Transcript Annual Users Meeting at HIMAC

Observation of Fast Scintillation of Cryogenic PbI2 with VLPCs
1*
Moses,
1
Choong,
1
Derenzo,
2
Bross,
William W.
W.- Seng
Stephen E.
Alan D.
Robert Dysert,2 Victor Rykalin,3 Kanai S. Shah,4 and Misha Klugerman4
1
CA,
Lawrence Berkeley National Laboratory, Berkeley,
Fermi National Accelerator Laboratory, Batavia, IL,2
3
NICADD, Northern Illinois University, Dekalb, IL,
RMD, Inc., Watertown, MA4
4000
1000
0
1
2
3
4
Time (ns)
5
Predicted PET
Spatial Resolution
14
BaF
12
2
10
PbI
2
8
Gain
40 k – 60 k
Quantum Efficiency
~80%
Wavelength Range 400–750 nm
Rise Time
1 ns
Bias Voltage
6–8 V
Operating Temp.
7–9 K
Noise Floor
<1 p.e.
Material
Silicon
• Time-of-flight can localize the source
along the line of flight.
PbI2 BGO
Density (g/cc)
6.2
7.1
Atten. Len. (cm)
1.4
1.1
Photofraction
40%
43%
Wavelength (nm)
510
480
Decay Time (ns)
0.5, 1.8 300
Luminosity (ph/MeV) 3300 8200
2000
-1
VLPC Photodetectors at 8.5 K
Time-of-Flight PET
50% @ 0.5 ns
50% @ 1.8 ns
3000
0
VLPC / PbI2 Time-of-Flight PET?
PbI2 Decay Lifetime
5000
Resolution (mm fwhm)
Counts / 7.5 ps bin
PbI2 Scintillation Properties at 11 K
• Time-of-flight information reduces
noise in images.
• Time of flight tomographs were built
in the 80’s with BaF2 and CsF.
TOF Constrains Position of Source
LSO
6
BGO
4
Like BGO, but VERY Fast...
• PbI2 / VLPC Timing Similar to BaF2 / PMT
• Spatial Resolution Similar to BGO & LSO
• TOF Reduces Noise Variance 5x
• Visible Light Photon Counter
• Combines Advantages of
PMTs and APD Arrays
PbI2 Crystals
Experimental Setup
Averaged Oscilloscope Traces
2
0
0
5
10
15
20
25
Radial distance (cm)
“Large” Crystals
(2.5 mm cube,
2x2x4 mm3 block)
“Small” Crystal
(1 mm tall, 1 mm dia.,
right circular cylinder)
Cryostat
• PbI2 has an orange color, but is
transparent to it’s emissions.
• Micaceous - cleaves very easily!
• Mounted into plastic cassette.
• Surrounded with reflector
(TiO2 and Epotek 301-2)
137Cs
Fast PMT
PbI2 Crystal
& VLPC
(behind
window)
Lots of Material  Lots of Compton Scatter
Coincidence Timing Spectrum
Pulse Height Spectra
(using “Large” 2.5 mm Cube Crystal)
1200
Co-57 (122 keV)
Ge-68 (511 keV)
Cs-137 (662 keV)
Counts per Bin
Counts per Bin
1000
1000
600
400
100
200
10
0
0
-4
-3
-2
-1
0
1
2
3
4
Time (ns)
• Timed Against BaF2 Scintillator
• 100 cm Wire (not Coax) Between VLPC & Amp
Discussion & Conclusions
(using “Small” 1 mm Crystal)
10000
800
• ~1 ns Rise Time
• Pulse Height ~ Proportional to Energy
1 cm Cube of BaF2
100
200
300
ADC Bin
400
500
ŅPhotopeakÓ Position (ADC Bin)
shroud
(1225 keV)
Radioactive Source
• 2x4 array of VLPC pixels.
• Each pixel 1 mm dia.
• Shroud surrounds 8 pixels.
• Cassette w/ PbI2 crystal
plugs into shroud.
• 0.001˝ air gap between
PbI2 and VLPC.
• 5.8 V bias, Gain = 40,000
8 Element
VLPC Array
60Co
0.125˝ Window
(Stainless Steel)
cassette
VLPC
(662 keV)
• Good (1.2 ns fwhm) Timing Resolution Achieved
500
The theoretically achievable resolution is ~200 ps fwhm. Our timing
resolution was degraded by a 100 cm long wire (instead of a coaxial
cable) that was used to bring the VLPC signal out of the cryostat.
400
300
• Poor Energy Resolution Observed
200
100
0
0
100
200
300
400
500
600
700
Gamma Ray Energy (keV)
• Identifiable Photopeak (55% fwhm) for 57Co
• Photopeak Not Identified for 68Ge, 137Cs, or 60Co
• Inhomogeneous Light Collection Efficiency?
The 511 keV emissions of 68Ge produce signals of ~250 p.e. This
should produce a photopeak of ~15% fwhm. However, a broad
shoulder was observed rather than a photopeak. This is probably
due to poor optical quality of the PbI2 crystal. The material is
micaceous and cleaves easily, so it is likely that internal structure
and/or flaws make the light collection efficiency non-uniform, thus
preventing a clear photopeak from being observed.
• This Is a Long Way From a Viable PET Detector!
This work was supported by the Director, Office of Science, Office of Biological and Environmental
Research, Medical Science Division, U.S. Department of Energy under contracts DE-AC03-76SF00098
and DE-AC02-76CH03000