Search for spontaneous muon emission

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M. Giorgini
OPERA Collaboration Meeting, LNGS, 19-22/05/2003
Search for spontaneous muon emission from lead
nuclei with OPERA bricks
M. Giorgini, V. Popa Bologna Group
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Pions or muons could be emitted by nuclei through
the decays (A,Z) m nm (nm) (A1,Z1)
(An,Zn) (A,Z) p(p0) (A1,Z1) (An,Zn)
(a)
(b)

• Spontaneous m emission m emitted with nuclear
  fragments
• (b) Hyper-cold approximation m emitted before
  nuclear fragments

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• The fission fragments should remain nearly at
  rest
• Most of the available energy would be used to
  produce the m or p
• As the associated neutrino takes a fraction of
  the available energy,
• the energy spectrum of emitted muons should be
  similar to the
• spectrum of electrons in b decay

Q-value m (MeV)
Nuclear mass
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• In the case of Pb spontaneous fission, the
  emission of the
• following particles is energetically allowed
• m (prompt muons), allowed for Z 72
• p delayed muons, allowed for Z 76

For Pb, these decays were considered possible
searched for with small detectors (for ?o
emission, 123g Pb and 109h counting time) but
only upper limits obtained
The expected branching ratios are very small, so
LARGE sources and LONG measuring times are
needed!
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OPERA
nuclear emulsions for tracking particles huge
amount of Pb and long exposure time

LNGS
low natural radioactivity very low cosmic muon
flux local neutron flux ? 1.8.10-6 neutrons/s/cm2

GOOD CONDITIONS FOR
a potential discovery a good upper limit for
these rare processes
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1 OPERA brick (8.23 kg of Pb) at LNGS for 1 year
should
establish the background contribution validate
the analysis procedure
As t1/2 gtgt exposure times T, the 90 C.L.
sensitivities are given by 2.3 ln 2
T N0 t1/2

Assuming 60 efficiency
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MonteCarlo simulation of 1 OPERA brick
lead (1mm)
..56
base (200 mm)
emulsion film (50 mm)
m kinetic energy up to 20 MeV m- kinetic
energy up to 30 MeV Isotropic m emission
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20 MeV emitted m
20 MeV m
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Definition of candidates The m crosses
at least 1 , 2 , 3 emulsion layers The e crosses
at least 2 , 3 , 5 emulsion layers
Estimate of the detection efficiency () for
emitted m
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Estimate of the detection efficiency () for
emitted m-
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BACKGROUND SOURCES

• a and b radioactivity from isotopes present in
  Lead
• a particles can be neglected
• particles can be reduced with mylar foils
  inserted between the lead and
• emulsion sheets ? (tests requested)
• 8.8 MeV a particles emitted by radioactive
  nuclides present in emulsions
• reducible by dE/dx measurements and range (74
  mm) considerations
• Local neutrons ? 1.8.10-6 neutrons/s/cm2 inducing
  the nuclear fission
• can be reduced with an appropriate candidate
  definition
• Radon induced background
• it could be reduced isolating the whole brick in
  hermetic bags and could be
• monitored using passive detectors (CR39, Lexan)
  insensitive to muons
• Background from the CNGS neutrino beam (for the
  future)
• The muon energy should be much higher and the
  beam should be monitored
• by large experiments.

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• Background from cosmic rays
• Cosmic ray muons from above
• due to their high energy and small energy
  losses, they can be removed
• with geometric considerations
• Pions produced by atmospheric muon
  interactions in a lead sheet,
• decaying into a muon
• the rate is 6.10-4 /year for 1 OPERA brick,
  so can be neglected
• Neutrons from cosmic rays ? 3.10-6
  neutrons/s/cm2
• can be neglected
• 
  
  

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CONCLUSIONS
We propose to perform a search for spontaneous
emission of muons from Pb using one (or more)
OPERA bricks This search would be complementary
and superior to other experiments looking for
such exotic radioactivity We need precise
measurements of lead radioactivity A test with
few OPERA bricks for 1-few years should be useful
for a good background study and for defining
appropriate analysis procedures Even in absence
of candidate events, we would obtain a
very significant limit for spontaneous muon
radioactivity