Dedicated Arrays: MEDEA GDR studies (Eγ= 10-25 MeV) Highly excited CN E*~ 250-350 MeV, 4 ≤ T ≤ 8 MeV

intermediate energy region 10 MeV/A ≤ Ebeam ≤ 100 MeV/A - large variety of emitted particles   γ-rays, π’s, n, p, e±, α, … heavy fragments - wide energy range   ∼ 1- 300 MeV

γ-ray spectrum

GDR

bremsstrahlung (nucleon.nucleon collisions)

for a precise determination of Compound Nucleus excitation energy

a study of various reaction products is needed

The limit of nuclear existence: limiting temperature of

Compound Nucleus

experimental observable: GDR yield vs. E*

MEDEA Multi Element Detector Array (GANIL-LNS)

4π array 180 BaF2 detectors: - γ ’s -  light charged particles 300 ≤ θ ≤ 1700

00 ≤ φ ≤ 3600 è Ω ∼ 3.7 π

forward wall 120 phoswich detectors : - light charged particles - heavier fragments

R

R = 22 cm, Δθ ∼ 150 crystal dimensions: Ø= 62mm, L ∼ 20 cm, V ∼ 1300 cm3

100 ≤ θ ≤ 300 00 ≤ φ ≤ 3600

R = 55 cm, Δθ ~150 (ΔE-E) plastic scintillators NE102A & NE115(τ ~ 2.4 ns & 230ns)

crystal dimensions: L= 2mm + 30cm same PMT, separation of signals via PSA ΔE threshold = 14,19,23,54 MeV for p, d, t and α

High granularity: high multiplicity events & angular resolution

Large crystal size: reduction of shower leakage through crystal surfaces

R

MonteCarlo Simulations: determination of optimal thickness of BaF2 for detection of γ-rays up to 300 MeV

efficiency

deposited energy

single crystal

9 crystals cluster

single crystal

NOT much improvement is observed from 20 to 22 cm

è MEDEA crystal size: L = 20 cm ~ 10 X0 (radiation length X0 = 2.05 cm)

~ Range 300 MeV protons ~ Range 1 GeV α

ΔEγ/Eγ = 12% (@ 662 keV line of 137Cs)

Radiation length X0: - typical scale for high-energy electromagnetic cascades: - high-energy electrons: mean distance to reduce to 1/e energy by bremsstralung - high-energy γ’s: 7/9 of mean free path for pair production

20 cm

Scattering chamber paraffine shield for γ calibration source

beam

½ BaF2 ball

phoswich wall D type BaF2 crystals

~ 10-5 mbar

BaF2 ball: discrimination of - γ-rays & neutrons : TOF

- γ-rays & light charged particles

Phoswich wall: identification of γ-rays & light charged particles:

PSA of anode output: -  fast: ΔE detector -  slow: Eres= E- ΔE detector

129Xe (@45MeV/A) + 197Au

Fast

(ΔE)

Slow (E)

129Xe (@45MeV/A) + 197Au

Z=1

Eres PMT

ΔE

forward backward particles particles

γ γ

Fast

Fast+Slow

p d t

Z=2

Investigation of Progressive disappearance of GDR with excitation energy

LNS Catania: MEDEA-MULTICS-MACISTE Setup

160

200

290

350

430

E* [MeV]

evidence for progressive quenching of collective motion (GDR)

onset of GDR quenching E*∼ 250 MeV

E*/A ∼ 2.5 MeV

Evolution of GDR yield with E*

statistical decay regime

competition between collective motion & particle decay

A=110-130

Dedicated Arrays: TAPS Low and High energy photons detection Relativistic Heavy Ions Physics

Relativistic Heavy Ions: study of nuclear matter

under extreme conditions

ρ ~ 2-3 ρ0 T ~ 300 MeV ⇓

nucleons are excited into resonances

which decay via mesons emissions

neutral mesons decay: π0 → γ + γ (99%) η → γ + γ (39%)

reconstruction of invariant mass requires: §  Eγ1, Eγ2 photons energies (up to 1 GeV)

§  Θ12 opening angle between photons (≤ 1800)

)cos1(2 12212 Θ−= γγ EEcmmeson

High energy y-rays develop electromagnetic showers

full reconstruction of em shower is needed mπ0 = 134.97 MeV

m η = 547.75 MeV

TAPS Two/Three Arm Photon Spectrometers (GSI-GANIL)

angular range: 100≤ θlab≤ 1650

target distance: 0.5 m ≤ D ≤ 3.5 m

64 scintillator module x·y = 51·42 cm2

γ-particle discrimination: - pulse shape analysis - TOF -  Veto detectors (particle suppression)

384 individual BaF2 detectors packed in arrays of 64 modules

precise determination of point of impact of electromagnetic showers

individual detector telescope L = 25 cm= 12 X0, Ø = 54 mm BaF2 gain monitor by N2-laser

Array of 64 plastic/BaF2 telescopes

0.01 X0

Veto

Response to electrons above critical energy Ec ≥ 13 MeV

electons generate EM Showers ⇒ cluster of detectors are needed

Ee = 43.5 MeV

position resolution: Δx,y ~ 3cm (from center of gavity of deposited energies)

Response to high-energy photons

Eγ = 186 MeV

Emean = 19 MeV

High-energy photons generate EM Showers ⇒ cluster of detectors are needed

Emean = 5.6 MeV

position resolution: Δx,y ~ 2.5cm (from center of gavity of deposited energies)

Response to charged particles p and α Ep = 55 MeV

Eα = 100 MeV Fa

st

Fast+Slow

projection

inelastically scattered p (@55MeV) on 12C

ΔE/E~2.4%

Discrimination against neutrons

- high-energy neutrons: (n,p) interactions TOF + response CPV + PSA - low-energy neutrons (<< 100 MeV): (n,γ) interactions TOF only

ε ~ 17 % for En ≥ 5 MeV

invariant mass spectrum

π0 production

to 12C excited

Dedicated Arrays: TAPS Low and High energy photons detection (KVI-Groningen)

Proton-proton scattering @ 190 MeV

study of virtual bremsstrahlung below pion-production threshold

Photon invariant (relativistic) mass:

2/ cEM γγ =

p + p → p + p + γ* → p + p + e+ + e-

σ ∼ 3-5 pb

liquid hydrogen target

SALAD - 2 wire chambers -  24 plastic scintillators (VETO of elastic channel)

Trigger 2 proton events in SALAD

(2 charged particle tracks) &

2 lepton events in TAPS (2 charged EM showers)

Virtual photons

The electron and nucleon interact by the electromagnetic force, the carrier of this is the virtual photon

as has different properties to ordinary photons.

Take for example two electrons:  These repel each other due to the electromagnetic force, we say that there is a mediator or exchange

particle which is transferred between them, the photon. 

If one imagines two ice skaters facing each other and one throws a ball to the other person both skaters will move apart, just as two

electrons would repel each other.

Virtual photons The electron and nucleon interact by the electromagnetic force, the carrier of this is the virtual photon as has different properties to

ordinary photons.

Take for example two electrons.  These repel each other due to the electromagnetic force, we say that there is a mediator or exchange

particle which is transferred between them, the photon.  If one imagines two ice skaters facing each other and one throws a ball to the other person both skaters will move apart, just as two electrons

would repel each other.

When delving inside the proton (or neutron) it is not the electron which actually 'probes' the nucleon but the photon.  An electron gives

some of its energy (and so loses some of its momentum) to the photon.  The more momentum which is transferred to the photon, the more energy it has and so the shorter the wavelength of the photon. One can imagine that a longer wavelength photon will only 'see' the

whole nucleon and so be elastically scattered, but for shorter wavelength photons it can 'see' the constituents of the nucleon, the quarks inside.  This is why physicists want to build larger and larger

accelerators, so that they can see more and more of the structure of particles.