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Solid State Detectors- 5
T. Bowcock
2
Schedule
1 Position Sensors
2 Principles of Operation of Solid State
Detectors
3 Techniques for High Performance Operation
4 Environmental Design
5 Measurement of time
6 New Detector Technologies
3
Time - the fourth coordinate
• Three Reasons time is used– to separate signal from background– particle identification– improvement in resolution
4
Using time
• Could use t to measure velocity directly– good spatial resolution easier to achieve
• In practice we tend to use it only for particle identification
5
TOF
Start time
6
Time of Flight
• The momentum of a particle is measured using its curvature in a magnetic field
• Its velocity is related to its momentum
• From time we can calculate velocity22
221 /)(1667 pmmLdt
(ps m)
7
TOF
-K
K-p
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E896 TOF
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Traditional Methods
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Scintillation
Intrinsically fast<0.1ps
11
TOF technology
• Normally use scintillator and photomultiplier tubes
• Resolution can be down to 100ps
• Solid State Detectors?
12
Timing with Si
• Signal rise time in our diode detectors was very slow
• few ns rise time– very high fields increase mobility– shallower detectors– impossible electronics
13
Microchannel plates
• Solid state photo-multiplierOften made of glasscan be made of solid Si
etching
14
Operation of MCP
15
MCP
• Can be made of Glass or Si
• Typical transit time about 50ps
• Resolutions of order 100ps or better– thin holes(down to 5 microns) – high fields– perhaps as low as 10ps– electronics
16
MCP
• Use either by themselves or in conjunction with scintillator
• Highly expensive
• Commercial product– night vision– time of flight systems for ion beams
17
Compare with Spark Chambers
• Cylindrical Spark Chamber (Pestov style)
Resistive glasslimits discharge
18
Note
• Calculation of tof usually requires precision tracking (mm or better)
• MCP could be used with r/o with high granularity– BUT– fine t resolution large distances– large distance imply large areas– cost becomes inhibitive
19
Counting Time
• Direct clock counting (1ns)
• Digital interpolation of GHz clock (0.2ns)
• Time Stretcher– best resolution available is about 1ps
• LeCroy
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Time Stretching
• Time to Amplitude converter– starts a ramp on a signal– stops when it receives another
• Wilkinson type “converter”– discharge by a constant current
• Second stage of TAC– final digitisation
• Factor of 500 attainable
21
Intrinsic limit
• At 1ps are we getting close to limit– waveguide on chips– sizes critical
• ps electronics not at all compact?
• Can’t put at Time stretcher on every channel
• Large dead time
22
TOF + telescope
• Not just used in HEP
• Galileo Probe
23
Streak Camera
• Can also be used as part of a streak camera
24
Other uses for timing information
• More “usefully”
• We can also use solid state detectors as drift detectors– remember gaseous drift detectors
25
PIN diodes
• Charge may be stored in a pnn+ diode structure
• At 4.2K trapping time>105s
- - -
p+
n+
n-doped Intrinsic
26
PIN diodes
• Stored charged can be released by application of a few V/m
• At 2V/m trapping time is only 10ps
• Capture happens at a characteristic distance of 1m
27
Tunneling from trap
Conduction Band
Finite Tunneling Probability
28
Silicon Drift Chamber
• Depleted from side over long distance• Until depletion “median line” conducts
n
Al
Al
-V
-V
p
p
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Silicon Drift
V V-dV V-2dV V-3dV
V V-dV V-2dV V-3dV
n
30
SSD Operation
Basic detector characteristics:
Rectangular active region 50 x 67 mm 9% of dead region (guard structures) Implanted P+ resistor HV divider Drift length 2 x 33.5 mm 200 anodes on each side of the detector (anode pitc 250 um) 3 lines of MOS , N+ and N+(AC) charge injection structures Precise openings in metalization for laser-induced charge injection Anode structures around the guard zone allows to study leakage currents in this region Detector is fabricated on 3.5 kOhm/cm NTD N Si
31
ALICE SDD
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SDD
• Many experiments use SDD– timing vital– drift possible– an extension of previous methodology
• Difficult to fabricate– double sided processing– handling
• Why not use strips?
33
Summary
• The time coordinate not used in comparison with x,y,x– space gives momentum and topology
• Time most useful for background rejections– c.f. emulsions– triggering (coarse requirement)
34
Summary cont’d
• Time TOF– particle ID– solid state only used as part of the
system(MCP)– pulses in diodes are slow
• Time can be used to give spatial information
35
New Technologies
• Si developments– Oxygenated Si– p-type Si
• Diamond• Polymer diodes• Deep Sub-Micron Processing• Nanotechnologies• Physics