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The LUNA experiment at Gran Sasso
Laboratory: studying stars by going
underground
Alessandra Guglielmetti
Università degli Studi di Milano and
INFN, Milano, ITALY
Laboratory
Underground
Nuclear
Astrophysics
Outline:
-Nuclear fusion reactions in stars: why measuring their
cross section?
-Why going underground to perform these experiments?
-The LUNA experiment: most important and recent results
- On-going measurements and future perspective: the
LUNA-MV project
Why studying nuclear fusion
reaction cross sections?
-Stars are powered by nuclear
reactions
-Among the key parameters
(chemical composition, opacity,
etc.) to model stars, reactions
cross sections play an
important role
- They determine the origin of
elements in the cosmos, stellar
evolution and dynamic
- Many reactions ask for high
precision data.
Neutrino production in stars
p+p
 2H + e+ + n
p + e- + p  2H + e+ + n
3He + p
 4He + e+ + n
7Be + e 7Li + n
8B
 8Be + e+ + n
13N
 13C + e+ + n
15O
 15N + e+ + n
17F
 17O + e+ + n
p-p
chain
CNO
cycle
Solar neutrino puzzle: solved!
Neutrino flux from the Sun can be used to
study:
• Solar interior composition
• Neutrino properties
ONLY if the cross sections of the involved
reactions are known with enough accuracy
Big Bang nucleosynthesis
Production of the lightest elements (D, 3He, 4He, 7Li, 6Li) in
the first minutes after the Big Bang
The general concordance between predicted (BBN theory)
and observed (stellar spectra) abundances gives a direct
probe of the Universal baryon density
CMB anisotropy measurements (WMAP/Plank satellites)
give an independent measurement of the Universal baryon
density
The agreement of the two results has to be understood in
terms of uncertainties in the BBN predictions
BBN reaction network
1.
2.
3.
4.
5.
6.
n

p+n 
D+p 
D+D 
D+D 
3H + D 
p + e- + n
D+g
3He + g
3He + n
3H + p
4He + n
7
Be
12
10
6
7
Li
Li
13
11
3
3
p
2
1
He
9
8
4
D
2
n
4
5
He
7
6
3
H
7.
8.
9.
10.
11.
12.
13.
+ 4H  7Li + g
3He + n
 3H + p
3He + D  4He + p
3He + 4He  7Be + g
7Li + p
 4He + 4He
7Be + n
 7Li + p
4He + D  6Li + g
3H
Apart from 4He, uncertainties are dominated by
systematic errors in the nuclear cross sections
Hydrogen burning
4p  4He + 2e+ + 2ne + 26.73 MeV
CNO cycle
Ne-Na cycle
Mg-Al cycle
The importance of going underground…
Sun:
kT = 1 keV
EC ≈ 0.5-2 MeV
E0 ≈ 5-30 keV
for reactions of H burning
kT but also E0 << EC !!
1
σ(E)  exp(-31.29Z1Z 2 μ/E ) S(E)
E
Cross sections in the range of pb-fb at stellar energies
with typical laboratory conditions reaction rate R can be as low as
few events per month
Rate and background
R has to be compared with background B
Bbeam induced : reactions with impurities in the target, collimators,…
secondary processes
Benv : natural radioactivity mainly from U and Th chains
Bcosmic : mainly muons
Laboratory for Underground
Nuclear Astrophysics
LUNA site
LNGS (1400 m rock shielding  4000 m w.e.)
LUNA MV
(2015->...)
LUNA 1
(1992-2001)
50 kV
LUNA 2
(2000…)
400 kV
Radiation
LNGS/surface
Muons
Neutrons
10-6
10-3
Cross section measurement requirements
3MeV < Eg < 8MeV:
0.5 Counts/s
1,00E+00
HpGe
1,00E+00
GOING
UNDERGROUND
1,00E-01
1,00E-02
1,00E-03
1,00E-01
1,00E-02
counts
counts
3MeV < Eg < 8MeV
0.0002 Counts/s
1,00E-04
1,00E-03
1,00E-04
1,00E-05
1,00E-05
1,00E-06
1,00E-06
0
2000
4000
6000
8000
10000
0
2000
4000
6000
10000
Eg[keV]
Eg [keV]
Eg<3MeVpassive shielding for
environmental background radiation
underground passive shielding is more
effective since μ flux,that create
secondary γ’s in the shield, is suppressed
8000

Pb
Cu
Background reduction - Si detectors - alpha
Underground+Pb vs. Overground: factor ~10-15 reduction in the range
200 keV - 2.5 MeV
3He(3He,2p)4He
•Fundamental reaction of the p-p cycle
•Measured down to the lower edge of the solar Gamow peak
•No resonances  no nuclear explanation for the solar
neutrino puzzle
LUNA 400kV accelerator
E
beam
I
max
 50 – 400 keV
 500 A protons
Energy spread  70 eV
I
max
 250 A alphas
Long term stability  5eV/h
14N(p,γ)15O
cross section influences
•CNO neutrino fluxSolar
metallicity
•Globular cluster age
S 14,1 /5
S 14,1 x5
Standard CF88
High resolution measurement
Solid target + HPGe detector
 single γ transitions
 Energy range 119-367 keV
 summing had to be considered
CNO neutrino flux decreases of a factor  2
Globular
Cluster
age increases
of 0.7 – 1 Gyr: new
High
efficiency
measurement
lower limit on the Age of the Universe
T>14 Gy
Gas target+ BGO detector

S0(LUNA) = 1.61 ± 0.08 keV b
 high efficiency
 total cross section
Energy range 70-230 keV
BBN reaction network
1.
2.
3.
4.
5.
6.
 p + e- + n
p+n  D+g
D + p  3He + g
D + D  3He + n
D + D  3H + p
3H + D  4He + n
n
7
Be
12
10
6
Li
7
Li
13
11
3
3
p
2
1
He
9
8
4
D
2
n
4
5
He
7
6
3
H
3H + 4H
7.
 7Li + g
3He + n
8.
 3H + p
3He + D  4He + p
9.
10. 3He + 4He  7Be + g
11. 7Li + p  4He + 4He
12. 7Be + n  7Li + p
13. 4He + D  6Li + g
The two Lithium problems
1) The BBN 7Li predictions are a factor 2-4 higher than observations: a
nuclear physics solution is highly improbable
(e.g 3He(4He,g)7Be measurement at LUNA)
2) The amount of 6Li predicted by the BBN is about 3 oom lower than the
observed one in metal poor stars (debated but still «true» for a few
metal poor stars)
BBN predicts 6Li/7Li= 2 * 10-5 much below the detected levels
of about 6Li/7Li= 5 * 10-2
Necessary to constrain nuclear physics input: 2H(a,g)6Li
2H(a,g)6Li:
available data
Upper limits (indirect meas)
No data in the BBN energy range!
2H(a,g)6Li
at LUNA: Experimental setup
Strong beam induced background due to:
1) Rutherford scattering of 4He beam on 2H target
2) 2H(d,n)3He reaction
3) Inelastic neutron scattering on different materials (Cu, Pb, Ge,…)
g background in the 2H(a,g)6Li RoI
The beam induced background weakly depends on the beam energy
About the experimental setup
2H(a,g)6Li
at LUNA: gamma spectra
An irradiation at one given beam energy can be used as a
background monitor for an irradiation at a different beam
energy, if the two ROIs do not overlap
Natural background subtracted
400 keV data (grey filled)
280 keV data (red empty)
rescaled to take into account
the weak energy dependence of
the beam induced background
2H(a,g)6Li
at LUNA: results
M. Anders et al.,
Phys. Rev. Lett. 113 (2014) 042501
New LUNA data
From the new data on the 2H(a,g)6Li reaction: 6Li/7Li = (1.5 ± 0.3) * 10-5
Standard BBN production as a possible explanation for the reported 6Li
detections is ruled out. “Non standard” physics solutions?
17O(p,a)14N
and 18O(p,a)15N reactions
• In AGB stars ( T=0.03-0.1 GK )
CNO cycle takes place in H burning
shell
• Measured 17O/16O and 18O/16O
abundances in pre-solar grain give
information on AGB surface
composition
• Information on mixing processes if
cross sections are well known
17O(p,a)14N
and 18O(p,a)15N reactions
18O(p,a)15N
17O(p,a)14N
Q = 1.2 MeV
Two narrow resonances at 70
and 193 keV
Main goal at LUNA:
70 keV resonance
Q = 4 MeV
Two narrow resonances at 95
and 152 keV
Main goals at LUNA:
• rescan excitation function
• 95 keV resonance (strength
and energy)
• Measure below 70 keV
17O(p,a)14N
and 18O(p,a)15N
experimental setup
Beam entrance
Silicon detectors
(8 in total)
proton beam from
LUNA 400 kV
enriched 17O or 18O
solid targets
8 silicon detectors
foils of Al Mylar to
stop backscattered
protons low alpha
particle energy (200250 keV for
17O(p,a)14N reaction)
Target position
The 17O(p,a)14N reaction: 70 keV
resonance
• Clear peak in the green ROI (determined from 193 keV resonance)
• Shape looks reasonable
• No obvious structures in the off resonance
Off-resonance
On-resonance
ROI1
7
6
5
Counts/C
4
3
2
1
0
-1
-2
100
200
300
400
Energy [keV]
500
600
700
The 17O(p,a)14N reaction: 70 keV
resonance
• Comparison with literature is not easy to make (several reanalysis, not all published)
• Our value is higher than previously reported important
astrophysical consequences
14
Resonance strength [neV]
12
10
Sergi2010
8
Sergi2015
Blackmon
6
Hannam
4
LUNA
2
0
The 18O(p,a)15N reaction
Data taking completed. Data analysis on going
5E+4
S-factor [MeV*barn]
151 keV
resonance
5E+3
217 keV
resonance
5E+2
5E+1
0.000
334 keV
resonance
95 keV
resonance
0.050
0.100
0.150
0.200
0.250
Effective CM energy [keV]
0.300
0.350
0.400
The 22Ne(p,g)23Na reaction
NeNa cycle of H burning:
-massive stars
-RGB and AGB stars
-classical novae and supernovae IA
Impact on the abundances of:
Ne, Na, Mg and Al isotopes
LUNA range
The 22Ne(p,g)23Na reaction
Two large HPGe detectors (135% and
90% relative efficiency)
15-20 cm thick lead shielding
5 cm additional copper shield for 55°
detector
The 22Ne(p,g)23Na reaction
Experiment carried out within the “green”
box
“red” resonances directly observed for the
first time
for the “black” resonances at 71, 105 and 215
keV an upper limit was determined
BGO phase almost concluded (71 and 105 keV
res + DC component)
F. Cavanna et al., Phys. Rev. Lett. 115 (2015) 252501
LUNA 400 kV new program 2016-2019:
a bridge toward LUNA MV
13C(a,n)16O
– neutron source for the s-process (formation of
heavy elements)
12C(p,g)13N
and 13C(p,g)14N – relative abundance of 12C-13C in
the deepest layers of H-rich envelopes of any star
2H(p,g)3He
– 2H production in BBN
22Ne(a,g)26Mg
– competes with 22Ne(a,n)25Mg neutron source
for the s-process (formation of heavy elements)
6Li(p,g)7Be
– improves the knowledge of 3He(a,g)7Be key
reaction of p-p chain
LUNA MV- scientific program
13C(a,n)16O
and 22Ne(a,n)25Mg : neutron sources for the s-process
(formation of heavy elements)
12C(a,g)16O:
key reaction of Helium burning: determine C/O ratio and
stellar evolution
12C+12C:
energy production and nucleosynthesis in Carbon burning. Global
chemical evolution of the Universe
Reactions occuring at higher temperature than those belonging to
Hydrogen burning or BBN
Higher energy machine needed!
12C(a,g)16O
– Holy Grail of Nuclear Astrophysics
Stellar Helium burning in Red Giant Stars
the He burning is ignited on the 4He and 14N ashes of
the preceding hydrogen burning phase (pp and CNO)
relevant questions:
Energy production and time scale
of Consequences
Helium burning:
 late stellar
evolution
4He(2a,g)
12C(a,g)
16O(a,g)20Ne
 composition of C/O White dwarfs
 Supernova type I explosion
Neutron
sources
forII
s process:
 Supernova
type
nucleosynthesis
14N(a,g)18F(+n)18O(a,g)22Ne(a,n)
22Ne(a,g)
Oxygen-16
Element abundances in the solar system
Big Bang
1E+11
Nuclear Astrophysics ambitious task is to
explain the origin and relative abundance
of the elements in the Universe
1
H
1E+10
Abundance relative to 106 Si
H-burning &
He-burning
4 He
1E+09
1E+08
a - elements
a
Type II SN
12 16O
C
1E+07
20
Ne
1E+06
56
Fe
40
Ca
1E+05
Fe- peak
Type I SN
1E+04
N=82
r -process peak
Type II SN
1E+03
19
F
1E+02
1E+01
118
Sn
N=82
s- process peak
AGB stars
N=126
r -process peak
Type II SN
N=126
s- process peak
AGB stars
138
Ba
208
195
1E+00
Pt
1E-01
Pb
232Th
238
U
1E-02
1E-03
0
50
n source reactions
100
Mass Number
150
200
250
13C(a,n)16O
experimental status of the art
Heil 2008
Big uncertainties in the R-matrix extrapolations. Presence of
subthreshold resonances.
A low background environment is mandatory for any new study
Neutron energy 2.5-3 MeV (in the energy range foreseen)
22Ne(a,n)16O
experimental status of the art
Jaeger 2001
Precise measurement of the known resonances down to the one at
Ea = 831 keV to be performed at first, followed by a detailed
search for unknown resonances down to Ea ~ 600 keV.
Neutron energy 50-450 keV (in the energy range foreseen)
LUNA MV project
LUNA MV accelerator will be installed in the south part
of Hall C of LNGS laboratory (OPERA location)
Dimensions of the hall: 27x12.5x5.5 m3
OPERA decommissioning started in Jan 2015. Should be finished by
October 2016
LUNA MV project
Accelerator:
Intense H+, 4He+, 12C+ e 12C++ beams in the energy range: 350 keV-3.5 MeV.
Two beam lines with all necessary elements (magnets, pumps, valves,...).
Total budget (about 3.9 Meuro) from LUNA MV «Premium projects» (total
5.3 Meuro) of the Italian Research Ministry
Tender assigned to HVEE in December 2015.
Timeline:
Accelerator built and tested by HVEE by 11/2017.
Accelerator delivered to LNGS by 01/2018
Accelerator installed and tested at LNGS by 07/2018.
Then first experiments…
Experimental program for the first five years of operation to be defined
by July 2016
LUNA MV project
Building & shielding:
Reactions to be studied at LUNA MV will produce a small amount of
neutrons…in a very low background laboratory…
GEANT4 simulations with different materials in order to find the best
compromise among performance as neutron shield, price, easiness of
decommissioning, thickness (maximize internal space, … )
LUNA MV project
The «simple» solution of a 80 cm thick
concrete shielding has been selected
(Fn)av < 1.5 10-6 n cm-2 s-1
To be compared with
Fn (LNGS) = 3.0 10-6 n cm-2 s-1
Validation through independent
calculation by MCNP code
presently underway
Timeline: Engineering of shielding & building concluded by
06/2016
LUNA MV project-commissioning
measurement 14N(p,g)15O
Use of neutrino flux
as a probe of solar
interior composition
(metallicity)
CNO neutrino play
a key role: Borexino
can detect them
Necessary to better
constrain nuclear
physics inputs i.e.
14N(p,g)15O
LUNA MV project-commissioning
measurement 14N(p,g)15O
Already measured at LUNA 400kV
down to 70 keV (110 keV with
angular distribution).
Target production known.
R matrix extrapolation to Gamow
peak energies affected by high
uncertainties at higher energies
(g.s. transition in the figure)
Measure in the range 200 keV - 1.5 MeV with both accelerators and same
experimental setup to minimize systematic effects
Study angular distribution with HPGe detectors in far geometry.
Reduce the nuclear physics uncertainty from 7% to 5% and measure CNO
neutrinos (15O) with 10% uncertainty allow to determine solar metallicity
with 17% accuracy (now >30%)
The LUNA collaboration
• A. Boeltzig*, G.F. Ciani*, A. Formicola, S. Gazzana, I. Kochanek, M. Junker |
INFN LNGS /*GSSI, Italy
• D. Bemmerer, M. Takacs, T. Szucs | HZDR Dresden, Germany
• C. Broggini, A. Caciolli, R. Depalo, R. Menegazzo, D. Piatti | Università di
Padova and INFN Padova, Italy
• C. Gustavino | INFN Roma1, Italy
• Z. Elekes, Zs. Fülöp, Gy. Gyurky| MTA-ATOMKI Debrecen, Hungary
• O. Straniero | INAF Osservatorio Astronomico di Collurania, Teramo, Italy
• F. Cavanna, P. Corvisiero, F. Ferraro, P. Prati, S. Zavatarelli | Università di
Genova and INFN Genova, Italy
• A. Guglielmetti, D. Trezzi | Università di Milano and INFN Milano, Italy
• A. Best, A. Di Leva, G. Imbriani, | Università di Napoli and INFN Napoli, Italy
• G. Gervino | Università di Torino and INFN Torino, Italy
• M. Aliotta, C. Bruno, T. Davinson | University of Edinburgh, United Kingdom
• G. D’Erasmo, E.M. Fiore, V. Mossa, F. Pantaleo, V. Paticchio, R. Perrino*, L.
Schiavulli, A. Valentini| Università di Bari and INFN Bari/*Lecce, Italy