§ § Charged particles detectors Arrays (2)

Charged particles detectors Arrays (2)

Low Energy Heavy Ions Reactions (~ 5-10 MeV/A)

Ancillary Detectors (EUROBALL)

§ Internal conversion electrons: MiniOrange ICEMOS

§ Heavy fragments detection:

- recoiling ions: RFD

- fission fragments: SAPHIR

- binary reactions products: BRS

Internal conversion electrons

Internal conversion

competing

g -decay:

if g-decay is inhibited the energy

can be given directly to an electron of the atom

b

ex

E

E

E

= -

E_ex = energy of nuclear state E_b = electron binding energy

decay of isomeric state

of ¹¹³In

conversion electrons are the only

monenergetic electrons

N.B. Following decay by internal electron conversion the atom is left with a vacancy, filled by electrons from higher shells Þ emission of X-rays

e

t

l l

l =

+

total decay probability of nuclear state

g-decay internal conversion

Example:

b

decay of

Hg

continuous

b- spectrum internal conversion K and L lines (from 279.2 keV)

electron energies:

electron binding energies

5 3 . 2

3

1

E n Z

e

e

µ ´

= l

a l

internal conversion coefficient

a _{depends on}

- atomic number Z - transition energy E_e; - multipolarity M or E

E_e [MeV]

a

Þ conversion electrons give information on:

§ g-multipolarity

§ E0 transitions

(forbidden for EM transitions)

0⁺ ® 0⁺

can only go by conversion electrons detailed calculations

ICEMOS

(Internal Conversion Electron MiniOrange Spectrometer)

conversion electrons are selected

from the high atomic background of d-electrons using a magnetic lens

focusing only electrons of a given energy

source ^{detectorSi(Li)} thickness = 6 mm T = 77 K

ring of 6 permanent

magnets

SmCo₅ magnets lenght 4.1 cm width 4.3 cm

Radial field B = (85±19)mT

Very compact geometry: position inside EUROBALL scattering chamber

transmission peak e ~ 5%

Important requirement :

high g-ray efficiency (EUROBALL) & similar electron conversion efficiency (ICEMOS) to performe

electron – g coincidences

g spectrum

e^- spectrum electron-g coincidences

Example: Study of decay-out of SD band in

Nd

(Example of transition from NORMAL to SUPERFLUIDquantum mechanical system)

SD band

ND band

B. Aengenvoort et al., Eur. Phys. J. A1 (1998)359 establishment of multipolarity

of linking transitions

SUPER Deformed

(rigid rotor, normal system)

E

e

NORMAL minimum

SUPER deformed minimum FEEDING

of SD bands

DECAY-OUT of SD bands:

Coupling between ordered and chaotic states

NORMAL Deformed

(reduced moment of inertia, superfluid system)

Recoil Filter Detector

(18 detectors ~ 1p)

e ~ 65%

beam

important in the study of fusion reactions characterize by:

1. Many evaporation residua 2. Large fraction of fission

3. Large amount of particle emission

residues identification & v/c determination via TOF

18 elements in 3 rings:

- 6 + 6 + 12 detectors

- angular range 1.4°-6.7°

- e ~ 65%

individual element:

- thin mylar foil (0.5 -2 mm) - thin fast, plastic scintillator

è N.B. scattered beam does not hit directly the scintillators

- recoiling ions hitting the mylar foil knock out electrons;

- electrons are accelerated by 20 kV and focused on the plastic scintillators

D t ~ 0.1 ns

co un ts

E

[keV]

mean velocity correction

<b> ~ 2.8%

true velocity correction mylar foil

electrodes

electron trajectories

# electrons

µ

summed signal = # electrons x 20 kV ~ 100 x 20 kV = 2 MeV

Þ TOF: residue identification

Þ tracking of reaction products

Þ v/c determination

(better Doppler correction)

20 kV

A = 45

4 keV

157Gd(³⁶Ar,4n)¹⁸⁹Pb, E_beam = 173 MeV

189

Pb angular distribution

189

Pb energy distribution

particle evaporation after target

detected ions Angular & energy distributions

of recoiling ions are strongly modified passing through the target:

much broader distributions

Þ average

for v/c ³ 3 %

K. Sphor et al., Acta. Phys. B26 (1995)297

PHYSICS CASE for RFD

Robustness of Shell Correction at T>0 in SHE Nuclei

Super Heavy Nuclei

most striking manifestation of shell structure in nuclei:

- Z > 100 nuclei are at the limit of Coulomb instability

- large shell correction energy provides additional binding (up to 8 MeV)

Z > 100 Search for

limiting spin and excitation energy SHE nucleus can substain

Z=102:

No

2n

253

No +

No < 1%

48 Ca + ²⁰⁸ Pb à ²⁵⁶ No*

2n 1n 3n

E

= 215-219 MeV Q

~ -150 MeV

s(

No) ~ 3 µb

2n

No

• chemistry

o SHIP

H.W. Gaggeler et al., NPA502(1989)561c

Recoil Filter Detector

@ AGATA-Demonstrator

(INFN-Legnaro)

Additional detectors around AGATA:

HELENA Array

Large LaBr

Array

TOF TOF

Pu ls e H ei gh t (En er gy )

beam CN beam CN

26

Mg (@128 MeV) +

Sn à

Sm*

(GASP + RFD, DeAngelis et al., March 2009)

RFD has been succesfully used

to identify the heavy nucleus ²²⁰

Th

220

Th

fission

RFD- measurements with a continuous beam

scattered beam

recoils

Eloss vs Egamma

RFD signal Ampl.

g-projectile g-recoil

Fission Fragments

From spontaneus fission or induces fission (ex. n-rich Nuclei)

Study of g-decay in coincidence with fission fragments:

Þ

out of a cascade of 10 g-rays

132Sn

78Ni

r-process

Neutron (N)

Proton (Z)

Saphir

(Saclay-Acquitane-Photovoltaic cell for Isomer Research)

Solar cell Array

important in the study of

spontaneus fission or induced fission (ex. n-rich Nuclei)

Information on:

1. fragment mass 2. released energy

3. momentum for Doppler correction 4. angle of fragments

Alternative to:

-

Si surface barrier

(expensive, severe radiation damage) -

Gas detector (PPAC)

(difficult to handle, care for thin window) ^2.

4 cm 4 cm

monocrystalline polycrystalline

preamp Si p-type wafer

Ag Ag

(for charge collection) titanium-oxide

§ low resistivity: r < 3W/cm²

Þ NO need of bias voltage

§ large capacitance: C~20-30 nF/cm²

Þ small signal from preamp: V=Q/C

(difficulty in extrating light particle from noise)

best performances: A>50, E>30 MeV

fragment energy versus pulse height

x E

) ' (

) '

( a a M x b b M

E = + + +

a, a’, b, b’ = calibation coefficients

Pulse-height-defect :

the pulse height observed for heavy ions is substantially less than observed for light ions

at the same energy, as a result of:

- larger dE/dx in entrance window & dead layers - loss of energy by nuclear collisions

producing recoiling ions with low energy

(reduced efficieny for electron-hole pair production) - high rate of electron-hole pair recombination

along ion track (dense plasma)

reponse function of solar cell

252Cf spontaneus fission source

§ DM = 7-8 amu

§ DE_cell ~ DESurface Barrier

§ e ~ 100%

§ Dt ~ 20 ns

§ g-ray transmission ~ 100%

§ count rate: up to 5 kHz

Pulse height spectra

Solar cell Surface barrier

H = heavy fragments (low energy) L = low fragments (high energy)

H H

L L

massspectrum

252Cf source M=145

M=86

barrel geometry: e ~ 45%

Other use of solar cells:

Fission cross section measurements (French Nuclear Waste project)

barrel geometry: e ~ 45%

Structure of neutron-rich isotopes produced by ¹²C+²³⁸U nduced fission:

observation of 18 new isomeric states t~ few ns ® few µs

fragments stopped in Saphir:

isomeric decay at rest without Doppler 119In

g-spectra detected in Dt = 50 ns- 1 µs after detection of 2 fragments in Saphir

isomer t = 240 ns

decay time spectrum for 152 keV transition depopulating 25/2⁺ isomer

t = 240 ns

gate on 1020 keV

x

x

BRS (Binary Spectrometer) Multi-Nucleon Transfer a -cluster nuclei, …

24

Mg

24Mg

12

C

C

22Mg

Frontiers of Nuclear Physics

EXOTIC NUCLEI

48Ca

66Ni

64Ni

46Ca

Multi-nucleon transfer A~50

48Ca + ⁶⁴Ni @ 6MeV/A

Need of Spectrometer for Heavy Systems