Nuclear physics for geo-neutrino studies
Neutrino Geoscience 2010
Gran Sasso National Laboratory - Italy
6 - 8 October, 2010
Lino Miramonti
Università degli Studi di Milano
and
Istituto Nazionale di Fisica Nucleare
Gianni Fiorentini, Aldo Ianni, George Korga, Marcello Lissia, Fabio Mantovani, Lino
Miramonti, Stefano Nisi, Lothar Oberauer, Michel Obolensky, Oleg Smirnov,
Corrado Salvo, Yury Suvorov
Lino Miramonti
Neutrino Geoscience 2010
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Motivation
Free protons
In all experiments that use hydrocarbons as detection media, the
employed reaction for the geo-neutrino detection is the inverse β
decay on free protons:
 e  p  e   n  1.806 MeV
Incoming flux of geo-ν
Signal Rate: S  N p  i  si
i
The relevant quantity is the specific signals si
si  
E max
1.806 MeV
(E  )f i (E  ) dE 
Where
 σ(Eν) is the cross section
 f(Eν) is the neutrino decay spectra of geo-ν produced in each β decay.
The specific signal is affected by unknown uncertainties
a) Whereas σ(Eν) is affected by uncertainties of less than 1%,
b) It is difficult to assess the accuracy of neutrino decay spectra f(Eν), which are
determined from rather indirect measurements and
questionable theoretical assumptions.
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Neutrino Geoscience 2010
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Our goal
is to provide a framework for a direct measurement of the neutrino decay
spectra f(Eν)
in order to establish the accuracy of the specific signal si.
Geo-neutrinos are produced through
pure β and β-γ processes:
X  X ' e    e
X  X '* e    e
X ' n
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The neutrino decay spectrum f(Eν) is obtained as a superposition of spectra calculated
assuming the “universal shape” approximation:
f (E )   pn Funiv (E  , Q  E n )
n
Q − En is the maximal energy
that the neutrino can take
To determine the geo-neutrino neutrino decay spectra f(Eν), one has to know the:
i) Feeding probabilities pn of the different energy states of the final nucleus
and
ii) Shape of the neutrino spectrum for each transition.
Feeding probabilities pn are derived from measurements of the intensities In,m of γ lines.
The feeding probability of the lowest state p0 (Which produce the most energetic geo-neutrinos) is
determined indirectly by subtraction:
p0  1   I m,0
m 0
Neutrino spectra should actually be measured !
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Neutrino Geoscience 2010
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“Effective” Geo-neutrinos
The natural radioactivity of present Earth arises mainly from the decay (chains) of
nuclear isotopes with half-lives comparable to or longer than Earth's age: 238U, 235U,
232Th, 40K, and 87Rb.
“Effective” geo-neutrinos are produced only in 238U, 232Th decay chains.
i.e. Antineutrinos with energy above threshold for
inverse beta decay on free proton: Eth = 1.806 MeV
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238U
chain
232Th
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chain
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238U: 234mPa and 214Bi
Geo-neutrinos from
• U are expected to contribute ≈ 80%
232Th: 228Ac and 212Bi
• Th are expected to contribute ≈ 20%
[chondritic ratio for masses Th/U = 3.9]
Considering just the 3 decays of 234mPa, 214Bi and 212Bi one have ≈ 97% of the total signal
(the respective contributions being ≈ 31% , ≈ 46% and ≈ 20% respectively) .
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Neutrino Geoscience 2010
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How to measure geo-neutrino decay spectra
When a nucleus X decays, whatever the transition
involved, energy conservation provides a connection
between
• the neutrino energy Eν ,
• the kinetic energy of the electron Te,
• the total energy of the emitted gammas Eγ.
Q  E   Te  E 
X  X '*  e    e
X ' n
Where Q  M X  M X '  M e
is the Q-value of the decay
In order to measure the geo-neutrino spectrum, we need to measure the visible energy Evis
deposited by electrons and gammas E  T  E
vis
e

by mirror reflection, we obtain the number
of events as a function of neutrino energy
E  Q  E vis
214Bi
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The Counting Test Facility (CTF)
In order to measure the visible energy Evis we need a
detector able to collect the energy released by both
electrons and gammas (with a similar response to both
particles) calorimetric measurement.
The CTF is composed by 4 tons of liquid scintillator (PC + PPO)
enclosed in a transparent nylon vessel of 2 m in diameter. The
vessel is surrounded by a volume of water contained in a
cylindrical stainless steel tank. The light is recorded by 100 PM.
The CTF photomultipliers and read-out electronics allow the
measurement of arrival times and Pulse Shape Discrimination.
The time correlation of events allows to identify specific decay
sequences associated with:
214Bi-214Po (in the 238U chain),
212Bi-212Po (in the 232Th chain).
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Neutrino Geoscience 2010
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Results from a diffuse Rn source
in 2005 during some operations with the CTF there was a limited radon contamination
due to an accidental air leak.
Radon is the progenitor of both 214Bi and 212Bi
222Rn
(in the 238U chain -> 214Bi) has a τ1/2 = 3.824 days
220Rn (in the 232Th chain -> 212Bi) has a τ
1/2 = 55.6 s
In this case is possible to
study only 214Bi .
(τ1/2 of 220Rn is too short)
Data from this period have been used in order to:
1. determine the probability p0 of populating the lowest energy state (assuming
the universal allowed shape).
2. discuss the implications on the specific geo-neutrino signal s(214Bi).
3. determine if the spectrum of the pure β transition (that to the lowest state)
is deformed with respect to the universal allowed shape.
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Data selection
214
promt 
19.9 m
Bi 
214
delayed 
164.3 s
Po 

Bi-Po coincidence
The main selection criterion is that the coincidence time between
the prompt β decay of 214Bi and the delayed α decay of 214Po,
must be 2 μs < Δt1/2 <602 μs.
This yield 4.54 × 105 events.
Others quality cuts such as
• energy cuts on the prompt and delayed signal,
• radial position of the two signals and
• α/β discrimination
give an acceptance efficiency of 99.4%.
This yield 4.46 × 105 events.
To reduce systematic effects because the gammas are
only partially contained and because of deviations from
spherical symmetry, a Fiducial Volume cut at R=42cm is
applied leaving
3.14×104 events.
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1) Feeding probability of the lowest state
Events have been grouped into 65 bins from 0 to 3.4 MeV.
The populations of the 82 excited 214Po states are fixed at the values given in the table
of isotopes (ToI)
Data have been fitted from 3 to 65 leaving
free 3 parameters:
i. p0,
ii. light yield,
iii. normalization.
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At the minimum χ2/degrees of freedom = 61.7/(63 − 3) the best-fit value is
p0 = 0.177, with a statistical 1σ error of ±0.004.
The total systematic error is estimated as +0.003−0.001
The largest systematic uncertainties originate from
• Imperfect spherical symmetry of the
detector (because of the deformations of the
inner vessel),
• Non uniform distribution of the active PMTs.
Counting Test Facility
Table of Isotopes
p0  0.177  0.004  stat 
0.003
0.001
p0  0.182  0.006
sys 
The value is consistent with the one reported in Table of Isotope.
 CTF measures p0
 whereas Table of Isotope deduces p0 from what was not observed!
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2) Implications for the specific signal
The geo-neutrino signal s(214Bi) can be written as the sum of 2 contributions:
s( 214 Bi)  p0  0  p1  1
where the cross section is averaged over the neutrino energy distribution.
Assuming universal shape, that is
the cross sections are

n
  (E  )Funiv (E  , Q  E n ) dE 
 0  7.76 1044 cm2
 1  2.82 10
44
cm
2
with errors of an order
of half a percent
From the previous analysis, we find that
CTF
ToI
Lino Miramonti
0.023
s( 214 Bi)  1.42  0.03 (stat) 0.008
(sys)  1044 cm 2
s( 214 Bi)  1.46  0.05 (stat)  1044 cm 2
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3) Shape factor for the pure β transition
we release the assumption that the spectrum for the transition to the ground state
has the universal shape.
The electron kinetic-energy distribution ф(Te) is (Te ) 
p 
n
univ
(Te , Q  E n )
n

Te  Te
(Te )  p 0  univ (Te , Q) 1  y

Te


   p n  univ (Te , Q  E n )
 n 0
The dimensionless shape
parameter y describes the
deviation
from
the
universal formula.
We constrained p0 and p1 to the ToI values leaving unconstrained only y.
The best fit, gives
 p0 = 0.177,
 p1 = 0.008,
 y = −0.11 ± 0.06 (stat).
• At minimum: χ2 = 51.6/(65-5)
• The statistical evidence of
deformed shape is 2.4σ
This result shows the potentiality of detecting spectral deformations.
Interesting results can be obtained in reducing statistical and systematic errors.
A large improvement will be obtained by positioning suitable sources in the center of the CTF.
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Neutrino Geoscience 2010
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So far, we have estimated the 214Bi geo-neutrino specific signal by using CTF data
resulting from a limited radon contamination. Our estimate has a comparable error
with the one derived from Table of Isotope.
compareble error
CTF  ToI
Our method has 2 advantages:
• The pure β transition can be detected in CTF and its probability can be
measured directly.
• One can check the validity of the universal shape approximation for the most
important decay mode.
for more info see: Physical Review C 81 034602 (2010)
Next step is to reduce both statistical and systematic errors (i.e.):
• Statistical error Δp0/p0≈ 0.5% (being Δ<σ>0/<σ>0≈ 0.5% ); this requires a
statistics larger by a factor of about 20 or some 6 × 105 selected events.
• Systematic error; the largest improvement should be obtained by
concentrating the source near the center of the detector.
Dedicated Radon Source
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Neutrino Geoscience 2010
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Preparation of a concentrated Rn source:
Radon generator
We have built some quartz vials (transparent to UV light) with an
external diameter of 50 mm (that is the maximum diameter we
can introduce in CTF from the insertion system).
In a vial we have introduced scintillator (i.e. PC+PPO) spilled
from the CTF itself.
We have “contaminated” the scintillator with Radon gas
starting from a Radon generator.
Because the Rn spike source has to be oxygen free we have
built a system permitting to introduce Rn under nitrogen
pressure.
The total activity must be below 10 Hz (because the electronic of
the CTF) to avoid pile up events.
oxygen free system
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Neutrino Geoscience 2010
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The total number of collected Bi-Po events (≈ 2 weeks) are 3.25 × 105
Data from DIFFUSE and VIAL QUARTZ
The light yield, estimated
from the position of the alpha
peak, is compatible with the
scintillator, indicating that the
quenching effect is negligible.
214Bi
Data obtained with the
diffused Rn (blue) are
within 50 cm from the CTF
center for a total number
of events of 54415.
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Neutrino Geoscience 2010
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Data have been fitted with a preliminary improved MonteCarlo code but some
inputs have to be implemented due to the more complex geometry.
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Toward a measurement of 212Bi spectrum:
Pros and Cons of 212Bi
212Bi
spectrum is much less complicated than 214Bi; 212Po has
less than 10 excited states (to be compare with 82 of 214Po).
212Po
But unlike 214Bi, the radon progenitor in the 232Th chain, the
220Rn, has an half-live of only 55.6 s.
→ We can’t start from 220Rn has we did with 222Rn for 214Bi.
We decided to start from natural thorium dissolved in Nitric Acid at 2%.
Because Th is insoluble in PC we have used TriButyl Phosphate (TBP) to
form stable hydrophobic complexes (these complexes are soluble in
organic solvents) in order to “contaminate” the scintillator spilled from
the CTF.
• The concentration of Th in TBP is measured by ICP-MS (≈ 100 ppb → ≈
430 Bq/kg). The TBP concentration in the scintillator has to be as low as
possible in order to avoid (minimize) variation in light yield and
quenching effects.
• Fluorimetric measurements were performed in order to verify the
light yield → no significant variation for a TBP concen. < 5%.
Lino Miramonti
Neutrino Geoscience 2010
214Po
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Because we want a sufficient statistic avoiding as
much as possible light yield variations and
quenching effects we have to increasing the source
volume → cylindrical quartz vials.
This vial has an external diameter of 5 cm and about
20 cm high for a total volume of about 340 cm3.
The total activity is ≈ 0.5 Bq.
We have collected 1.82 × 105 events
in 5.6 days.
α
β
MC simulations are in progress in
order to extracts feeding probabilities
and specific signals of 212Bi.
Probably we need 2 different light
yield; inside and outside the vial.
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An interesting by-products: τ1/2 measurement of 214Po and 212Po
Table of Isotope reports:
τ1/2 (214Po) = 164.3 ± 2.0 μs (80% of references have been retrieved)
τ1/2 (212Po) = 299 ± 2 ns
(70% of references have been retrieved)
214Po
212Po
From CTF data:
(214Po)
τ1/2
= 163.6 ± 0.5 μs
τ1/2 (212Po) = 298.6 ± 1.1 ns
Lino Miramonti
Works are in progress to analyze
in more detail systematic errors.
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