Double Beta Decay in SNO

1
Double Beta Decay in SNO?
Art McDonald, on behalf of a research and
discussion group including Alex Wright, Jose
Maneira, Mark Chen, Mark Boulay, Aksel
Hallin, Hamish Leslie, Richard Ford, Ryan
MacLellan, George Ewan, Hamish Robertson also
interest from Clarence Virtue, Dick Hahn.

• A discussion of a variety of nuclei that might be
  placed in the SNO Acrylic
• Vessel in the future via a number of possible
  approaches such as
• Suspend them as nanoparticles in liquid
  scintillator.
• Dissolve or absorb them in liquid scintillator
  or water.
• Incorporate them in scintillating crystals (few
  cm dia.) suspended in water
• or liquid scintillator as in the CAMEO or
  CANDLES proposals.
• All suffer from the inherent energy resolution
  problem of scintillators
• (gt 5 FWHM). Backgrounds must be addressed as
  for all double beta
• measurements, but the possibility of 10 tonnes or
  so of material
• makes it interesting to have a look.

2
High Energy Zero Neutrino Double Beta Emitters
G Geochemical, C Calculated, Notable,
(1) is best in ranking.
Note that 0n numbers have significant nuclear
Matrix Element uncertainties
3
Backgrounds

• Two-Neutrino interference with Zero-Neutrino
  because of resolution

From a very nice Review by Elliott and
Vogel hep-ph/0202264
The ratio of 2 n to 0 n lifetimes is
very important for resolution gt 5 FWHM as
in Scintillators.
Full 0 n peak region
For the upper half of the peak region Peak
reduced by 2, Two-neutrino reduced by 16.
!
Resolution
4
Radioactive Backgrounds
Simulation of one year With 1 x 10-16 g/g of U
and Th in liquid Scintillator (Jose). Lines
indicate energies of 0 n decays. Note summing
of decay chain events So that 2.4 MeV and 2.6 Mev
gammas become higher energy Blobs.
5
KAMLAND Backgrounds
External
Internal U,Th as in previous Plot
Muon Induced
6
How to Get 10 Tonnes of Double Beta Material into
SNO ?

• Put it in as nanopowders
• Many materials can be made into nanopowders with
  sizes
• as small as 4 nm, that will stay in suspension
  for long periods
• of time. Commercial field is expanding greatly
  recently.
• A major limitation is Rayleigh scattering, but
  that can be
• reasonable for small sizes as it goes as the
  sixth power of
• the radius.
• The bulk absorption of the material must be
  relatively small since
• the absorption just depends on the amount of
  material traversed.
• Both Rayleigh and absorption can be acceptable
  for some
• materials at about 1 loading.
• Some metal oxides are very transparent for the
  optical region
• because the photons are not energetic enough to
  be absorbed
• by transitions across the insulator band gap.

7
For example TeO2 is very transparent as can be
seen for these 5 cm thick crystals used in
CUORICINO
8
Some attenuation and scattering measurements have
started at Queens using off-the shelf
nanopowders with fairly small size and high
loading. Samples (lt4 nm size) (½ loading in PC)
have recently been obtained. We are interested
in wavelengths around 350 to 450 nm to match
the emission from PPO (perhaps moved up to 420 by
further WLS bis-MSB)
9
For the samples studied so far, the measurements
at 380 nm are summarized as follows Sample
Rayleigh
Attenuation 90 Deg Scatt
Calculation
(cm-1) Measurement 4 nm 15 SiO2 in water
0.008 0.43 10 nm
30 SiO2 water 0.26
0.62 0.62 (normalized) 20
nm 30 SiO2 ethylene glycol 0.56
0.14 0.14 5-10 nm 20 ZrO2
water 0.36 (8 nm) 0.32
0.24
The attenuation length for Rayleigh scattering
is (Karker, 1969) aRayleigh W x 24 p3 x (n2
- 1)/(n2 2)2 x r(solution)/ r(particle) x
4/3 p r3/ l4, where I I0 e - a L for intensity
as a function of length L and W weight
fraction of the particles, n the ratio of the
indices of refraction of the particles and the
solution, r is the density, r is the particle
radius and l is the wavelength. For pure
Rayleigh scattering, the 900 scattering is
proportional to aRayleigh
10
The wavelength dependence is not quite l-4, but
is a power law close to -4. There may be some
extra absorption below 330 nm.
330 nm
400 nm
700 nm
11

• Using the measured attenuation for ZrO2 at 380
  nm (0.32 cm -1) and
• assuming that no more than ½ of that is
  absorption (0.16 cm -1), one
• obtains an absortion length of 6 cm for 20
  loading, or 30 cm for the bulk
• material. Perhaps not unreasonable, considering
  the picture of TeO2
• crystals.
• Therefore, for a 1 loading of nanopowders, the
  absorption length would be
• 30 meters.
• For 1 of ZrO2 as 4 nm particles in pseudocumene
  (ref. index 1.5)
• (or a mineral oil base mix to protect the AV),
  one would predict a
• Rayleigh scattering length of about 11 meters at
  380 nm wavelength.
• 1 loading is 10 tonnes.
• Possible materials for nanopowders
• - ZrO2, TeO2, transparent as discussed.
• - Nd2O3 absorbs in the optical. Might be
  acceptable in scintillation light region.
• - SeO2 is added to glass to make it red. Optical
  absorption??

12
Metallo-organics Hartmann, Schoenert et al MPIK,
Heidelberg
Nanoparticles Measurements starting at Queens
on 4 nm Nd2O3 nanoparticles 0.5 in Pseudocumene.
13
SUMMARY TABLE
For comparison of various cases, Assume 10 Tonnes
of material, 50 meV neutrino mass, Resolution
NHIT-1/2 with 450 NHIT/MeV
14
Nd (3.3 MeV No Problem from 2.6 MeV External
gammas) Rate for 10 tons dissolved in 1000 tonnes
of scintillator 1990 per year. S/B (2n / 0n) for
50 (100) meV is 2.3 ( 9.2). Absorption is a
question being investigated. Scaling 2.6 MeV
Background roughly from KamLAND figures For a
radius of about 3 meters, the expected rate of
2.6 MeV gammas would be 0.6 per year. For 50
meV neutrino mass, considering neutrinoless
decays Xe For 2 of natural Xe dissolved in a
central 3 m volume, (2.4 tonnes of Xe in a
balloon?) the expected zero-neutrino rate in that
fiducial volume would be 76 x 0.12 x 0.089
0.85 per year for 50 meV neutrino mass. At 85
enriched Xe the rate is 7.8 per year. Te For 2.4
tonnes of natural Te dissolved in a central 3 m
volume (balloon?), the zero-neutrino decay rate
would be 128 x 0.12 x .35 5.4 per year. The
Rayleigh scattering length would be 5 meters.
15
Tailored Nanocrystals for wavelength shifting or
scintillation in organics or Water??
We have ordered some 4 nm CdS in pseudocumene.
(Designed for 450 nm)
16
These very preliminary numbers imply that it
would be worth doing more detailed studies of
double beta decay in SNO with liquid
scintillator. We are certainly pleased with the
interest of the Milano group in
these possibilities and Barbara will present some
of their studies in the next talk (for the
specific case of Xe) with more detail than I have
presented. We will proceed with further
measurements and simulations and are very open to
additional collaboration in this activity.