1. PositronBG_2006-04-17.ppt 1 Positron BG on GLAST/EGRET GLAST Calibration/Analysis VRVS meeting April 17, 2006 Tsunefumi Mizuno Hiroshima University

2. PositronBG_2006-04-17.ppt 2 Purpose of this Study Expected BG level of LAT is close to extragalactic  -ray flux for E<100 MeV (Bill’s talk at DC2 kickoff meeting). Significant contribution from positron – they are Irreducible. EGRET didn’t observe rigidity dependence of BG, implying no significant contamination from charged particles. Is positron flux in DC2 BG model too high than real? Study by Simulation. Sreekumar et al. 1998 Bill’s talk at DC2 kickoff meeting

3. PositronBG_2006-04-17.ppt 3 Simulation ACD: Plastic scintillator of 2cm (EGRET)/1cm (LAT) e+ (normal incidence) Blanket: Carbon of 0.18 g cm -2 (EGRET)/0.39 g cm -2 (LAT) “Perfect” Tracker which is made of vacuum but has 100% detection efficiency Simulate very simplified LAT/EGRET and evaluate the BG induced by positron. ACD threshold: 10 keV(EGRET)/500 keV(LAT; corresponds to ~0.3 MIP)

4. PositronBG_2006-04-17.ppt 4 Result (1) EGRETLAT annihilation at blanket annihilation at ACD Deposit energy distribution in top ACD tile. Energy resolution is not taken into account for simplicity Positron annihilation at blanket (EGRET/LAT) and at ACD (LAT) induces irreducible BG.

5. PositronBG_2006-04-17.ppt 5 Result (2) e+ spectrum at geomagnetic equator (orbital maximum) Extragalactic diffuse  (Sreekumar et al. 1998) BG due to e+ (LAT) BG due to e+ (EGRET)

6. PositronBG_2006-04-17.ppt 6 Possibility of observing orbital variation It will be nice if we can bound the BG induced by positron with orbital variation. orbital max. e+ spectrum (geomag. equator) orbital minimum (high lat. region) orbital max e+ spectrum for DC2 orbital minimum Extragalactic diffuse  orbital variation Please note: e-/e+ flux below 150 MeV is uncertain and need to be validated (see also our talk on March 13 th ), so please don’t take this plot as the prediction for real flight. LAT EGRET

7. PositronBG_2006-04-17.ppt 7 Conclusion BG induced by positron is ~1/10 or lower of extragalactic  -ray flux for EGRET. No wonder why EGRET didn’t observe significant orbital variation of  -ray flux. (Sreekumar et al. 1998) BG due to e+ is about 1/2~1/3 of extragalactic  -ray flux for LAT at orbital maximum. Part of them is due to annihilation at blanket and the others is due to annihilation at ACD. (Please note that LAT has thicker blanket and higher ACD threshold than those of EGRET.) EGRET observation and DC2 LAT simulation are consistent. It is not that BG flux model for DC2 is much higher than real. Orbital variation would be 1/3~1/4 of extragalactic diffuse  -ray flux and could be measurable and constrain the BG level.