1. Laboratori Nazionali del Sud of INFN
The 3rd ELIMED Workshop
7-10 September 2016
Laboratori Nazionali del Sud of INFN
Gamma ray beams for Nuclear Astrophysics: first results of tests and simulations of the ELISSA array
Marco La Cognata

2. - High power laser system, 2 x 10PW maximum power
- Gamma radiation beam, high intensity, tunable energy up to 20MeV, relative bandwidth , produced by Compton scattering of a laser beam on a 700 MeV electron beam produced by a warm LINAC
Magurele, Romania
Photon scattering on ultra relativistic electrons

3. Nuclear Astrophysics @ ELI-NP
Nuclear physics with high power lasers:
Fission & Fusion
Production of exotic nuclei
Nuclear reactions in plasmas & electron screening
Nuclear excitations in plasma …
Photo dissociation reactions
Nuclear spectroscopy and cluster studies
Photofission
Investigation of GDR and Pigmy Dipole Resonance
Nuclear astrophysics
Direct measurements: reactions in explosive environments
Indirect measurements: investigation of radiative capture reactions
Industrial & medical applications, material science…

4. Nuclear Astrophysics @ ELI-NP
For the first time high intensity (>107 γ/s) high resolution (5 10-3) will be available, making it possible to measure photodissociation reactions of astrophysics importance.
A few physical cases to be ELI-NP
He-burning in stars
12C(α,γ)16O through the 16O(γ,α)12C reaction [indirect measurement]
3α12C+γ through the 12C(γ,3α) reaction [indirect measurement]
Si-burning in stars and presupernova phase
- 24Mg(γ,α)20Ne

5. Nuclear Astrophysics @ ELI-NP
For the first time high intensity (>107 γ/s) high resolution (5 10-3) will be available, making it possible to measure photodissociation reactions of astrophysics importance.
A few physical cases to be ELI-NP
He-burning in stars
12C(α,γ)16O through the 16O(γ,α)12C reaction [indirect measurement]
3α12C+γ through the 12C(γ,3α) reaction [indirect measurement]
Si-burning in stars and presupernova phase
- 24Mg(γ,α)20Ne
p-process (production of proton rich nuclei along the stability valley)
96Ru(γ,α)92Mo
74Se(γ,p)73As
Indirect measurements: the detailed balance principle is used to deduce the cross section of interest from the one of the time-reversed process.

6. NA experiments with gamma beams
A SSD array is very flexible as it can be used to measure photodissociation on many nuclei, including noble gases (using a gas cell) and long lived unstable nuclei (such as 7Be, 14C or 26Al)
For nuclear astrophysics, typical gamma energies are right above the particle emission thresholds
Gamma energies around 10 MeV are typically necessary
Particles are emitted with energies ranging from hundreds keV up to few MeV
Consequences on the design of a SSD array:
Large area coverage is necessary as beam intensity & cross section are relatively low
Detector granularity is not an issue as two or three particles at most are emitted per reaction event
Low threshold detectors are necessary, to keep detection efficiency as large as possible
No PSD or DE techniques are viable for particle ID

7. The ELI-NP SSD array Requirements: Low detection threshold
Low energy particles (< few MeV)  no ΔE detector, ToF, PSD
High energy and angular resolution for kinematic particle ID
24Mg(γ,α)20Ne
Typical cross section to be measured
~3 104 events per day expected at 11 MeV

8. A physical case
Silicon burning is analogous to neon burning in that it proceeds by photodisintegration and rearrangement, but it involves many more nuclei and is quite complex.
The reaction 28Si + 28Si  (56Ni)* does not occur owing to the large Coulomb inhibition. Rather a portion of the silicon (and sulfur, argon, etc.) “melt” by photodisintegration reactions into a sea of neutrons, protons, and alpha-particles. These lighter constituents add onto the remaining silicon and heavier elements, gradually increasing the mean atomic weight until species in the iron group are most abundant.
The nucleosynthesis is governed by the slowest
reaction in the network:
24Mg(g,a)20Ne
Its cross section is affected by a factor of 2
uncertainty.
Equilibrium  synthesis of the most tightly
bound nuclei  formation of an “iron” core
 pre-supernova conditions

9. Drawings of the detector
The SSD array covers about 80% of 4π
Thanks to the use of charge-partition position sensitive detectors the number of electronic channel is ~300

10. Simulated spectra
Particle yield for a 11 MeV gamma beam impinging on a 24Mg target with C backing
Energy and angular resolution of detectors, gamma beam energy spread and emittance, straggling into dead layers were accounted for
Signal is much stronger than background (3 orders of magnitude)
Kinematic separation viable

11. Background (GEANT4) Neutron background includes:
Neutrons emitted in photodissociation reactions off target nuclei (see picture on the left)
Neutrons emitted in reaction triggered by scattered photons (negligible)
Electromagnetic background includes:
Electrons and gammas from Compton scattering
Electron-positron pairs
Photons from nuclear or atomic deexcitation
The picture on the right shows the gamma per second hitting the whole detector  negligible rate & damage

12. LNS

13. The ELI-NP detectors
#35 X3 position sensitive detectors in barrel configuration
Energy resolution (FWHM) ~ 0.3%
Angular resolution 1 mm or ~ 0.4 deg
8 QQQ3 DSSSD as end cap detectors
Energy resolution (FWHM) ~ 0.3%
Angular resolution 3 mm or ~ 0.8 deg

14. Standard electronics & GET
16 - fold preamplifier + shaper with timing filter and discriminators, multiplexed output
 Since low energy gamma rays are used, max 3 particles per event are emitted
Multiplexing well suited!
Above: GET belonging to ASFIN collaboration for a future DACQ for nuclear physics experiments with SSD
In collaboration with the CHIMERA
LNS we are implementing
GET for nuclear astrophysics

15. Tests @ LNS Design goal: - Low threshold (few hundredes keV)
- Angular resolution better than 1 cm
- Energy resolution better than 1%
Experiments:
- alpha sources
- a 11 MeV 7Li beam from the INFN-LNS tandem was delivered onto on Au (about 100 µg/cm2) and 12C (about 60 µg/cm2) targets
Grids with equally spaced slits of different sizes were used to estimate position resolution

16. Experimental spectra
To measure the detection threshold, we have performed a run using a standard 3-peak alpha source and a americium source shielded by a 17 µm thick Al foil. This degrader shift the energy peak to 1 MeV and, due to energy straggling, the energy range spanned reached zero.
 300 keV threshold achieved owing to a hot collimator close to detectors. Lower threshold achieved in other chambers.
Position resolution at high energies (11 MeV, left panel) and low energies (1 MeV, right panel).
 Resolution better than 1 mm at high energies and about 6 mm at low energies

17. THANKS FOR YOUR ATTENTION
Conclusions
- The advent high-intensity & high resolution gamma-ray beam facilities is a great opportunity for nuclear physics and astrophysics, as a number of reaction of key astrophysical importance can be measured for the first time
- ELI-NP cutting-edge features must couple with high-performance detectors
- For nuclear astrophysics, particles of energies as low as few hundreds keV are emitted, making it necessary a careful detector implementation
- GEANT4 simulations have been performed, implementing the detector configurations are updating the physics wherever necessary (especially in the case of photodissociation reactions)
- A SSD array in barrel configuration (plus end cap detectors) turns out to be a very suited tool for nuclear astrophysics studies
THANKS FOR YOUR ATTENTION

18. Collaboration
M. La Cognata, A. Anzalone, V. Crucillà, G.L. Guardo, M. Gulino, D. Lattuada, R.G. Pizzone, S. Romano, C. Spitaleri, A. Taffara, A. Tumino
C. Matei, D. Balabanski, S. Chesnevskaya, D. M. Filipescu, O. Tesileanu, Yi Xu