Engineering LYSO Single Crystals for Performance

LYSO:Ce for Positron Emission Tomography (PET)

LYSO:Ce has a set of interesting features, including high density, gamma emissions which are suitable for PMTs, and a high quality scintillation performance. It was grown using the Czochralski method. It could be improved, however, by a quicker Decay Time and lower Afterglow.

PET scanner ring

PET scanner ring

NA Density 1/µ (511 keV) λemission (nm) Light Yield (ph/MeV) Energy Resolution Decay time
BGO 7.1 10.5 mm 480 8200 15% 300 ns
LuAP:Ce 8.3 10.6 mm 365 11000 9% 60 + 600 ns
LSO:Ce 7.4 11.5 mm 420 30000 9% 40 ns + afterglow
LYSO:Ce (10%Y) 7.1 12.2 mm 420 32000 8% 40 ns + afterglow

Czochralski growth

Czochralski growth

LYSO Performance Enhancement by Co-Doping

Its higher output of light and reduced afterglow improves its level of performance. Its Ce4+ stabilization must be considered, and is proved with XANES1.

As a result of the stabilization of Oxygen vacancies, trapping is less efficient. The compensation mechanism can be altered using Ce4+, however.

The uncontrolled co-doping contact results in unstable crystal growth. This produces a spiral shaped, meaning that cracks may be more liable to occur and production yield is low. This may be explained by a reduced surface tension, as a result of high impurities or doping.

Standard co-doping is not suited to industrial production as poor performance results from a low co-doping content. Furthermore, unstable crystal growth results from a high co-doping content.

Crystal Composition Pulse Height (137Cs – 662 keV) XRL Rel. intensity
Light Yield (photons / MeV) Energy Resolution
LYSO:Ce 28,000 8.9% 1
LYSO:Ce,Mg 33,000 8.4% 1.12
LYSO:Ce,Ca 34,000 8.5% 1.19

Stabilization of Ce4+ centers

Stabilization of Ce4+ centers

Significantly suppressed afterglow

Significantly suppressed afterglow

Reduced decay time

Reduced decay time

Engineered LYSO for Large Scale Industrial Production

The composition’s progressive optimization can be achieved by modifying the characteristics of crystal growth. This is achieved via the precise control of both co-dopants' and activator-dopants' concentrations.

During the growth, the oxidizing agent (such as MOx) can be used. This is a source of oxygen, and can be used for decomposition in the melted bath. The number of defects related to oxygen can be reduced without producing undesired contamination.

There are several consequences for the growth. Surface tension is increased, crystal growth is stabilized (even at sizeable diameters), cracking is eliminated, and the quality of the crystals improves.

Large Diameter Controlled Growth

Performance of Engineered LYSO

Composition Standard LYSO Engineered LYSO
Light Yield (Photons/MeV) 28,000 38 – 42,000
Decay time 43 – 45 ns 34 – 37 ns
Energy Resolution 8.9% 7 – 8%
Afterglow High ~ GOS ceramics

Consistently Improved Timing Resolution

Timing Resolution of Engineered LYSO*

Timing Resolution of Engineered LYSO*

*Measured with 4x4x20 mm LYSO pixels, ends-on coupling Two Photonis XP20Y0 PMT’s @ -900 V, 100 µCi 22Na source

Conclusions and Perspectives

There are limitations to standard co-doping which can be improved. Afterglow, Decay Time, and Light Yield could all be improved, and Ce4+ could be stabilized for charge compensation. Unstable crystal growth results from uncontrolled co-doping, and co-doping is not able to be applied directly to industrial production.

LYSO can be engineered for industrial production as there isn’t any pollution to impact upon scintillation. The oxidizing agent method can be used and doping concentrations can be controlled. This opens up a range of opportunities for the preparation of scintillators.

Full production sizes of engineered LYSO are available. Decay Time has been reduced to 34 ns and Light Yield is over 40000 Ph/MeV (gamma 662 keV). The Afterglow of commercial GOS ceramics is similar to that of engineered LYSO, providing a for the CT or PET systems’ markets.

References

  1. S. Blahuta, A. Bessière, B. Viana, P. Dorenbos and V. Ouspenski, IEEE Transactions On Nuclear Science 60, 3134-3141 (2013).
  2. M. Spurrier, P. Szupryczynski, H. Rothfuss, K. Yang, A. A. Carey and C. L. Melcher, J. Crystal Growth 310, 2110-2114 (2008).

This information has been sourced, reviewed and adapted from materials provided by Saint-Gobain Crystals.

For more information on this source, please visit Saint-Gobain Crystals.

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