Transcript Can Radiogenic Heat Sources inside the Earth be located by

Models of the Earth distribute U- and Th masses mainly
between the continental crust and the lower mantle.
As has already been discussed here a number of detectors
stationed at appropriate geographical sites can separate the
crust and mantle contributions.
60
50
40
Thorium
30
Uranium
20
10
0
Baksan
Kamioka
Gran Sasso
Hawaii
Adopted from
F.Mantovani,
L. Carmignani,
G. Fiorentini,
M. Lissia
Phys. Rev.
D69, 013001
(2004)
Can Radiogenic Heat Sources
inside the Earth be located by
their Anti Neutrino incoming
Directions?
G. Domogatsky, V. Kopeikin, L. Mikaelyan, V. Sinev
Here we analyze directional separation of e
signals arriving from the crust and the lower
mantle with only one detector.
Crust
Upper mantle
Lower
mantle
Liquid
core
Hard core
Geoneutrinos from
the Crust and the
Lower Mantle enter
the detector from
different directions.
NEUTRON DETECTION
We consider CH2 ,  = 0.8 g/cm3
Liquid Scintillation Detector and
e + p => e+ + n
Geoneutrino detection reaction.
The Geoneutrino signature is delayed
coincidence between the positron and neutron
signals.
Positron spectrum boosted by two 511 keV annihilation quanta
is shown below:
250
90Sr-90Y
calibration
source
Counts per MeV
200
150
100
U+Th
50
Th
0
1.0
1.5
2.0
Positron energy released, MeV
2.5
NEUTRINO DIRECTIONS
Parallel geoneutrino beam (along Z-axis)
This geometry was studied earlier at CHOOZ
with reactor antineutrinos.
1) Neutron initial direction is strongly
correlated with the incoming e direction.
qen
pe
pn
p
E = 2.5 MeV
qmax  26º
Angular distribution of reaction (1) neutrons relative to the
incident geoneutrino direction weighted with the reaction
cross section
8000
8000
= 0.967
cos cos
qnq n=
0.967

4000
4000
0
0
0.8
0.8
0.9
0.9
cos q n

cos qn
1.0
1
N=1
N=3
<Z>=1.32 cm
<Z>=0.66 cm
N=5
N=8
<Z>=1.58 cm
X, cm
<Z>=1.72 cm
X, cm
2). In first few collisions
with scintillator atoms the
memory is partially
conserved and neutron is
displaced from the
reaction point in + Z
directions. After 7 - 8
collisions the memory is
lost and neutrons slow
down and diffuse
symmetrically around the
displaced center.
CONCLUSIONS
We present a first attempt to analyze directions of a
multidirectional low energy flux. In this attempt
only two geoneutrino sources have been taken into
account: the continental crust and lower mantle.
We haven’t analyzed perturbations due to possible
fluctuations of U and Th concentrations in the
detector’s immediate vicinity.
Clearly more work is needed to come to more
accurate results.
At this preliminary stage of analysis we can summarize
the results as follows:
Present understanding of radiogenic sources and their
distribution in the Earth’s reservoirs is based on a shaky
ground of cosmogenical and geochemical arguments with
an obvious deficit of direct experimental evidence
Information obtained with one 30-kton target mass
detector using directional separation of incoming
geoneutrino flux is useful but limited: it can give only some
indications against the orthodox Earth’s model predictions,
or can provide it’s rough confirmation.
More definite information can be obtained only with ~
4 times larger detector
Later neutrons diffuse and are captured symmetrically around
the displaced center. For parallel e beam the average neutron
displacement is calculated as:
dz = 1.72 cm
now we find:
< Renx > = < Reny > 0  /N1/2 ,
< Renz > = 1.72  /N1/2,
N is the number of detected reaction (1) events and  is vector
R component’s Gaussian dispersion.
Dispersion   20 cm is considerably larger than the
displacement dZ = 1.7 cm and thus e statistics (N) should be
sufficiently large.
ASSUMING NOW N = 2500 and  =
20cm (as in the CHOOZ experiment) we
get:
dZ = 1.7  04 cm
Thus neutron displacement can be found
at 4 st. deviation level (which is exactly the
CHOOZ result).
Geoneutrinos from the lower mantle
We generate 105 MC
events and find the
average displacement dLM:
dLM = 1.20 cm
With ~30 kton target mass 4000
geoevents can be accumulated in 5 years
of data taking.
N = 4000 is considered here as
maximally thinkable events sample.
dLM = 1.20 cm  20 / 40001/2 = 1.2  0.32
GEONEUTRONOS from the CRUST
Here we consider hypothetical case where
continental crust source forms a uniform 6000
km diameter and 40 km thick circular region
centered around the geoneutrino detector
The vertical component of the flux is small here
and the neutron displacement is also small:
dCr = < Renz >  0.29 cm  /N1/2
e from the Crust and the Lower Mantle
The average displacement of neutron cloud in the vertical
direction is given by the expression:
dLM+Cr = LM dLM + (1 – LM)dCr  /N1/2,
where dLM = 1.2 cm, dCr = 0.29cm and LM = FLM / (FLM + FCr)
is the lower mantle fraction in the total geoneutrino incoming
flux; N = 4000, the maximal achievable events sample
considered here,
/N1/2 = 0.32 cm.
1.5
1.50
d Z, cm
dz, cm
11.00
0.5
0.50
00.00
0.00
0
0.20
0.40
0.60
0.80
0.2 0.4 LM0.6 0.8
LM
1.00
1
Vertical displacement of neutron cloud dLM +Cr vs relative
contribution LM of lower mantle to the total (crust + lower
mantle) geoneutrino flux (solid line). Shaded is the (68% CL)
uncertainty region, the dark gray area between vertical lines
represents model’ prediction for detector installed in BNO.
One can see that separation method considered
here is not very sensitive. Only sufficiently large
displacements dLM+Cr, larger than 1 cm if found,
can indicate contradiction to the predictions of the
orthodox Earth’s model. In case experiment
favours lower displacements, and thus indicates low
contribution of the mantle geoneutrino flux, the
dominant role of the crust geoneutrinos predicted
by the model can roughly be confirmed.
Only with much larger number of collected events
(N ~ 2104 ) and therefore with much larger detector
more definite conclusions could be obtained.
P.S.Detector calibrations
Detection of small displacements discussed
above requires adequate calibration
procedures.
While usual method, based on inserting
neutron- and gamma sources into the
fiducial volume can and should be exploited,
the use of sufficiently more strong source is
highly desirable.
We propose for calibration purposes a movable
~ 1 MCi 90Sr-90Y antineutrino source.
90Sr
(T1/2 = 28.6 yr) decays to the ground state
of 90Y(Emax = 2.28 MeV, T1/2 = 64h).
If installed at the distance of 30 m from the 30
kton detector center, the source can generate
about 2105 events of reaction (1) per year.
Two circumstances make this source attractive:
First, it will irradiate the detector with flux of known intensity,
known energy spectrum in the geoneutrino energy range and of
known angular structure
250
At a distance of 30 m
there will be
~ 200 000
events/(year·30 kt)
Counts per MeV
90Sr-90Y
200
calibration
source
150
100
Th
U+Th
50
0
1.0
1.5
2.0
2.5
Positron energy released, MeV
and, second, the sources are produced commercially and used
to supply heat for Radioisotope Thermoelectric Generators
(RTGs)
We note that proposed
calibration method could
also be used in other low
energy experiments
employing large liquid
scintillation detectors.
For each neutrino event positron and neutron
capture positions are reconstructed and
positron –neutron vector ReNi is found:
ReNi.= RNi – Rei
The Reconstruction procedure is based on light
and (or) time signals from PMTs.
Neutrino direction is found be neutron
displacement in the e direction
Information of neutrino incoming directions is
derived from vector ReNi. analysis.