NaI calibration and neutron observation during the charge exchange experiment

1. NaI calibration and neutron observation during the charge exchange experiment Giovanni Signorelli, INFN Pisa MEG collaboration meeting, PSI 9 Feb 2004 • Improving the NaI energy resolution (as low as reasonably achievable!) • Common noise reduction • Intercalibration • Clustering algorithm • Observation of the prompt signal of “high energy” neutrons (8.9 MeV) • A “matter-of-fact” evidence, in Xe and in NaI • Comparison with cross sections • First look at and requirements of a MC for neutrons in LXe.

2. NaI calibration procedure • Common noise reduction • Crystal intercalibration • Clustering for energy summation 23%  11% FWHM @ 55 MeV

3. Common noise reduction Correlation between channels due to electronics, noise in cables, ADCs… • Algorithm: • Simplif. From E.Frlez, D. Pocanic, S.Ritt NIM A463 (2001) • Take the ADC of the channels which see pedestal • Make the average • Subtract it from all channels (second pedestal correction) • The pedestal ’s shrink from 56 to 23. • It’s not perfect but compatible with the ALARA principle

4. Crystal intercalibration • ROUGH CALIBRATION • Cosmic ray runs can be used to inter-calibrate crystals • Muons triggered by crystal pairs • Position of the Landau peak • FINE TUNING • Problems for crystals at the center (the crystal are not uniformly spanned by cosmics?) • Refined with monoenergetic gammas

5. Energy clustering E = Ei iC The cluster C includes the element of the detector with the maximum energy plus all the fired elements connected to another member of the cluster by a side or a corner

6. Results Reconstructed peak • The resolution is acceptable • The peak position is well reproduced s

7. A better NaI helps • A cleaner separation of the two NaI peaks helps in reducing the tails on the Lxe distributions • An improved collinearity requirement shows the real performance

8. Neutron observation during the experiment • Evidence for a prompt signal from neutrons • 8.9 MeV neutron in coincidence with the 129 MeV gamma • Neutrons from the Am/Be source (10 MeV) • Comparison with cross sections (physics) • Inelastic scattering • Xe level excitation • First look at and requirements of a MC for neutrons in LXe. • Geant 3.21 + GCALOR • Geant 4 • Possible use of neutrons for calibration/monitoring purposes (Angela) • Availability – switchability • Probe of the entire detector

9. Evidence • Runs triggered with one of the detectors only (&S1 &RF…) • Emeasured> 110 MeV  selection of the - p   n events • No timing cut (implies an energy/position cut!) Xe NaI 50% efficiency

10. Neutron-induced prompt signal in Xe For fast neutrons (110 MeV) the total and scattering cross sections are similar for all isotopes = 1 barn  = 72 cm in LXe

11. Neutron cross section

12. Processes • A COMPLETE MONTECARLO CALCULATION IS NEEDED FOR COMPUTING THE NEUTRON EFFECTS IN THE CALORIMETER : • efficiency for fast and thermal neutron detection • determination of the energy spectrum in the calorimeter • energy released as a function of time • energy density (x,y,z) • dependence on threshold and n-energy • ALL THE RELEVANT NEUTRON CROSS-SECTIONS CAN BE INCLUDED IN GEANT 3.21 AND ARE INCLUDED IN GEANT4 • information from medical physics…….! • KERMA COEFF. (Kinetic Energy Released per unit Mass) and • tr /  (mass energy transfer coefficient) • tabulated for neutrons

13. MC for neutrons in liquid Xenon • Though the most reliable simulation today is GEANT4, some quick results were obtained with GEANT 3.21 + GCALOR • 8.9 MeV neutron simulated impinging a 10 x 10 cm2 window of the Lproto (time cut-off at 600ns) coming from the LH2 target • GCALOR (MICAP, En < 20 MeV) takes care of n cross sections (ENDF VI B) • N,n n,2n … • If the residual nucleus is left in an excited state the deexcitation photon is generated (this is not done in the n,Xe  n’Xe case. Bug? We generated these photons by hand) • Some refinement still possible • In GEANT4 the code for the neutron transportation is automatically embedded in the packageand is “benchmarked” with a comparison to real data!

14. Neutron Monte Carlo event sample • 8.9 MeV neutron • 10 x 10 cm2 window • Coincidence with the 129 MeV photon Incoming neutron

15. MC spectrum • A neutron edge is present • Low energy lines due to Xe and/or other nuclear levels • High energy tails: n capture and isotope production • The comparison with the data is good but not excellent Xe levels 2.2 MeV capture on protons

16. Conclusion • A calibration procedure for the NaI has been estabilished and coded in the (Pisa version of the) analyzer, obtaining a fairly good E resolution for this detector • The neutron prompt signal was identified in Xe and NaI and the understanding of the process is under way. We’ll do our best to reproduce the experimental result… • A new window is open, a new handle is present. To us the difficult task to exploit it (calibration, monitoring…)!

17. …timing

18. gamma n2n neutron n 1 n 2 n

19. Xe 129 TOT SC

20. Xe 129 n2n n3n Initial energy degradation and neutron duplication

21. Xe 129 nuclear level excitations n1 n2 etc. Levels 0.039 0.236 0.318 MeV energy degradation and kinetic energy into  energy

22. Xe 129 n

23. Xe 132 nuclear level excitations n1 n2 etc. Levels 0.628 0.1.298 01.44 MeV

24. Xe 132 n