

80

100

0

20

40

60

80

200 400 600

em: 424 nm ex: 310 nm

PWO

400 600

em: 402 nm ex: 358 nm

LYSO

300 400 500

em: 335 nm ex: 310 nm

LaCl3(Ce)

300 400

em: 301 nm ex: 265 nm

CeF3

250 500 750

em: 540 nm ex: 322 nm

CsI(Tl)

20

40

60

80

100

Wavelength (nm)

Inte

nsity (

a.u

.)

Tra

nsm

itta

nce

(%

)

Figure 2. The excitation (red) and emission (blue) spectra (left scale) and the transmittance(green) spectra (right scale) are shown as a function of wavelength for ten crystal scintillators.The solid black dots are the theoretical limit of the transmittance.

and LaCl3 which are a cylinder with a length of 1.5 X0 and areas at two ends equaling to 1.5×1.5X2

0 to match the 2 inch diameter of the PMT cathode.Figure 2 shows a comparison of the transmittance, emission and excitation spectra as a

function of wavelength for ten samples. The solid black dots in these plots are the theoreticallimit of the transmittance, which was calculated by using corresponding refractive index asa function of wavelength taking into account multiple bounces between the two parallel endsurfaces and assuming no internal absorption [11]. Most samples, except LaBr3 and LaCl3,have their transmittance approaching the theoretical limits, indicating a negligible internalabsorption. The poor transmittance measured for LaBr3 and LaCl3 samples is probably due toscattering centers inside these samples.

It is interesting to note that BaF2, BGO, NaI(Tl), CsI(Tl) and PbWO4 have their emission

1000

2000LaBr

3:Ce

Source: Cs-137

By bialkali PMT

E.R.: 3.7% LaCl3:Ce E.R.: 5.1% NaI:Tl E.R.: 7.4% CsI:Tl E.R.: 7.9% CsI:Na E.R.: 8.9% LSO:Ce E.R.: 9.1%

1000

2000

0 200 400 600

LYSO:Ce E.R.: 9.3%

0 200 400 600

BGO E.R.: 10.8%

0 200 400 600

BaF2

E.R.: 13.9%

0 200 400 600

CsI E.R.: 25.1%

0 200 400 600

CeF3

E.R.: 28.5%

0 200 400 600

PWO E.R.: 75%

Channel Number

Counts

Figure 3. 137Cs γ-ray pulse hight spectra measured by a Hamamatsu R1306 PMT are shownfor twelve crystal samples. The numerical values of the FWHM resolution (E.R.) are also shownin the figure.

XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) IOP PublishingJournal of Physics: Conference Series 293 (2011) 012004 doi:10.1088/1742-6596/293/1/012004

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0

1000

2000

3000

4000

0 100 200 300 400 500

0 3810 190 2210 420 2150 440 208 3330 101 301.9 7.3 31

L.O = F + S ( 1 - e-t/τs )

F S τs

LaBr3(Ce)LSOLYSOCeF3CsIPWO

Lig

ht O

utp

ut (p

.e./M

eV

)

Time (ns)

-1000

-500

0

500

1000

1500

2000

2500

0 1000 2000 3000 4000

0 2093 1220

L.O = F + S ( 1 - e-t/τs )

F S τs0 2604 245

0 350 302

0 2274 693

98 1051 6550 1190/380 24/570

NaI(Tl)CsI(Na)CsI(Tl)

LaCl3(Ce)BaF2

BGO

Lig

ht

Ou

tpu

t (p

.e./

Me

V)

Time (ns)

Figure 4. Light output measured by using a XP2254b PMT is shown as a function of integrationtime for six fast (Left) and six slow (Right) crystal scintillators.

spectra well within the transparent region showing no obvious self-absorption effect. The UVabsorption edge in the transmittance spectra of LSO, LYSO, CeF3, LaBr3 and LaCl3, however,cuts into the emission spectra and thus affects crystal’s light output. This self-absorption effectis more serious in long crystal samples used in high energy and nuclear physics experiment asdiscussed for LSO and LYSO crystals [12]. It is well known that a good light response uniformityis crucial to maintain a small constant term for a crystal calorimeter.

Figure 3 shows the 137Cs γ-ray pulse hight spectra measured by a Hamamatsu R1306PMT with bi-alkali cathode for twelve crystal samples. Also shown in these figures are thecorresponding FWHM energy resolution (E.R.). γ-rays spectroscopy with a few percentsresolution is required to identify isotopes for the homeland security application. It is clearthat only LaBr3 approaches this requirement. All other crystals do not provide sufficient energyresolution at low energies.

Figure 4 shows light output in photo-electrons per MeV energy deposition as a function of theintegration time, measured by using a Photonis XP2254b PMT with multi-alkali photo cathode,for six fast crystal scintillators (Left): LaBr3, LSO, LYSO, CeF3, undoped CsI and PbWO4

and six slow crystal scintillators (Right): NaI(Tl), CsI(Na), CsI(Tl), LaCl3, BaF2 and BGO.The corresponding fits to the exponentials and their numerical results are also shown in these

Table 2. Emission Weighted Quantum Efficiencies (%)

Emission LSO/LYSO BGO CsI(Tl)Hamamatsu R1306 PMT 12.9±0.6 8.0±0.4 5.0±0.3Hamamatsu R2059 PMT 13.6±0.7 8.0±0.4 5.0±0.3

Photonis XP2254b 7.2±0.4 4.7±0.2 3.5±0.2Hamamatsu S2744 PD 59±4 75±4 80±4

Hamamatsu S8664 APD 75±4 82±4 84±4

XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) IOP PublishingJournal of Physics: Conference Series 293 (2011) 012004 doi:10.1088/1742-6596/293/1/012004

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0

0.05

0.1

0.15

0.2

300 400 500 600 700

Photonis PMT, XP2254B

BGO: Q

E

=4.7 ± 0.2%

LSO/LYSO: Q

E

=7.2 ± 0.4%

CsI(Tl): Q

E

=3.5 ± 0.2%

Wavelength (nm)

Quantu

m E

ffic

iency

Hamamatsu PMT, R1306

BGO: Q

E

=8.0 ± 0.4%

LSO/LYSO: Q

E

=12.9 ± 0.6%

CsI(Tl): Q

E

=5.0 ± 0.3%

LSO/LYSO

BGO

CsI(Tl)

0

0.25

0.5

0.75

1

300 400 500 600 700 800

Hamamatsu APD, S8664-55

BGO: Q

E

=82 ± 4%

LSO/LYSO: Q

E

=75 ± 4%

CsI(Tl): Q

E

=84 ± 4%

LSO/LYSO

BGO

CsI(Tl)

Wavelength (nm)

Qu

an

tum

Eff

icie

ncy

Hamamatsu PD, S2744

BGO: Q

E

=75 ± 4%

LSO/LYSO: Q

E

=59 ± 3%

CsI(Tl): Q

E

=80 ± 4%

Figure 5. Left: Quantum efficiencies of a Hamamatsu 1306 PMT with bi-alkali cathode (opencircles) and a Photonis 2254B PMT with multi-alkali cathode (solid dots) are shown as a functionof wavelength together with the emission spectra of the LSO/LYSO, BGO and CsI(Tl) samples,where the area under the emission curves is proportional to their corresponding absolute lightoutput. Right: The same for a Hamamatsu S8664 Si APD (open circles) and a HamamatsuS2744 Si PIN diode (solid dots).

figures. The undoped CsI, PbWO4, LaCl3 and BaF2 crystals are observed to have two decaycomponents. Despite its poor transmittance the cerium doped LaBr3 is noticed by its bright fastscintillation, leading to the excellent energy resolution for the γ-ray spectroscopic applications.The LSO and LYSO samples have consistent fast decay time (∼40 ns) and photo-electron yield,which is 6 and 230 times of BGO and PbWO4 respectively.

Since the quantum efficiency of the PMT used for the light output measurement is a

0.9

1

1.1

1.2

NaI(Tl)

T.C.: -0.2 ± 0.1 % / oC

LaBr3(Ce)T.C.: 0.2 ± 0.1 % /

oC

CsI(Tl)

T.C.: 0.4 ± 0.1 % / oC

CsI(Na)

T.C.: 0.4 ± 0.1 % / oC

CsI - Pure

T.C.: -1.4 ± 0.1 % / oC

BaF2

T.C.(220 nm): 0.1 ± 0.1 % / oC

BaF2

T.C.(300 nm): -1.9 ± 0.1 % / oC

0.9

1

1.1

1.2

15 20 25

CeF3

T.C.: 0.0 ± 0.1 % / oC

15 20 25

LaCl3(Ce)T.C.: 0.1 ± 0.1 % /

oC

15 20 25

LSOT.C.: -0.2 ± 0.1 % /

oC

15 20 25

LYSO

T.C.: -0.2 ± 0.1 % / oC

15 20 25

BGO

T.C.: -0.9 ± 0.1 % / oC

15 20 25

PWO

T.C.: -2.5 ± 0.1 % / oC

Temperature (oC)

Norm

aliz

ed L

ight O

utp

ut

Figure 6. Light output temperature coefficient obtained from linear fits between 15◦C and25◦C for twelve crystal scintillators.

XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) IOP PublishingJournal of Physics: Conference Series 293 (2011) 012004 doi:10.1088/1742-6596/293/1/012004

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function of wavelength, it should be taken out to directly compare crystal’s light output.Figure 5 shows typical quantum efficiency as a function of wavelength for a PMT with bi-alkali cathode (Hamamatsu R1306) and a PMT with multi-alkali cathode (Photonis 2254B),a Si APD (Hamamatsu S8664) and a Si PIN PD (Hamamatsu S2744). The emission spectraof LSO/LYSO, BGO and CsI(Tl) crystals are also shown in these figures. Table 2 summarizednumerical result of the emission weighted average quantum efficiency for several readout devices.The light output values in Table 1 are listed with the PMT quantum efficiency taken out. Thelight output of LSO and LYSO crystals is a factor of 4 and 200 of that of BGO and PbWO4

respectively.Scintillation light yield from crystal scintillators may also depends on the temperature.

Fig 6 shows light output variations for twelve crystal samples between 15◦C and 25◦C. Thecorresponding temperature coefficients obtained from linear fits are also listed in the figure. Thenumerical result of these fits is also listed in Table 1.

3. LSO/LYSO Crystal CalorimeterLarge size LSO and LYSO crystals are routinely grown in industry [12]. Figure 7 shows four longcrystal samples of 2.5 × 2.5 × 20 cm3 from CTI Molecular Imaging (CTI), Crystal Photonics,Inc. (CPI), Saint-Gobain Ceramics & Plastics, Inc. (Saint-Gobain) and Sichuan Institute ofPiezoelectric and Acousto-optic Technology (SIPAT). Figure 8 shows the spectra of 0.51 MeVγ-rays from a 22Na source observed with coincidence triggers [12]. The readout devices used area Hamamatsu R1306 PMT (Left) and two Hamamatsu S8664-55 APDs (Right). The FWHMresolution for the 0.51 MeV γ-ray is about 12% for the PMT readout, which can be comparedto 15% for the BGO sample. With the APD readout, the γ-ray peaks are clearly visible. Theenergy equivalent readout noise was less than 40 keV for these long LSO and LYSO samples.

LSO/LYSO crystals is also found to be much more radiation hard than other crystalscommonly used in high energy and nuclear physics experiment, such as BGO, CsI(Tl) andPbWO4 [13]. Their scintillation mechanism is not damaged by γ-ray irradiation. Radiationdamage in LSO and LYSO crystals recovers very slow under room temperature but can becompletely cured by thermal annealing at 300◦C for ten hours. The energy equivalent γ-ray induced readout noise was estimated to be about 0.2 MeV and 1 MeV respectively in a

Figure 7. A photo shows four long crystal samples with dimension of 2.5× 2.5× 20 cm3.

XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) IOP PublishingJournal of Physics: Conference Series 293 (2011) 012004 doi:10.1088/1742-6596/293/1/012004

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0

550

1100

CTI-LSO-L Source: Na-22

PMT: R1306 Gate = 200 ns

L.O = 1100 p.e./MeV

E.R. = 11.3%

0

200

CPI-LYSO-L

L.O. = 1020 p.e./MeVE.R.= 21.2%

0

400

800

SG-LYSO-L

L.O. = 1090 p.e./MeV

E.R. = 11.3%

0

400

800

0 100 200 300 400 500 600 700 800 900

SIPAT-LYSO-L

L.O. = 1080 p.e./MeV

E.R. = 11.4%

Channel Number

Co

un

ts

100

200

300

source: Na-222 × Hamamatsu S8664-55 CTI-LSO-L

E.R. = 27.0%

L.O. = 1580 p.e./MeV

100

200

300CPI-LYSO-L

E.R. = 42.9%

L.O. = 1310 p.e./MeV

100

200

300SG-LYSO-L

E.R. = 25.5%

L.O. = 1610 p.e./MeV

0

150

300

0 200 400 600 800 1000

SIPAT-LYSO-L

E.R. = 26.3%

L.O. = 1590 p.e./MeV

Channel Number

Counts

Figure 8. The spectra of 0.511 MeV γ-rays from a 22Na source, measured by a HamamatsuR1306 PMT (Left) and two Hamamatsu S8664-55 APDs (Right), with a coincidence trigger forfour long LSO and LYSO samples from CTI, CPI, Saint-Gobain and SIPAT.

78

79

80

81

82 CTI-LSO-L

EWLT

emission

300oC, 55.3%

102 rad, 55.1%

104 rad, 52.5%

106 rad, 50.6%

74

76

78

80CPI-LYSO-L

EWLT

emission300

oC, 52.5%

102 rad, 52.2%

104 rad, 50.5%

106 rad, 48.4%

80

81

82

83

84SG-LYSO-L

EWLT

emission

300oC, 56.4%

102 rad, 56.1%

104 rad, 53.8%

106 rad, 51.8%

T

ran

sm

itta

nce

(%

)

80

81

82

83

84

350 400 450 500 550 600 650

SIPAT-LYSO-L

EWLT

emission

300oC, 58.5%

102 rad, 58.1%

104 rad, 55.5%

106 rad, 53.6%

Wavelength (nm)

0.85

0.875

0.9

0.925

0.95

0.975

1

1.025

1.05

ID/Seed end coupled to PMT

CTI-LSO-L 1

CPI-LYSO-L 1

SG-LYSO-L 1

SIPAT-LYSO-L 1

0.85

0.875

0.9

0.925

0.95

0.975

1

1.025

1.05

10 102

103

104

105

106

ID/Seed end coupled to APD

CTI-LSO-L 1

CPI-LYSO-L 1

SG-LYSO-L 1

SIPAT-LYSO-L 1

Norm

aliz

ed A

vera

ge L

ight outp

ut

Irradiation Dose (rad)

Figure 9. Left: The longitudinal transmittance spectra are shown as a function of wavelengthin an expanded scale together with the photo-luminescence spectra for four LSO and LYSOsamples before and after the irradiation with integrated doses of 102, 104 and 106 rad. Right:The normalized light output is shown as a function of the integration dose for four long LSOand LYSO samples with PMT (top) and APD (bottom) as the readout devices.

XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) IOP PublishingJournal of Physics: Conference Series 293 (2011) 012004 doi:10.1088/1742-6596/293/1/012004

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radiation environment of 15 rad/h and 500 rad/h for LSO and LYSO samples of 2.5× 2.5× 20cm3 [7]. Figure 9 (Left) shows an expanded view of the longitudinal transmittance spectrafor three samples before and after several steps of the γ-ray irradiation with integrated doseof 102, 104 and 106 rad. Also shown in the figure is the corresponding numerical values ofthe photo-luminescence weighted longitudinal transmittance (EWLT ). Figure 9 (Right) showsthe normalized average light output as a function of integrated dose for five long samples fromvarious vendors. It is interesting to note that all samples show consistent radiation resistancewith degradations of both the light output and transmittance at 10 to 15% level after γ-rayirradiation with an integrated dose of 1 Mrad.

4. SummaryIn a brief summary, LSO/LYSO crystals are an excellent material for a total absorptionelectromagnetic calorimeter for a future high-energy physics experiment in a severe radiationenvironment. Assuming the same APD-based readout scheme used for the CMS PWOcalorimeter, the expected energy resolution of a LSO/LYSO crystal based electromagneticcalorimeter would be

σE/E = 2%/√

E ⊕ 0.5% ⊕ 0.001/E, (1)

where the stochastic term is dominated intrinsically by the fluctuation of shower leakage asindicated by GEANT simulations. This represents a fast calorimeter over a large dynamicrange with very low noise. Thanks also to the LSO/LYSO low temperature coefficient, such acalorimeter is less demanding to the experimental environment. Because of the crystals excellentradiation hardness, a LSO/LYSO crystal calorimeter is capable of making precision measurementfor electrons, photons and jets and thus provides a great physics discovery potential in a severeradiation environment, like the HL-LHC.

AcknowledgmentsThis work is partially supported by the U.S. Department of Energy Grant No. DE-FG03-92-ER40701 and the U.S. National Science Foundation Award PHY-0612805 and PHY-0516857.

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