Download Imaging Nuclear Reactions

Imaging Nuclear Reactions
Zhon Butcher
2006 REU Program
Cyclotron Institute
Mentor: Dr. Robert Tribble
Applications of Nuclear Imaging


Space Telescopes – Cosmic radiation identification
and direction of origin.
Imaging reactions in the nuclear physics laboratory.
How Imaging Works in the Lab
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

Several detectors are placed around the reaction site
covering a given solid angle.
Detectors determine particle identity and position.
The resulting image gives a picture of the reactions
that took place in the chamber.
Particle Identification

Telescopes: Front detector and rear detector.



Front detector picks up energy loss as the particle passes through.
Rear detector picks up residual energy.
Particle identification determined by:
dE mz 2

dx Etot
Methods for Position Determination

Many small detectors coupled with a large amount of
electronics (clustering).

Resistive strip detectors.

Double sided strip detectors.

Resistive sheets.
1-D Position Sensitive Detector
Q1
Q2
Qtot
x
Q1
*L
Qtot
Resistive Strip Detectors


Consist of many
resistive strips placed
alongside one another.
Good resolution in the
X direction, poor
resolution in the Y
direction (or vice versa
depending on
orientation).
PSSDs
Double Sided Strip Detectors

Two sheets of strips placed one in front of the other
so the strips form a grid.


Results in better position resolution
Washington University team had detectors with 32
strips in each direction.

64 strips per detector x 4 detectors = 256 channels for
position reconstruction
Double sided PSSDs
Resistive Sheets


A single resistive sheet
spans the entire active
area of the detector.
Advantages



Fewer signals to process.
Less electronic equipment.
Detector Types:


Duo-lateral: Generates two
signals from each face of
the detector, two from the
front and two from the
back.
Tetra-lateral: Generates
five signals, one from each
corner of the resistive side,
and one signal from the
back.
Tetra Lateral Detectors
1 MW
Bias
10 kW
10 kW
10 kW
10 kW
10 kW
10 kW
10 kW
10 kW
Schematic
diagram of the
detector
Particle impinging position calculated by:
Y
(C  D)  ( A  B) L
*
( A  B  C  D) 2
X
(C  B)  ( A  D) L
*
( A  B  C  D) 2
Signal Processing
Preamplifier
Spectroscopy
Amplifier
Preamplifier
Spectroscopy
Amplifier
ADC
Detector
Computer
Preamplifier
Spectroscopy
Amplifier
Preamplifier
Spectroscopy
Amplifier
Preamplifier
Rear signal
Timing
Amplifier
Discriminator
Gate
Generator
How Silicon Detectors Work
Current Through Semiconductor
Doped Semiconductor

What is doping?
Doping is the integration of impurities into
the lattice structure of the semiconductor.

This allows extra electron and hole energy
levels which will increase the conductivity of
the semiconductor.
Experiment



To characterize the Micron
Semiconductors tetra-lateral
detectors in terms of energy
and position resolution as well
as non-linearity in position
reconstruction.
Three tetra-lateral type PSDs
were investigated. One 200 mm
and one 400 mm thick detectors
with a resistive strip around the
active area, and one 200 mm
without a resistive strip.
Optimal strip resistance is
approx. 1/10th the resistance of
the detector active area.
Setup


The detectors were
placed in a vacuum
chamber with a
radioactive source.
(241Am and 228Th
were used)
The distance between
the source and the
detector was approx
25cm for 241Am and
10cm for 228Th
Calibration Masks

Two masks were used to cover the detectors.
Position Reconstruction 200mm
Position reconstruction of impinging alpha particles for
the 200 mm thick detector with and without a resistive
strip.
Without resistive strip:
With resistive strip:
Position Reconstruction 400 mm
Position reconstruction of impinging alpha particles
with and without a mask for the 400 mm thick
detector with a resistive strip.
Without mask:
Slit mask:
Holes mask:
Energy Resolution

Energy Spectrum of
alpha decay from 228Th
with 400mm detector:
Energy Resolution:
Approx 10%
Results


The position resolution was determined to be around
3-4 mm and energy resolution of 8% for both the
400 mm and 200 mm thick detectors with the
resistive strip.
The resistive strip has a major contribution in
reducing the position reconstruction distortion.*
*For more information see T.Doke et.al. NIM A261 (1987) 605
Conclusion
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
The position resolution for the tetra-lateral PSDs
strongly depends on the resistivity of the resistive
sheet, electrode termination resistors, the filter
components of the preamplifiers, and the shaping
times of the amplifiers.
The measurements done were employing the use of
Indiana University preamplifiers and CAEN amplifiers
(3 ms shaping time). Further investigation of these
dependencies is ongoing.
Acknowledgements
Special thanks to:
 Dr. Robert Tribble
 Dr. Livius Trache
 Dr. Adriana Banu
 Matthew McCleskey