§ Detection of γ -rays from nuclear decay: 0.1 < E < 20 MeV § § § §

(1)

Detection of γ-rays from nuclear decay:

0.1 < E _γ < 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

(2)

α

n p

γ 70

rotazione smorzata

T ≠ 0

T=0

rotazione discreta

α

n p

γ α

n p

γ 70

γ-decay of Compound Nucleus

40 Ar

~ 5 MeV/A

124 Sn ¹⁶⁴ Er

GDR

132

Ce

γ-decay below n-threshold

§  E

_γ

< 8 MeV

§  FWHM ~ 2-10 keV

§  M

_γ

~ 20-30 (within few ps)

Giant Dipole Resonance

§  E

_γ

~ 15 MeV

§  FWHM ~ 5-7 MeV

§  P

_γ

/P

_part

≈ 10

^-3

163

Er

(3)

3 6

E

_γ2

E

_γ1

10

^-15

sec γ detector

Δt ∼ 10

^-9

sec

γ-ray patterns reveal nuclear structure

Single-particle generation of spin leads to an

irregular level structure Collective rotation

leads to regular

band structures

(4)

there is NO detector

providing simultaneous measurement of high-energy and low-energy γ-rays

with optimal conditions

Low-energy:

Ge detectors

good energy resolution

High-energy:

Scintillators (NaI, BaF

₂

, …)

high efficiency, good timing, particle discrimination

Arrays

dedicated or coupled

Arrays of segmented Ge detectors … ?

Pulse shape & tracking (particle-γ discrimination ?)

Present status

Future

Z

32 83

53 56

(5)

@ 1.33MeV

Low-energy γ-rays

662 keV 482 keV

FWHM=1keV

FWHM=40 keV

Ge

NaI(Tl)

E

_γ

[keV]

C ou nt s

108m,110m

Ag

The energy resolution depends directly on the number N of charge carriers

R 2 . 35 N

in a scintillator ~ 100 eV/photoelectron ⇒ N= 10

⁴

@ 1 MeV ≈

in a semiconductor ~ 3 eV/photoelectron ⇒ N=3x10

⁵

@ 1 MeV

(6)

n energy delay

(after 30 cm)

High-energy γ-rays photopek efficiency

ε

_a

=ε

_p

(Ω/4π)

for 10 cm source

ε p

_{4.2x4.2 cm}2

7.5x7.5 cm²

absolute efficiency

Ge

from ORTEC BaF

₂

10⁴

14cm × 18 cm

(7)

High Purity Ge Detectors

impurity concentration N ~ 10

¹⁰

atoms/cm

³

600 µm 0.3µm

eN d ² ε V

≈

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 (ε_a= 1.2 x 10^-3)

active region d

V~2500-4500V

15%

150%

(8)

Best choice HpGe detector, from ORTEC

(9)

Energy resolution versus Temperature

FW H M [ ke V ] FW H M [ ke V ]

Temperature [K] Temperature [K]

@122 keV @1332 keV

band structure effect

(10)

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

t

out

t e

E

E =

⁻

if C

₁

R

₁

=C

₂

R

₂

=τ

τ ~ 15 µs

is a good compromise between reduced pile-up and good energy resolution

(depending on large charge collection)

mV

V

(11)

Preamplifier

R=input resistance C=input capacitance+

detector capacitance + cables …

τ = RC

operation mode

for time information, high rates, …

operation mode

for energy information

t_c

charge collection time ~100 ns

τ =RC

decay time

~ 50 µs

Amplifier

(RC-CR shaping)

RC-integrator (low-pass filter)

) 1

( ...

/τ t out

out in

e E E

E iR E

− −

= +

=

CR-differentiator (high-pass filter)

/τ

...

t out

out in

Ee E

C E E Q

= −

+

=

(12)

Ge Response function

(+ Anti-Compton Shield)

P/T~20%

P/T~60%

Anular detector

used material: BGO (Bi

₄

Ge

₃

O

₁₂

)

§  density ~ 7.3 g/cm

³

§  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

(13)

incident γ

Compton scattering angular distribution

) cos 1 )(

/ (

1 ²

'

γ

θ

γ

γ = + −

c m E E E

e

§ Detection of γ -rays from nuclear decay: 0.1 < E < 20 MeV § § § §