Development of Yttrium Doped PWO Crystals for Physics Applications

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Development of Yttrium Doped PWO Crystals for
Physics Applications

• Qun Deng
• on Behalf of PWO Group
• Shanghai Institute of Ceramics
• Chinese Academy of Sciences

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Outline

• Segregation coefficient of the yttrium in PbWO4
  crystal.
• Performance of the yttrium doped PbWO4 crystals.a
• The transmittance and the birefringence.b
• Progress on stability.

a most results published in NIM A480 (2002),
468 b http//www.hep.caltech/edu/zhu/cms_020305.p
pt
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Crystal Growth
Bridgman Technology
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Typical Bridgman Furnace with 28 Crucibles.
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Yttrium Distribution in Crystal

• The segregation coefficient ke, defined as the
  ratio of the dopant concentration in the bulk
  crystal (Ccrystal) to that in the melt (Cmelt),
  describes the ability of the dopant to be
  incorporated into the solid phase,

(1)

• Assuming a slow, steady state growth process, the
  distribution of a dopant concentration in a
  crystal can be expressed as

(2)
g is defined as the ratio of the volume of
solidification part of the ingot to the whole
volume of the melt.

• The solution of Equation 2 is

(3)

• Taking logarithm, Equation 3 can be written as a
  linear equation

(4)
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Yttrium Distribution in Crystal(cont.)

• The Glow Discharge Mass Spectroscopy (GDMS) was
  used to determine yttrium concentration in
  crystals.
• A fit to the GDMS data extracts the yttrium
  segregation coefficient ke in PbWO4.
• Ke 0.91 ? 0.04

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Performance of the yttrium doped PWO crystals

• Longitudinal transmittance
• Emission
• Light output uniformity
• Decay kinetics
• Radiation damage
• Color centers

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Longitudinal Transmittance
Grown along c axis
Progress observed during crystal development
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Longitudinal Transmittance at 360 nm
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Emission
9 krad/h
Excitation Emission not affected by ? ray
radiation.
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Light Output Uniformity
Light response Uniformity is not affected by
radiation
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Decay Kinetics
90 and 95 of light output in 50 and 100 ns
respectively
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Radiation Damage
Damage is dose rate dependent, reaching
equilibrium under constant dose rate 5 to 15
loss of light output at 15 rad/h
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Radiation damage test at Protvino
Dose rate dependence also observed under hadron
irradiation
BteV-int-2001/20
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Summary of Light Output Measurements
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Color Centers
C1 3.07 eV (400 nm) / 0.76 eV, C2 2.30 eV
(540 nm) / 0.19 eV
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Transmittance and Birefringence

• PWO crystal is anisotropic, its crystallographic
  axis a and b is equivalent, while axis c is
  different with a and b
• PWO crystals grown along the c axis have lower
  theoretical limit in longitudinal transmittance
• PWO crystals grown along the c axis are isotropic
  transversely.

Bridgman grown along the c axis
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PWO Crystals Grown along c Axis
PWO crystals grown along the c axis are isotropic
transversely
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PWO Crystals Grown along c Axis(cont.)
Good longitudinal uniformity in the transverse
transmittance
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Stability
35 rad/h
35 rad/h
Crystals Produced in 1999
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Impurities in Crystal
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Results of K,Na doping
Na 20 ppm K 20 ppm
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Stability (cont.)

• Light yield increases under irradiation(instabilit
  y) was observed, it can be explained by
  preexisting color centers in the crystal which
  were bleached by scintillation light
• Color centers result from shallow trap defects
  concentrated in later part of crystal
• SOLUTION stoichiometric tuning and raw material
  purification.

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Longitudinal Transmittance at 420 nm
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Radiation Damage results of 2001
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Summary

• The concentration of yttrium ions in PbWO4
  crystals is rather uniform, the segregation
  coefficient is 0.91?0.04.
• The scintillation light of yttrium doped PbWO4
  crystals has a broad distribution with a peak at
  420 nm, the luminescence spectra and longitudinal
  light response uniformity are not affected by the
  ? ray irradiations.
• Yttrium doping is effective in reducing slow
  scintillation component, the ratio between light
  outputs integrated in 100 and 1000 ns is about
  95.
• The yttrium doped PbWO4 crystals have good
  radiation hardness.

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Summary (cont.)

• The radiation induced absorption in all yttrium
  doped samples can be decomposed to two common
  color centers peaked at 400 nm (3.07 eV) and 540
  nm (2.30 eV) with widths of 0.76 eV and 0.19 eV
  respectively.
• Because of the birefringence PWO crystals grown
  along the c axis are isotropic transversely. Also
  because of the birefringence PWO crystals grown
  along the c axis have lower theoretical limit in
  longitudinal transmittance, which is slightly
  more profound in the short wavelength region.
• Crystal instability is essentially eliminated by
  stoichiometric tuning and raw material
  purification.