Search for spontaneous muon emissionfrom lead nuclei with OPERA bricks

M. Giorgini, V. Popa Bologna Group

OPERA Collaboration Meeting, LNGS, 19-22/05/2003

M. Giorgini

Pions or muons could be emitted by nuclei through the decays :

(A,Z) ± + () + (A1,Z1) + …… + (An,Zn)

(A,Z) ±() + (A1,Z1) + …… + (An,Zn)

(a) (b)

(a) Spontaneous emission : emitted with nuclear fragments(b) Hyper-cold approximation : emitted before nuclear fragments

• The fission fragments should remain nearly at rest • Most of the available energy would be used to produce the or • 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 decay

Q-v

alue

+ (

MeV

)

Nuclear mass

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!

In the case of Pb spontaneous fission, the emission of the following particles is energetically allowed :

± (prompt muons), allowed for Z ≥ 72

± delayed muons, allowed for Z ≥ 76

nuclear emulsions for tracking particleshuge amount of Pb and long exposure time

OPERA

low natural radioactivityvery low cosmic muon fluxlocal neutron flux 1.8.10-6 neutrons/s/cm2

LNGS+

=GOOD CONDITIONS FOR :

a potential discovery …a good upper limit for these rare processes

1 OPERA brick (8.23 kg of Pb) at LNGS for 1 year should :

establish the background contributionvalidate the analysis procedure

As t1/2 >> exposure times T, the 90% C.L. sensitivities aregiven by

2.3 ln 2 * TN0 t1/2

=

Assuming 60% efficiency

lead (1mm)

base (200 m)

emulsion film (50 m)

…..

MonteCarlo simulation of 1 OPERA brick

+ : kinetic energy up to 20 MeV- : kinetic energy up to 30 MeVIsotropic emission

15 MeV -

15 MeV +

20 MeV emitted +20 MeV +

Definition of candidates :

The ± crosses at least 1 , 2 , 3 emulsion layersThe e± crosses at least 2 , 3 , 5 emulsion layers

Estimate of the “detection efficiency” (%) for emitted +

Estimate of the “detection efficiency” (%) for emitted -

BACKGROUND SOURCES : and radioactivity from isotopes present in Lead : particles can be neglected particles can be reduced with mylar foils inserted between the lead and emulsion sheets ? (tests requested)

8.8 MeV particles emitted by radioactive nuclides present in emulsions:reducible by dE/dx measurements and range (~74 m) 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 monitoredby large experiments.

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

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 otherexperiments 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 verysignificant limit for spontaneous muon radioactivity