LECTURE 35 γ DECAY PHY492 Nuclear and Elementary Particle Physics

LECTURE 35
γ DECAY
PHY492 Nuclear and Elementary Particle Physics
Last Lecture
Shell-model pictures for beta decays (example)
Fermi transition 14O(0+)→14N(0+)+e++νe
electron, neutrino spin : anti-parallel
allowed transition
→ total spin S = 0
→
L=0
changes of spin and angular momentum of nucleon → ΔS= 0, ΔL = 0
changes of spin and parity of nuclear states → ΔJ= 0, ΔP = 0
2s1/2 2s1/2 1d5/2 1d5/2 1p1/2 1p1/2 1p3/2 1p3/2 1s1/2 1s1/2 14O(0+) April 11, 2014 14N(0+) PHY492, Lecture 35 2 Gamma decay
Shell-model pictures for gamma decays (example)
E1 transition 14O(1-)→14O(0+) + γ
gamma ray carries a certain angular momentum (and parity)
2s1/2 2s1/2 1d5/2 1d5/2 1p1/2 1p1/2 1p3/2 1p3/2 1s1/2 1s1/2 14O(1-) April 11, 2014 14O(0+) PHY492, Lecture 35 3 Gamma transitions
Ji Gamma emission is a form of
electromagnetic radiation and
includes two possibilities:
Electric (E) radiation
Magnetic (M) radiation
γ
Characterized by
the angular momentum L
and the change of the parity ΔP
Photon ( spin-1 vector meson )
carries a total angular momentum L
Jf L≥1
transition Ji=0+ → Jf=0+
is strictly forbidden. Transitions with different multipolarities are called;
L = 1 ( dipole )
L = 2 ( quadrupole )
L = 3 ( octupole ) etc
April 11, 2014 PHY492, Lecture 35 4 Selection Rules
Ji The allowed values of L are
restricted by the conservation equation
γ , L between
the photon total angular momentum L
Jf and the spins of the initial ( Ji ) and final ( Jf ) states.
Ji = Jf + L
L Ji + Jf ≥ L ≥ | Ji – Jf |
Ji Jf Parity
electric dipole :
qr magnetic dipole : qr×v +q r -q P = -1
r → -r P = +1
r → -r April 11, 2014 PHY492, Lecture 35 P = (-1)L
(for electric transitions)
P = (-1)L+1
(for magnetic transitions)
5 Multiporarity
We can determine which radiation types ( multipolarity )
are allowed for any specific transitions. exceptions) 6.0 MeV
examples) 2.1 MeV gs 12Be
0+ (L=2,ΔP=No)
April 11, 2014 (E0)
2+ E2 0+ 0.35 MeV gs -  internal
conversion
-  internal
pair production 5/2+ M1 or E2 3/2+ 21Ne
(L=1 or 2,ΔP=No)
PHY492, Lecture 35 gs 0+ 16O
(L=0,ΔP=No)
6 Transition Rates
The transition probability per unit time is given by Eγ : the photon energy
BE(L) [e2fm2L]
: reduced transition probabilities
BM(L) [(µN/c)2fm2L-2] which contain structure information
*) if the nuclear structure factor is the same,
lifetimes (∞ 1/T ) for EL (or ML) transitions have
the photon energy dependence of (Eγ)-(2L+1)
April 11, 2014 PHY492, Lecture 35 7 Weisskopf units
If one can assume that the radiation results from the transition of
a single proton from an initial orbital state ( pure shell-model state )
to a final state with the angular momentum L=0,
the transition strength is approximated by ( Weisskopf Units )
With R = R0 A1/3 ( R0 = 1.21 fm ) There is a substantial decrease in
decay rates with increasing L, E > M by two orders of magnitude April 11, 2014 PHY492, Lecture 35 8 Interactions of Photons in Matter
Photons have a high probability of
being absorbed or scattered through
large angles by the atoms in matter
A collimated monoenergetic beam of I
Photons per second traversing a thickness
dx of matter will lose
dI = - I0 · dx / λ
Ι = I0 exp (-x/λ) (λ : mean free path )
a : the photoelectric effect
photon is absorbed by the atom as a whole
b : Rayleigh
photon scatters coherently from the atom
c : Compton scattering
photon scatters from an atomic electron
d : pair production (nucleus)
e+, e- pair production, related electron bremsstahlung
e : pair production (atomic electrons) April 11, 2014 PHY492, Lecture 35 9 Gamma-ray detectors
To measure total photon energies,
one has to choose a proper material
for detectors, to maximize the
photoelectric effect.
NaI Z dependence of cross sections
Counts Z5 … photoelectric effect
Z …Compton scattering
Z2 … pair production
high Z material Ge Typical detectors
-  NaI scintillator ( ZI = 53 )
J high efficiency
L reasonable resolution (several %)
-  Ge detector (solid-state detector) (ZGe=32)
J high resolution (less than 1%)
L low efficiency
April 11, 2014 PHY492, Lecture 35 photon energy 10 Gamma-ray spectroscopy with RIB
In RIB experiments,
(rare isotope provided as beam),
one has to measure gamma-rays
emitted in flight
observed gamma-ray energies Eγ obs
are Doppler-shifted Eγ obs
42S θ
Eγ
To obtain the energy Eγ
in the rest frame of the beam, one needs
to make Doppler-shift corrections ;
Eγ
= γ ( 1 - β cos θ ) Eγ obs ; β = v/c, v beam velocity Gamma-ray detector must be
segmented to determine
gamma-ray emission angle θ
Eγ
Eγ obs
Dopplercorrections H.Scheit et al.
PRL77(96)3967
April 11, 2014 PHY492, Lecture 35 11 Gamma-ray detector arrays at NSCL
SeGA
W.F. Mueller et al, NIM A 466, 492 (2001) 18 Segmented Ge Array CAESAR 192 CAESium iodide ARray April 11, 2014 APEX 24 NaI scintillator bars PHY492, Lecture 35 12