surface characterisation of germanium detectors t.engert 1,2, i.kojouharov 1, j.gerl 1, p.nolan 2,...
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Surface characterisation of Germanium detectorsSurface characterisation of Germanium detectors T.EngertT.Engert1,21,2, I.Kojouharov, I.Kojouharov11, J.Gerl, J.Gerl11, P.Nolan, P.Nolan22, Th.Krings, Th.Krings33
11GSI, 64291 Darmstadt, GermanyGSI, 64291 Darmstadt, Germany22 University of Liverpool, UK University of Liverpool, UK
33SEMIKON, Detector GmbH, GermanySEMIKON, Detector GmbH, Germany
AbstractAbstract The project deals with the surface characterisation of The project deals with the surface characterisation of Germanium (Ge) detectors through all necessary Germanium (Ge) detectors through all necessary processing steps, from a raw crystal to a final processing steps, from a raw crystal to a final detector diode. The aim is to improve the surface detector diode. The aim is to improve the surface quality in order to obtain the best performance as a quality in order to obtain the best performance as a γγ-ray detector. -ray detector. It is motivated by the need for lower contaminations It is motivated by the need for lower contaminations and reduced mechanical defects. This is achieved by and reduced mechanical defects. This is achieved by applying new mechanical treatment procedures at an applying new mechanical treatment procedures at an better cost-efficiency ratio.better cost-efficiency ratio.
Detector surface scanned with AFM Detector surface scanned with AFM
Lapped-Ge Grinded-Ge Lapped-Ge Grinded-Ge
Ra [nm] 285,83 nm 660,08 nmRa [nm] 285,83 nm 660,08 nm
Rq [nm] 358,88 nm 820,37 nmRq [nm] 358,88 nm 820,37 nm
Rz [µm] 4,41 µm 9,82 µmRz [µm] 4,41 µm 9,82 µm
Rt [µm] 3,1 µm 8,61 µmRt [µm] 3,1 µm 8,61 µm
Depletion Depletion voltage voltage
Voltage [V]Voltage [V]
Cu
rren
t [A
]C
urr
ent [
A]
Operational Operational voltage voltage
γγ -ray -ray en
ergy resolution
energy resolu
tion
Edge chips
Dimples
Scratches
Mechanical defects define the voltage-current characteristics
Ge-detector characteristicsGe-detector characteristics The operational region (depletion voltage to plateau voltage) The operational region (depletion voltage to plateau voltage) has to be reached for the required energy resolution. has to be reached for the required energy resolution.
PreparationPreparation For the characterisation For the characterisation three different machined three different machined germanium crystals with germanium crystals with different surface qualities different surface qualities were prepared. were prepared.
CharacteristicsCharacteristics The surface analysis was The surface analysis was realized by an Atomic realized by an Atomic Force Microscope AFM Force Microscope AFM (right) and a Profiler (left).(right) and a Profiler (left).
Roughness of Ge-surface in x- and Roughness of Ge-surface in x- and y-direction scanned with a profilery-direction scanned with a profiler
Difficult Difficult geometrical geometrical shapes!!!!!shapes!!!!!
Excellent surface Excellent surface quality!!!!!quality!!!!!
Ø 3,5mm
Ø 2mm
Ge-crystal shaped with the Ge-crystal shaped with the ultrasonic-grinding machineultrasonic-grinding machine
ResultsResults Mechanical surface parameters Mechanical surface parameters (mean values) recorded with AFM (mean values) recorded with AFM and Profiler:and Profiler:
Better cost-efficiency ratio!!!!!
DetectorsDetectors On the basis of these mechanical On the basis of these mechanical results obtained with ultrasonic results obtained with ultrasonic grinding, novel types of planar grinding, novel types of planar Ge detectors are proposedGe detectors are proposed
Spectroscopy tests Spectroscopy tests planned with radioactive planned with radioactive sources sources
241241 Am and Am and 137 137 CsCs
Further investigations Further investigations To get more information about the To get more information about the surface structure and contaminations surface structure and contaminations from these Germanium detectors (left) from these Germanium detectors (left) and from the raw crystals, and from the raw crystals, measurements with ameasurements with a Scanning Electron Microscope are in Scanning Electron Microscope are in preparation. preparation.
pp++ contact contact
nn- - contactcontact
[l] length= 8 mm[l] length= 8 mm
[b] width= 8 mm[b] width= 8 mm
[h] height= 10 mm[h] height= 10 mm
This kind of This kind of sophisticated sophisticated mechanical mechanical treatment treatment
allows:allows:Ultrasonic grinding Ultrasonic grinding methodmethod
> micro structure with a > micro structure with a tolerances of 2 µmtolerances of 2 µm
> degree of roughness is a > degree of roughness is a factor of 4 times better than factor of 4 times better than with standard grinding techniqueswith standard grinding techniques
> and a factor of 2 times > and a factor of 2 times better compared to standard better compared to standard lapping techniques. lapping techniques.
Germanium surface machined
with a different feed parameter
Germanium surface machined
with the best mechanical parameters
Plateau Plateau voltage voltage
Measurement principle: A diamond
needle (Profiler & AFM) scans
the intrinsic surface
Safe treatment of Safe treatment of the crystals!!!!!the crystals!!!!!
AFMAFM
ProfilerProfiler
Dimensions are 32,5x32,5x15 mm3
with rounded corners and a
special guard ring