Detection of γ-rays from nuclear decay:
0.1 < Eγ < 20 MeV
§  Basic concepts of radiation interaction & detection
§  Compound Nucleus reactions and γ-ray emission
§  High resolution detectors: the semiconductor Ge’s
§  Present Ge Arrays: EUROBALL
§  Future Arrays: AGATA
SALTARE E PASSARE A LEZIONE II
γ-decay of Compound Nucleus
124Sn
164Er
Giant Dipole Resonance
§  Eγ ~ 15 MeV
§  FWHM ~ 5-7 MeV
§  Pγ/Ppart≈ 10-3
40Ar
~ 5 MeV/A
132Ce
70
GDR
α
γ
p
n
γ-decay below n-threshold
§  Eγ < 8 MeV
§  FWHM ~ 2-10 keV
§  Mγ ~ 20-30 (within few ps)
T≠ 0
rotazione
smorzata
T=0
163Er
rotazione
discreta
γ-ray patterns reveal
nuclear structure
6
γ detector
Δt ∼ 10-9 sec
Eγ1
10-15 sec
3
Eγ2
Collective rotation
leads to regular
band structures
Single-particle generation
of spin leads to an
irregular level structure
Present status
there is NO detector
providing simultaneous measurement
of high-energy and low-energy γ-rays
with optimal conditions
Low-energy:
Arrays
High-energy:
good energy resolution
dedicated
or coupled
high efficiency, good timing,
particle discrimination
Ge detectors
Scintillators (NaI, BaF2, …)
56
53
83
32
Z
Future
Arrays of segmented Ge detectors … ?
Pulse shape & tracking
(particle-γ discrimination ?)
Counts
Low-energy γ-rays
Ge
FWHM=1keV
NaI(Tl)
FWHM=40 keV
482 keV
662 keV
Eγ [keV]
108m,110mAg
@ 1.33MeV
The energy resolution depends directly on the number N of charge carriers
in a scintillator
~ 100 eV/photoelectron ⇒ N= 104 @ 1 MeV
in a semiconductor ~ 3 eV/photoelectron ⇒ N=3x105 @ 1 MeV
2.35
R≈
N
High-energy γ-rays
photopek efficiency
εp
7.5x7.5 cm2
n energy
delay
(after 30 cm)
4.2x4.2 cm2
for 10 cm source
absolute efficiency εa=εp(Ω/4π)
BaF2
14cm × 18 cm
Ge
104
from ORTEC
High Purity Ge Detectors
impurity concentration N ~ 1010 atoms/cm3
15%
0.3µm
150%
600 µm
active region
d
Characteristics
size
shape
n-type
Ø~10cm, L~9cm
coaxial
less sensitive
to radiation damage
operating temperature
rates
2εV
d≈
eN
V~2500-4500V
energy resolution
time resolution
efficiency*
< 85 K
~ 10 kHz
to prevent pile-up
2 keV at 1.332 MeV
(0.2 %)
4-5 ns (with CFD)
200-300 ns total rise time
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)
See also:
Best choice HpGe detector, from ORTEC
Energy resolution versus Temperature
@122 keV
FWHM [keV]
FWHM [keV]
@1332 keV
Temperature [K]
band structure
effect
Temperature [K]
Pulse shaping
FET
Preamplifier : FET (at 130 K,
to minimize noise)
energy
Amplifier:
CR-RC shaping circuit
time
if C1R1=C2R2=τ
true pulses
from preamp
mV
τ ~ 50µs
t
Eout = E e
t
−τ
τ
pile-up
after shaping
V
τ ~ 15 µs
τ ~ 15 µs is a good compromise
between reduced pile-up
and good energy resolution
(depending on large charge collection)
Preamplifier
τ = RC
R=input resistance
C=input capacitance+
detector capacitance +
cables …
Amplifier
(RC-CR shaping)
RC-integrator (low-pass filter)
Ein = iR + Eout
...
Eout = E (1 − e −t /τ )
operation mode
for time information,
high rates, …
CR-differentiator (high-pass filter)
operation mode
for energy information
Ein =
Q
+ Eout
C
...
Eout = Ee −t /τ
tc
charge collection
time ~100 ns
τ =RC
decay time
~ 50 µs
Ge Response function
(+ Anti-Compton Shield)
Anular detector
used with
heavy metal
collimators
in front
P/T~20%
P/T~60%
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
Compton scattering
angular distribution
incident γ
'
Eγ =
Eγ
1 + ( Eγ / me c 2 )(1 − cosθ )
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)
Study of Detector response
133Ba
in-beam spectrum
after
unfolding
Backscattering peak:
θ=π ⇒ (Εγ)min ~mec2/2 =256 keV
important for θ≥ 1100
Accurate study of
detector response
is done with
MonteCarlo GEANT simulations