Experimental Approaches to Sterile
Neutrinos Using Low Energy Neutrinos
Jonathan Link
Center for Neutrino Physics
Virginia Tech
NOW 2012
9/14/12
The LENS-Sterile Proposal
Scintillation Lattice:
Voxels separated by clear
films channel light down
the coordinate axis by total
internal reflection.
Proposal to insert a Mega-Curie 51Cr
source in the center of the LENS detector
to observe multiple wavelengths of large
Δm2 oscillations in a few meters.
LENS: R.S. Raghavan, Phys. Rev. Lett. 37, 259 (1976).
115In
+ νe → 115Sn* + e→ 115Sn + 2γ
Jonathan Link
Spatial resolution of order
of cell size over root 12.
9/14/2012
The LENS-Sterile Proposal
Raju Raghavan
1937-2011
Proposal to insert a Mega-Curie 51Cr
source in the center of the LENS detector
to observe multiple wavelengths of large
Δm2 oscillations in a few meters.
LENS: R.S. Raghavan, Phys. Rev. Lett. 37, 259 (1976).
115In
+ νe → 115Sn* + e→ 115Sn + 2γ
Jonathan Link
9/14/2012
Gallex, Sage, 51Cr and Sterile Neutrinos
Giunti and Lavender
(Mod. Phys. Lett. A22, 2499)
noted that the low ratio
of observed to expected
ν +71Ge interactions in
the Gallex and SAGE
source experiments:
R = 0.88 ± 0.05
may be due to sterile
neutrino oscillations.
Now commonly known as the “Gallium Anomaly”
Jonathan Link
9/14/2012
The Gallium Anomaly
νe disappearance in Gallex and SAGE from 51Cr and 37Ar source
C. Giunti and M. Laveder, Phys. Rev. C83, 065504 (2011).
Jonathan Link
9/14/2012
Electron Capture Neutrino Sources
Electron capture isotopes decay to two bodies and as such
produce a mono-energetic beam of neutrinos at low energies.
51Cr
+ s-shell e- → 51V + νe (+ X-ray)
Sources such as this have played a critical role in the calibration
of radiochemical experiments as a proxy source of solar neutrinos
with a well known flux.
Advances in detector technology such as:
Borexino’s ability to do real time detection of 7Be neutrinos
have created new opportunities for groundbreaking neutrino
physics using electron capture sources.
Jonathan Link
9/14/2012
51Cr
as a Mono-Energetic Neutrino Source
K shell capture
L shell capture
90% of the time the capture goes directly to the ground state of 51V
and you get a 750 keV neutrino.
10% of the time it goes to an excited state of 51V and you get a
320 keV photon plus a 430 keV neutrino.
Jonathan Link
9/14/2012
Advantages of 51Cr
1. Can be easily produced with thermal neutron capture (50Cr has
a ~17 barn neutron capture cross section).
2. Has a long, but not too long, lifetime (39.9 days).
Longer lifetimes require more neutrons to get high rates
Shorter lifetimes lose too much rate in shipping and handling
3. Has one, relatively easy to shield, gamma that accompanies
10% of decays.
5 cm of tungsten reduce 320 keV γ rate from 1 MCi to 1 Hz
19 cm are needed to reduce 1 Ci of 1 MeV γ to 1 Hz
4. Mega-Curie scale sources have been produced by both Gallex
and SAGE.
Jonathan Link
9/14/2012
The High Flux Isotope Reactor (HFIR) at ORNL
HFIR operates at 85 MW with 23 operating days each fuel cycle.
Jonathan Link
9/14/2012
The High Flux Isotope Reactor (HFIR) at ORNL
Thermal neutron flux of 2.5×1015 /cm2/s in the target region.
40 times larger
neutron flux than
what was used by
Gallex
4 times higher mean
capture cross section
than what was used
by SAGE
Jonathan Link
9/14/2012
Solar Neutrino Detectors & Source Sterile Searches
What works for LENS may work for other low energy solar
neutrino detectors. (ES rate ~102× larger than 115In rate in LENS)
Mono-energetic neutrinos → known neutrino energy →
You don’t need a charged current process
You still need good spatial resolution to fix L/E
Candidate detectors include:
Large liquid noble gas scintillating detectors: Clean,
XMASS, Xenon100
Large LS detectors: Borexino, SNO+, Kamland
All these detectors would use electron elastic scattering
NC detection is another interesting idea
(Formaggio et al, Phys. Rev. D 85, 013009)
Jonathan Link
9/14/2012
Large Detectors and Centrally Located Sources
A centrally located source maximizes the interaction rate per MCi.
With no oscillation the event rate is a flat function in radius.
Initial 2 MCi Source for a 70 day Run
(3+1)
(3+2)
Models from the fit of Kopp,
Maltoni & Schwetz
arXiv:1103.4570 [hep-ph]
A source inside the detector would need to be well shielded.
Jonathan Link
9/14/2012
Real Time Detectors Require Serious γ Shielding
gammas/sec·10 keV
The 320 keV gamma (10% of decays) is a non-issue compared to
the internal bremsstrahlung in ~0.05% of decays.
Spherical
energy (keV)
Possible source and W-alloy
shielding configuration…
But what is the activity of W?
Jonathan Link
9/14/2012
Low-Background Counting of Tungsten Alloy
Measured at the Kimballton Underground Research Facility (KURF)
Signal Sample
214Pb
234Pa
Background
238U
214Bi
< 5mBq/kg (from 234Pa)
232Th <40 mBq/kg (from 228Ac)
40K < 220 mBq/kg
214Bi = 150±50 mBq/kg
228Ac
40K
214Bi
214Bi
208Tl
Background Subtracted
Jonathan Link
9/14/2012
SNO+ Source Deployment Case Study
Jonathan Link
9/14/2012
Signal to Noise Ratio as a Function of Radius
Detector BG
grows as r2
Source BG
falls as e−r/λ
Assuming a uniform detector BG out to the fiducial radius.
Jonathan Link
9/14/2012
SNO+ Sensitivity
Sensitivity based on a χ2
fit to signal and BG over
the full energy range.
90% CL contours
GLoBES
1.
Not that sensitive to
backgrounds
2.
Source normalization
and spatial resolution
are critical to large
Δm2 resolution.
3.
Statistics limited
measurement.
3+1 contours from Kopp,
Maltoni & Schwetz
arXiv:1103.4570 [hep-ph]
Jonathan Link
9/14/2012
Borexino Sensitivity
(See Aldo Ianni’s Talk)
Borexino is the only detector where we
know this will work.
One can use the vast solar phase to
subtract backgrounds.
10 MCi Source
8.25 m
Source Under Detector
Jonathan Link
9/14/2012
Conclusions
1. Mega-Curie scale sources of electron capture isotopes
are a excellent source of low energy, mono-energetic
neutrinos.
2.
51Cr
is likely the best source candidate for the future
experimental program.
3. Sources as strong a 2 MCi could likely be produced at
HFIR.
4. Such a source could be used for a sensitive search for
eV sterile neutrinos with large, low-background
scintillating detectors like Borexino and SNO+
Jonathan Link
9/14/2012
Question Slides
Jonathan Link
9/14/2012
Electron Capture Neutrino Sources
In 1973 Luis Alvarez proposed using a 65Zn source to calibrate
Ray Davis’ chlorine detector.
Since then several such source have been proposed:
Isotope
τ½
Eν Max
65Zn
244 d 1.3 MeV
51Cr
27.7 d
750 keV
152Eu
13 y 1.05 MeV
37Ar
34.9 d
812 keV
Production Mechanism
Gammas
Thermal neutron capture
770 & 345 keV (50%)
Thermal neutron capture
320 keV (10%)
Unknown
121 keV -1.7 MeV (100%)
40
37
Fast neutron Ca(n,α) Ar
Internal Brem. only
Jonathan Link
Notes
Proposed by Alvarez
Proposed by Raghavan, used by Gallex and SAGE
Proposed by Cribier and Spiro
Proposed by Haxton, used by SAGE
9/14/2012
The Gallex Sources
• Made in the Siloé reactor in Gernoble, France (35 MW)
• Two sources produced from the same enriched Cr (38.6% 50Cr)
1.67 MCi
1.89 MCi
The average temperature
across the Cr was ~525 K,
which gives a flux averaged
cross section of ~16 barns.
(My production estimates are
scaled from these numbers
assuming that their entire
neutron flux is thermal.)
Jonathan Link
9/14/2012
The Sage Source
• Made in the BN-350 fast breeder reactor at Aktau, Kazakhstan.
• Irradiated 512.7 g of Cr (enriched to 92.4% 50Cr)
• Fast neutron flux of 5×1015/(cm2 s) was locally moderated near
the Cr to give an average cross section of about 4 barns.
• Longer exposure: 90 days at 520 MW and 16 days at 620 MW.
Source strength of 516 kCi.
Jonathan Link
9/14/2012
Source Production Scaling from Siloé to HFIR
Using
1. the initial amount of 50Cr,
2. the source strength after irradiation, and
3. the 51Cr decay rate,
The survival lifetime of 50Cr (τ50) in the Siloé Reactor is calculated
to be about 13,500 days.
Similarly, τ50 for locations the HFIR core are calculated, accounting
for the differences in core temperature and thermal neutron flux.
Location
Siloé (GALLEX)
Small Vertical Experiment Facility
Large Removable Beryllium Facilitiy
Flux Trap Target
n Flux
(cm−2s −1)
5.20E+13
6.18E+14
1.40E+15
2.13E+15
τ50
(days)
13488
984
434
286
Cr-50
Exposure Activity
(grams) Time (days) (MCi)
13715
908
408
272
23.83
23
23
23
1.721
1.504
1.506
1.502
This does not include 51Cr production from non-thermal neutrons.
Jonathan Link
9/14/2012
Check of the HIFR Production Model
In the 1980’s ORNL studied 51Cr production in HFIR using rods
of natural chromium in the Small VXF and Large VXF locations.
With a 5.7 cm diameter rod in the Large VXF location they got
0.0673 MCi.
While the 3.1 cm rod in the Small VXF yielded 0.0707 MCi.
Fraction of n to Cr-51 Production
HFIR Location
0.2806
n Flux
−2 −1
Large Vertical Experiment Facilities
Small Vertical Experiment Facilities
(cm s )
4.24E+14
6.18E+14
53Cr
(9.5%) has a larger n capture x-section than 50Cr (4.4%)
τ Prod.
(days)
Grams
Cr-50
1435.43
984.30
392.2
112.1
Exposure
Time (days)
41
41
Activity
(MCi)
0.1780
0.0737
True
Activiy
0.0673
0.0707
The thermal neutron attenuation length in natural chromium is
4.5 cm, so the difference from expectation at the large VXF may
be due in part to self-shielding.
Jonathan Link
9/14/2012
Chromium Self-Shielding
Chromium scans from large VXF
Self-shielding was also
likely an effect at Siloé.
The neutron attenuation
length in Siloé was
about 4.1 cm.
The chromium was in
two parallel boxes 50
cm high by 12.6 cm
long by 1.4 cm wide.
Jonathan Link
9/14/2012
Sterile Neutrino White Paper
For more information see the Light Sterile Neutrinos: A White Paper
(arXiv:1204.5379 [hep-ph])
Outline:
1. Theory and Motivation (editors Barenboim & Rodejohann)
2. Astrophysical Evidence (Abazajian & Wong)
3. Evidence from Oscillation Experiments (Koop & Louis)
4. Global Picture (Lasserre & Schwetz)
5. Requirements for Future Experiments (Fleming & Formaggio)
6. Appendix: Possible Future Experiments (Huber & Link)
Written from an international perspective for an audience including
both the scientific community and funding agencies.
Visit http://cnp.phys.vt.edu/white_paper/
Jonathan Link
9/14/2012