11th ICATPP Conference onAstroparticle, Particle, Space Physics, Detectors and Medical

Physics Applications Villa Olmo, Como, 5-9 October 2009

Search for a muon spontaneous emission from heavy nuclei

M. Giorgini et al., Bologna University and INFN

(a) (b)

A

A

A

1

n

+

A

A

A

1

n

+

AHC

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

“Exotic radioactivity” predicted for heavy nuclei

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

(A,Z) → ± + () + (A1,Z1) + ……. + (An,Zn)(A,Z) → ± (0) + (A1,Z1) + ……. + (An,Zn)

D.B. Ion et al., Spontaneous muon emission as a new natural radioactivity, Rev. Roum. Phys. 31 (1986) 209

The expected branching ratios are very small, so MASSIVE

“sources” and LONG measuring times are needed

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

± (prompt muons), for Z ≥ 72± → delayed muons, for Z ≥ 762± (prompt muon pairs), for Z ≥ 91

± → delayed muon pairs, for Z ≥ 100

Spontaneous fission of heavy nuclei

238U (Z=92) First 3 channels allowed207Pb (Z=82) First 2 channels allowed

Kinematical considerations• The fission fragments remain nearly at rest • Most of the kinetic energy would be carried out by the • As the associated neutrino takes a fraction of the available energy, the energy spectrum of emitted muons could be similar to the spectrum of electrons in decay

These decays have been searched using small different detectors…

but only poor upper limits were obtained [D.B. Ion et al., Rev. Roum. Phys. 32 (1987) 299]

Available kinetic energy in a decay :

85-90 MeV <E> 40 ÷ 50 MeV25-30 MeV <E> 10 ÷ 15 MeV

238U207Pb

LNGS+

=

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

low natural radioactivitybackground contributions known and estimatedambient neutron flux 1.8.10-6 neutrons/s/cm2

GOOD CONDITIONS FOR :

a good upper limit for these rare processes

OPERA

!!!☻a potential discovery …

We propose a search of muon emission from heavy nuclei using some “bricks” of the OPERA detector

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

establish the background contribution

First step: use of some OPERA “bricks”

base (200 m)

emulsion film (43 m)

validate the analysis procedurereach a good sensitivity level

lead (1mm)

…..

BACKGROUND SOURCES AND REJECTION

8.8 MeV particles emitted by radioactive nuclides (212Po) present in emulsions:reducible by dE/dx measurements and range (~74 m) considerations

and radioactivity from isotopes present in Lead : particles can be neglected

Radon induced background :it could be reduced isolating the brick in hermetic bags and could be monitored using radon detectors

Background from the CNGS neutrino beam :the induced muon energy is much higher, so the tracks could be easily recognizable

Background from cosmic rays :

Cosmic ray muons from above : due to their high energy and small energy losses, they can be rejected 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

Ambient neutrons ( 1.8.10-6 neutrons/s/cm2) may induce nuclear fissions :flux reduced by the shield; the effects it can be further reduced with “appropriate” candidate definitions

Shielding for background reduction

55 c

m

15

cm

10

cm

70 cm

30 cm10

cm

55 cm

5 bricks

Copper

Lead

The background reduction is a crucial point : shields + analysis cuts

Shield surrounded by polyethylene/paraffin for neutrons Plastic shields for reduction of radon inside the “cave”

MonteCarlo simulation of events in an OPERA “brick”

20 MeV -

20 MeV +

1500 MCevents

(microtracks)

N 4 4 4 4 6 6 6 6

Ne 0 2 4 6 0 2 4 6

5 MeV - - - - - - - - -

7 MeV - - - - - - - - -

10 MeV - 0.6 0.5 0.4 0.3 - - - -

15 MeV - 31 28 23 17 1 1 0.9 0.7

20 MeV - 56 50 40 30 29 26 21 15

25 MeV - 70 60 49 36 48 42 34 25

30 MeV - 78 66 54 36 60 51 42 28

Next step : reconstruction efficiency

Geometrical efficiency g (%)

N , Ne = minimum number of emulsion films* crossed by the -, e-

* means a microtrack (43 m)

Next step : reconstruction efficiency

This search takes advantage of the fast (~20 cm2/h) scanning microscopes used for the OPERA experiment

CONCLUSIONS

We propose to perform a search for spontaneous emission of muons from Pb using one or more OPERA bricks

A test with few OPERA bricks for 1-few years exposure should be useful for a good background study and for defining appropriate analysis procedures

This search would be complementary and superior to otherexperiments looking for such exotic radioactivity

Even in absence of candidate events, we would obtain a significant limit for spontaneous muon radioactivity