Semiconductor Detectorssleoni/TEACHING/Nuc-Phys-Det/PDF/... · 2012-12-21 · Semiconductor...
Transcript of Semiconductor Detectorssleoni/TEACHING/Nuc-Phys-Det/PDF/... · 2012-12-21 · Semiconductor...
Semiconductor Detectors
Solid state Ionization Counters
1/40 eV
4 valence atoms
No need to go to very low Temperature with Si:
Intrinsic carriers concentration
ISi ∼ 10-9 A, IGe ∼ 10-3 A,
Leakege current
At room temperature: Measurable with Si detectors NOT Measurable with Ge detectors
ISi ∼ 10-9 A, IGe ∼ 10-3 A, Leakege current
n-type p-type
To control the electrical conduction
p-type in contact with n-type give rise to a p-n junction
n-type p-type
Increase of depletion region by applying REVERSE BIAS ⇒ E = Eint+ eVext Ge: Vext = 1000-3000 V, Si: Vext = 300 V
∇2ϕ = −ρ /ε
Ε(x) = − dϕdx
∇2ϕ = −ρ /ε
p-type n-type
Used for charged particles: 1 MeV e-, range = 1 mm 5 MeV α, range = 0.02 mm
Electric contact
77 K (−196 °C) with LN2
High Purity Ge Detectors impurity concentration N ~ 1010 atoms/cm3
600 µm
0.3µm
eNVd ε2
≈
Characteristics size Ø~10cm, L~9cm shape coaxial n-type less sensitive to radiation damage operating temperature < 85 K rates ~ 10 kHz to prevent pile-up energy resolution 2 keV at 1.332 MeV (0.2 %) time resolution 4-5 ns (with CFD) 200-300 ns total rise time efficiency* up to 200% * relative to 7.5x7.5 cm NaI(Tl) for 1.33 MeV γ-rays emitted by 60Co source at 25 cm from detector (εa = 1.2 x 10-3)
active region d
V~2500-4500V
15%
150%
See also: Best choice HpGe detector, from ORTEC
Signal Pulse Shape depends on interaction point
V+
V-
V+
V-
Planar geometry
Coaxial geometry
most severe in p-type detectors
Trapping Effect: - Reduction of pulse amplitude due to capture of carrier by trapping centers - Deterioration of energy resolution due to variable amount of of charge lost per pulse
Energy resolution versus Temperature FW
HM
[ke
V]
FWHM
[ke
V]
Temperature [K] Temperature [K]
@122 keV @1332 keV
band structure effect
T(LN2) = 77 K (−196 °C)
Pulse shaping
true pulses
from preamp τ ~ 50µs
after shaping
FET
Preamplifier : FET (at 130 K, to minimize noise) Amplifier: CR-RC shaping circuit
pile-up
energy
time
τ
τ
t
out etEE −=if C1R1=C2R2=τ
τ ~ 15 µs τ ~ 15 µs is a good compromise
between reduced pile-up and good energy resolution
(depending on large charge collection)
mV
V
Preamplifier
R=input resistance C=input capacitance+ detector capacitance + cables …
τ = RC
operation mode for time information, high rates, …
operation mode for energy information
tc charge collection time ~100 ns
τ =RC decay time ~ 50 µs
Amplifier (RC-CR shaping)
RC-integrator (low-pass filter)
)1(...
/τtout
outin
eEE
EiRE
−−=
+=
CR-differentiator (high-pass filter)
τ/
...t
out
outin
EeE
ECQE
−=
+=
γ-ray interaction
γσ EZpp ln2≈
ionization occurs in limited regions of the absorber
Ge
µ
ppCph σσσµ ++=
Linear attenuation coefficient (probability per unit path)
γ
γσEE
ZC
ln≈
54
5.3
−=
≈
n
EZ n
phγ
σ
I/I0
t
e-µt
Detector response We detect recoil electrons
and NOT photons !
)(256.02
/21
22
2
cmEifMeVcm
cmEE
EEE
ee
eCEgap
>>=≈
+=−=
γ
γ
γγ
Egap
Important characteristics: § energy resolution: δEγ/Eγ = FWHM/Eγ
§ peak-to-total: P/T = Areapeak/Areatotal
Egap
Ge Response function (+ Anti-Compton Shield)
P/T~20%
P/T~60%
Anular detector
used material: BGO (Bi4Ge3O12) § density ~ 7.3 g/cm3
§ Z = 83 § 3 times more efficient than NaI ⇒ ideal for very compact geometry (small spaces) N.B. in some cases NaI nose is used to improve the light output far away from PM tube
used with heavy metal collimators in front
incident γ
Compton scattering angular distribution
)cos1)(/(1 2'
θγ
γγ −+=
cmEE
Ee
high-energy γ-ray: forward scattering low-energy γ-ray: forward & backward
NaI nose: improvement of light output far away from PM tubes (low-energy γ-rays) BGO back-catcher: improvement of high-energy Compton scattering (high-energy γ-rays)