1. Search for spontaneous muon emission from lead nuclei with OPERA bricks M. Giorgini, V. Popa Bologna Group OPERA Collaboration Meeting, LNGS, 19-22/05/2003 M. Giorgini

2. Pions or muons could be emitted by nuclei through the decays : (A,Z)  ± +  (  ) + (A 1,Z 1 ) + …… + (A n,Z n ) (A,Z)  ± (   ) + (A 1,Z 1 ) + …… + (A n,Z n ) (a)(b) (a)Spontaneous  emission :  emitted with nuclear fragments (b) Hyper-cold approximation :  emitted before nuclear fragments

3. 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-value  + (MeV) Nuclear mass

4. 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

5. nuclear emulsions for tracking particles huge amount of Pb and long exposure time OPERA low natural radioactivity very low cosmic muon flux local neutron flux  1.8. 10 -6 neutrons/s/cm 2 LNGS + = GOOD CONDITIONS FOR : a potential discovery … a good upper limit for these rare processes

6. 1 OPERA brick (8.23 kg of Pb) at LNGS for 1 year should : establish the background contribution validate the analysis procedure As t 1/2 >> exposure times T, the 90% C.L. sensitivities are given by 2.3 ln 2 * T N 0 t 1/2 = Assuming 60% efficiency

7. 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 MeV Isotropic  emission

8. 15 MeV  - 15 MeV  +

9. 20 MeV emitted  + 20 MeV  +

11. Definition of candidates : The  ± 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  +

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

13. 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/cm 2 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.

14. 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/cm 2 : can be neglected

15. 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