TY  - THES
A3  - Fleck, Ivor
AB  - The detection of higher energetic gamma rays (≥ 1 MeV) is of increasing importance in
medical imaging and nuclear medicine. Especially proton therapy treatment could benefit
from the ability to measure prompt gammas emitted subsequent to the irradiation of the
patient with high-energetic sub-atomic particles like protons. Such an imaging modality
would help monitor the treatment process and ensure correct particle range and optimal dose
delivery to the tumor while sparing surrounding healthy tissue.
One potential gamma detector for medical applications is the Compton camera – a two-layer
detection system, where an incoming gamma scatters in a first detection layer and is absorbed
in a second layer. In the first layer, a Compton electron is created, which carries a large part
of the momentum information about the incoming gamma. A coincidence measurement of
energy and position of both the electron and the absorbed gamma enables to reconstruct the
gamma source location to lie on the surface of a cone. Knowledge of the electron momentum
direction enables to confine the origin to an arc. The real reconstructed source position
is obtained by the measurement and superposition of many of these cones or arcs, respectively.
In this work, a novel detection concept for the Compton scattered electron is presented and
investigated, which is based on the coincident measurement of Cherenkov photons created
by that electron in an optically transparent radiator material. The photons are emitted
along the surface of a cone with a characteristic opening angle that mainly depends on the
refractive index of the material and the velocity of the electron. The intersection of this
Cherenkov cone with a photon sensitive detector area forms a ring or an ellipse, which can
be used to reconstruct the cone and the momentum direction of the electron. The number
of emitted photons yields information on the electron energy, while the size of the ellipse
contains information on the scattering vertex position.
A first proof of this concept is provided in this thesis. In a first test set-up, a successful
coincident measurement of Cherenkov photons on an array of Silicon-Photomultipliers (SiPMs)
was performed. The photons were created by electrons from a Sr-90 source inside radiator
materials of different types and thicknesses. A coincidence time resolution of 242 ps could be
achieved using signal read-out based on an application specific integrated circuit (ASIC). The
number of detected photons could be counted with a charge integrating measurement and
analysis method using an oscilloscope. The width of the distribution of the measured patterns
was quantified and was in good agreement with predictions. All results were compared
with calculations, which were performed under consideration of electron energy and range,
detection efficiency of the SiPM, detector geometry and absorption properties of the radiator.
A sensitivity of the measured pattern to the thickness of the sample and to the position of
the electron source was observed from accumulated coincident events. These patterns also
allowed for a reconstruction of the electron source position with an accuracy better than
1 mm. In the scope of the development of the set-up and measurement method, all detector
components were investigated to find the optimal parameter settings and the most suited
radiator materials.
With an improved set-up with a different ASIC and cooled detectors a coincident light
detection on single photon level was possible. An extensive correction algorithm allowed for
a compensation of time walk effects and inherent time differences between individual ASIC
channels. The ability to count the number of detected Cherenkov photons per event and per
Silicon Photomultiplier (SiPM) channel was implemented using the Time over Threshold
(TOT) information of the SiPM signals. The average number of detected photons per event
was measured for various sample thicknesses and the results were compared to calculations
and simulations performed with Geant4.
After these first successful coincidence measurements, the detection principle was applied
to the detection of Compton scattered electrons and photo electrons created by 511 keV
photons from a Na-22 source in UV transparent Polymethyl Metacrylate (PMMA). A detection
efficiency on the order of 0.001 was found. Simulations indicate a strong increase in the
efficiency to about 3 % for higher gamma energies. The number of detected Cherenkov
photons from Compton electrons was counted and compared with simulation results. The
measured coincidence pattern from accumulated events showed response to a shift of the
gamma source position.
The ability to detect Cherenkov photons from Compton electrons in coincidence could be
successfully demonstrated. In future works, the patterns of individual events need to be used
to reconstruct the Cherenkov cone and the electron momentum direction. The achievements
in this thesis constitute a vital step towards an application of this electron detection principle
for medical purposes and could help realize prompt gamma detection in particle therapy
treatment using a Compton camera.
AU  - Bayerlein, Reimund
DA  - 2020
DO  - 10.25819/ubsi/4298
KW  - Compton-Kamera
KW  - Cherenkov Light
KW  - Medical Imaging
KW  - Silicon-Photomultiplier
KW  - Photon Detection
KW  - Detector Physics
KW  - Compton Camera
LA  - eng
PY  - 2020
TI  - Coincident detection of Cherenkov photons for medical applications
TT  - Koinzidente Detektion von Cherenkov Photonen für medizinische Anwendungen
UR  - https://nbn-resolving.org/urn:nbn:de:hbz:467-16853
Y2  - 2024-11-22T09:57:04
ER  -