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