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How the DADL PSD Works
Both sides of the detector are used to obtain the energy of
incoming particles. Resistive Strips help to guide the charge
produced by the incoming particles across the detector to the
collecting edges, eliminating distortion. The gathered charge
is then used to determine the energy produced by the
particles as well as the particle locations.
Front
1
F2
Back 2
B2
Guide
wires for
charge
F1
Front 2
To determine the energy (E) and location (X,Y) from the
charge (Q), several equations must be used:
EBack = QBack1 + QBack2
And:
X α QBack1 - QBack2
QBack1 + QBack2
Y α QFront1 - QFront2
QFront1 + QFront2
These equations are then applied to a computer program to
produce images based on what is hitting the detector.
Edge 2
L2
L1
Guide
Wires
L1
There are a variety of detectors that can be used to
measure particles emitted from nuclear reactions. The various
materials used in these detectors can be solids, liquids or
gasses. The composition could contain elements spanning the
periodic table from hydrogen through barium. Different
detectors have varied attributes such as stopping power,
energy resolution, cost and ease of handling. A combination
of different detectors are often combined to create detector
arrays. One material that is commonly used in detectors is
silicon.
This is a position image using a 228Th source, which emits αparticles. These α-particles have enough energy to cause a
charge to be produced as they go through the PSD. αparticles are also weak enough that they are easily handled,
allowing preliminary tests to be preformed on the detector.
228Th
Alpha Spectra
Crystalline silicon is very
versatile as a detector
material. As
semiconductor, it has a
band gap. The band gap
can be adjusted by
implanting ions into the
silicon crystal. This doping results in n-type or p-type wafers
which are used to create the detector.
ETotal = EFront + EBack
L1
Robin Dienhoffer, Texas A&M University
Advisor: Dr. Sherry Yennello, Texas A&M University
Silicon Detectors
EFront = QFront1 + QFront2
Edge 1
Alpha Position
Radiation Detectors
Charge
collecting
strips
B1
Back
1
Dual-axis Duo-Lateral
Position Sensitive
Detectors
228Th
Charge
Collection
L2
L2
The detector acts much like a uniform resistor. This means the
further the charge travels to reach the collection edge, the
less charge. So:
When L1 = L2, both edges receive the equal charge, or Q1 = Q2
When L1 > L2, Edge 1 receives less charge, or Q1 < Q2
When L1 < L2, Edge 1 receives more charge, or Q1 > Q2
When a voltage (bias) is applied to the crystal , the band
gap is enlarged thus increasing the region which is depleted
in charge carriers. The depletion layer is where the particle is
actually detected. The energy deposited results in a charge
which is collected at the surface of the detector, resulting in a
measurable signal.
Over the years, a variety of approaches have been used
to determine the position of a particle measured by a silicon
detector. Discrete detectors require many channels of
electronics to achieve good position resolution. Another
avenue is resistive detectors which measure the position by
charge splitting. To achieve both horizontal and vertical
position, tetra-lateral detectors are the most advanced,
commonly used devices. However, these result in some
distortion in the signal requiring a complex algorithm for
correction.
Six very clear peaks are visible. These are the 5.432 MeV, 5.686
MeV, 6.051 MeV, 6.288 MeV, 6.778 MeV and 8.784 MeV. These
are the six peaks that must be visible for a Silicon detector to
be used.
Beam Position
Our solution to these challenges was to improve the
technology available by creating a new, state of the art,
detector- now known as the Dual-axis, Duo-lateral Position
Sensitive Detector, or simply, a DADL PSD.
This is a position image using the Cyclotron. A gold target was
used with a silver, 15 MeV beam. Since this reaction produces
higher energies, a clearer image is produced than when the
228Th is used.