Tritium beta-decay experiments: a (p)review
OR
KATRIN
A next generation experiment
with sub-eV sensitivity for the electron neutrino mass
M. Charlton1, A.J. Davies1, H.H. Telle1, D.L. Wark2, J. Tennyson3, P.J.
Storey3 and P.T. Greenland3
1Department
of Physics, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
3Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
2Rutherford Appleton
OUTLINE OF TALK
Motivation
General description of the experiment
The KATRIN collaboration
Crunch Areas of the Experiment and the UK role
NEUTRINOS HAVE MASS
Oscillations prove they are massive
mass eigenvalues, m1, m2 and m3
hierarchical
or
m1 << m2 << m3 or
degenerate?
m1  m2  m3
NO ABSOLUTE SCALE FROM m
AN ABSOLUTE SCALE …….
A finite measured value for m(e) would be vitally important
An improved limit as proposed by KATRIN – implications
for cosmology and astrophysics
OTHER POSSIBLE METHODS
Astronomical Measurements
Need model-dependent assumptions to arrive at mass
Neutrinoless Double Beta-Decay
Neutrino must be a Majorana particle for there to be
mass sensitivity
THE PROCESS
T2 
3HeT+
+ e– + e(bar)
The distortion of the beta-spectrum due to m(e)  0
is only appreciable near the endpoint, Eo since the
count rate rises rapidly in this region, varying as
dN/dE  (E – Eo)2
THE BETA-DECAY SPECTRUM OF TRITIUM
RECENT MAINZ DATA
10 -YEARS OF NEUTRINO
MASS FROM TRITIUM
EXPERIMENTS
THE CHALLENGE
Measure the kinetic energy of an 18.6 keV electron
with sub-eV resolution!
So E/Eo  5 x 10-5 or better ……
(NB “normal” electron spectrometers have resolutions of 10-3 or
worse)
THE SOLUTION
Magnetic Adiabatic Collimation with Electrostatic
Filter - the MAC-E Filter
Exploits the properties of charged particle orbits in
slowly-varying (in magnitude) magnetic fields
CONSERVED QUANTITIES (adiabatic invariants - non.rel.)
BA = constant
E/B = constant
B is magnetic field, A is the area enclosed by the orbit and E is
the component of the kinetic energy perpendicular to B.
……. E/Eo = BA/Bmax …so BA is the magnetic field in the
analysing plane (5 x 10-4 T and Bmax is the maximum field (10 T)
 E/Eo = 5 x 10-5 ……..as required
SCHEMATIC OF THE KATRIN EXPERIMENT
ESTIMATES OF THE SENSITIVITY OF KATRIN - New mode
of operation under discussion; possible limit around 0.1 eV
THE KATRIN COLLABORATION
More than 50 scientists ()
from
Mainz, Karlsruhe, Fulda, Moscow/Dubna, Prague, Washington
and the UK
Development complete …. 2006
Testing starts ……………. 2007
Overall cost (excluding manpower) around €25M
CRUNCH AREAS OF THE EXPERIMENT (I)
SOURCE
Workhorse source will be the Windowless Gas Tritium Source
Change conditions …e.g. pressure, local magnetic fields
Modelling – most parameters can be measured or calculated with
high precision and used to model many aspects of β-particle
interactions
Trapped ions/electrons; local potentials
In-situ measurements with electrons (gun or 83Kr) …..
….. energy loss and scattering of β-particles
UK INPUT
Molecular physics – (Tennyson, UCL)
Monitoring – (Telle, Swansea)
Modelling – (various, UCL)
WINDOWLESS GAS TRITIUM SOURCE
___________ Analysis of T2




(KATRIN requirements) __________
For the KATRIN neutrino experiments T2 gas of isotopic purity  0.99
is required (impurities constitute other H-isotopomers, contents of
3He, 4He have to be negligible)
Monitoring and regulation of T2 purity to about ±0.005-0.010
Measurement of feed gas impurity concentrations at the inlet to the T2
source (at pressure of ~100-1,000mbar)
For a mixture of say 99% T2 and 1% H2 on finds, at chemo-thermal
equilibrium, fractional amounts of [T2] ~ 0.9899, [HT] ~ 1.00310-2
and [H2] ~ 710-5
 detection sensitivities of the order <10-5 ( 10-2 mbar) needed
Analytical method of choice:
RAMAN SPECTROSCOPY
_______ Raman analysis of T2 (KATRIN requirements) _______
 Raman
λLaser
Spectral pattern
 isotopomer species identification
Spectral intensities
 quantitative information
J’
V’=1
J”
v”=0
Raman excitation
with J = 0, 2
Detection limits realised by Karlsruhe
group using ASER Raman set-up:
~110-5 H2 in D2 (no T2 measurements yet)
 at the borderline of requirement for
purity control
Estimated detection limit using proposed
new set-up with pulsed laser:
< 210-6 H2 in T2
____________ Raman spectroscopy of H2 / HT / T2 _____________
Fraction
1
Q
Typical dynamic range of
ICCD detectors ~64,000
 the weakest H2 lines
would not be detected,
or the strongest T2 lines
would be saturated
S
O
~10-2
<10-4
[ nm ]
CALCULATED RAMAN SPECTRA
(for Laser=532nm, T=300K)
Line widths of cw and pulsed
Nd:YAG lasers (532nm) less
than typical spectral resolution
of an ICCD-coupled spectrograph
 all rotational lines except
in the Q-branch resolved
_____ Raman analysis of T2 (initial attempt at Karlsruhe) _____
ASER = actively stabilised external resonator
(used to enhance ILaser by ~ 250)
“Problems” with this initial, complex set-up:
(a) optical isolator needed to avoid laser damage by back reflections
(b) modulator required for locking control of ASER
(c) ASER difficult to keep in resonance in the long-term
(d) large Rayleigh scattering background contribution to Raman signals
KATRIN - Molecular Physics issues
Need reliable model of 3HeT+ final state over wide (for molecules) energy
range.
Many issues covered by Froelich, Saenz & co-workers, but:
•
Excitation to electronic continuum in both resonant and non-resonant
processes must be considered.
•
Nuclear motion continuum should be treated at better than the reflection
approximation (particularly for resonances).
•
Cross-section requirements for collisions arising in the source.
Will be done theoretically.
Question: Is it possible to design experimental tests of theory?
CRUNCH AREAS OF THE EXPERIMENT (II)
SPECTROMETER(S)
MAC-E filter type; very high energy resolution (~ 5 x 10-5)
Sources of background need to be understood – modelling of
internal discharges
Voltage “standard” needed (and stability)
Calibration – stand-alone MAC-E filter for 83Kr; electron gun
for (source − spect)
UK INPUT
Modelling – (Davies, Swansea)
SCHEMATIC VIEW OF THE KATRIN SPECTROMETERS
SWANSEA CONTRIBUTION TO THE MODELLING
•
Design of spectrometer must eliminate electrical breakdown
due to desorption of gas from surfaces and field emission. Great
care to be taken in design and preparation of interior surfaces.
•
Modelling at Swansea will involve evaluation of precise 3D
electric and magnetic field distributions in critical regions,
especially near interior surfaces.
•
Simulation of low-pressure breakdown processes resulting
from gaseous emission from interior surfaces will enable critical
breakdown paths to be determined.
•
Monte-Carlo techniques will also be used to investigate the
interaction of beam electrons with the background gas.
•
Simulation and modelling work at early stage of design will
help minimise potential breakdown and background problems.
CONCLUDING REMARKS
KATRIN will be sensitive to m(e) to below 1 eV and maybe
as low as 0.1 eV
Hope for a factor of 10 improvement over current best direct
limit of 2.2 eV
It will be difficult and systematics will have to be chased hard
KATRIN is most likely the “end-of-the-road” for this type of
spectrometer-based experiment