Dedicated Arrays: MEDEA

Dedicated Arrays: MEDEA

GDR studies (E

= 10-25 MeV) Highly excited CN

E*~ 250-350 MeV, 4 ≤ T ≤ 8 MeV

intermediate energy region

10 MeV/A ≤ E_beam ≤ 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

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

ΔE_γ/E_γ = 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

BaF

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: E_res= E- ΔE detector

129Xe (@45MeV/A) + ¹⁹⁷Au

Fast ( Δ E)

Slow (E)

129Xe (@45MeV/A) + ¹⁹⁷Au

Z=1

E

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

[MeV] E* 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:

π

→ γ + γ

η → γ + γ

reconstruction of

invariant mass requires:

§  E_γ1, E_γ2 photons energies (up to 1 GeV)

§  Θ₁₂ opening angle between photons

(≤ 180⁰)

) cos 1

(

2 _{1 2 12}

2 = E_γ E_γ − Θ

c m_meson

High energy y-rays develop electromagnetic showers

 full reconstruction of em shower is needed

m_π0 = 134.97 MeV m _η = 547.75 MeV

TAPS

Two/Three Arm Photon Spectrometers (GSI-GANIL)

angular range: 10⁰≤ θ_lab≤ 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

64

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 E_c ≥ 13 MeV electons generate EM Showers

⇒ cluster of detectors are needed E_e = 43.5 MeV

position resolution: Δx,y ~ 3cm

(from center of gavity of deposited energies)

Response to high-energy photons

E_γ = 186 MeV

E_mean = 19 MeV

High-energy photons generate EM Showers

⇒ cluster of detectors are needed

E_mean = 5.6 MeV

position resolution: Δx,y ~ 2.5cm

(from center of gavity of deposited energies)

Response to charged particles p and α

E_α = 100 MeV

Fast

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 E_n ≥ 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:

/ c2

E M_γ = _γ

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.