Cosmic-Ray Background Flux Model for Hard X-ray/Gamma-ray Balloon and Satellite Experiments

1. Cosmic-Ray Background Flux Model for Hard X-ray/Gamma-ray Balloon and Satellite Experiments Tsunefumi Mizuno, Tuneyoshi Kamae (Stanford Linear Accelerator Center), Masanobu Ozaki (JAXA) and Yasushi Fukazawa (Hiroshima Univ.) Abstract: In high energy astrophysics observations, it is crucial to reduce the background effectively to achieve a high sensitivity, for the source intensity is quite low. Most of background events are generated via an interaction between cosmic-rays and the instrument, and an accurate cosmic-ray flux model and computer simulation are necessary to develop the background refection method and evaluate the remaining background level. The model needs to cover the entire solid angle, the sensitive energy range, and abundant components. Here we built a cosmic-ray model based on existing data and theories, and validated the model with data of the GLAST Balloon Flight Engineering Model launched in Palestine, Texas in 2001 summer. The model successfully reproduced the data. An application to a balloon-borne hard X-ray polarization mission called PoGO is also presented. Cosmic-Ray Background Flux Models: • Overview: • Cosmic-rays in or near the earth atmosphere consist of the primary and secondary components. The primary cosmic-rays are generated in and propagate through the interstellar space. They are decelerated by solar wind as they enter the solar system, and suffer low energy cutoff due to geomagnetism as they enter the earth magnetosphere. When primary particles penetrate into the air and interact with molecules, they produce relatively low energy particles, i.e., secondary cosmic-rays. In order to use as input in instrument simulation programs, we modeled cosmic-ray spectra. Followings are the features of our models: • We have modeled spectra of abundant components, i.e., proton, alpha, electron, positron, gamma, positive and negative muons. • Spectra are expressed in analytic functions for flexibility and speed-up in simulation. • Models take into account the solar modulation and geomagnetic cutoff effects. • Models cover the entire solid angle and wide energy range (typically 10 MeV-100 GeV). Models of each particle type: proton primary: used our formula and modeld the spectra based on AMS/BESS data. (Fig. a and b) proton secondary: modeled the AMS data with analytic function region by region (Fig. b). The upward and downward spectra are identical in 0<theta_m<0.6, and we assumed the uniform angular distribution. alpha primary: the same as proton primary. (secondary is negligible) e-/e+ primary: used our formula and model the spectra based on a compilation of measurements by Webber (1983). (Fig. c and d) e-/e+ secondary: the same as proton secondary (Fig. c and d). gamma primary: modeled the data compiled in Sreekumar et al. (1998). gamma 2ndary: referred to data at Palestine, Texas (Fig. e, f and g). In order to predict the flux in other region, we adopted the rigidity dependence by Kur’yan et al. (1979) of R-1.13. muons: Modeled the data by Boezio et al. (2000) (Fig. h). Not expected in satellite orbit. Primary Charged Particle Spectra: Based on the solar modulation theory by Glesson and Axford (1968) and experimental data by AMS, we expressed the primary charged particle spectra as below. The formula well reproduces the spectra in regions up to theta_m=0.8 (Fig. b). We assume the uniform angular distribution except the shield by earth. Primary spectra before being affected by modulation Solar modulation formula given by Gleeson and Axford (Fig. a) Our reduction factor to reproduce the geomagnetic cutoff in AMS data. A parameter r=12.0 for proton/alpha and 6.0 for electron/positron. (Fig. b, c and d) (a) (c) proton primary Angular distribution of 2ndary gamma (g) (e) 2ndary downward gamma (not expected in satellite orbit) Phi=650MV Phi=800MV Phi=1100MV e-/e+ spectra in geomagnetic equator proton primary and 2ndary (h) (b) (d) (f) 2ndary upward gamma atmospheric muons e-/e+ spectra in high goemag. lat. region Application to experimental data: Count rate of each TKR layer, ACD detected a hit Polarized Gamma-ray Observer (PoGO): (b) GLAST Balloon Experiment: In order to validate the flux model, we analyzed the data of GLAST Balloon Experiment (Thompson et al. 2002) and compared it with simulation prediction. The primary objective of the experiment was to validate the basic design of GLAST LAT in space-like high counting environment, but it also provided us with data to study cosmic-ray environment. For this experiment we have developed a Geant4 based instrument simulator (Fig.a) and used the cosmic-ray models described above as input. After applying some modification to the model (e.g., taking into account the air attenuation for primaries and modifying the proton/e-/e+ secondary spectra based on balloon measurements), we successfully reproduce data within 10%. (Fig. b, c and d) muons gamma upward gamma downward alpha data PoGO is a new balloon-borne instrument to measure the polarization from astrophysical objects in 25-100 keV range with unprecedented sensitivity, and under development by an international collaboration including Japan. It utilizes the design of well-type phoswich counter (Fig. e), providing us with a large effective area and an extremely low background. To examine the performance of PoGO, we have developed the Geant4-based Monte-Carlo simulation program with our cosmic-ray model. The expected background level is below 10mCrab in 25-100 keV range (Fig. f and g). (e) e-/e+ passive/active collimator plastic scintillator (detection part) proton secodary proton primary side BGO Count rate of each TKR layer, ACD not detected a hit (c) data gamma upward gamma downward bottom BGO protons e-/e+ (a) Zenith angle distribution of gamma-ray candidate XGT (f) (g) downward gamma upward gamma ACD (d) 100mCrab (incident) Support Structures TKR data CAL gamma upward Flux (c/s/cm2/keV) e-/e+ gamma downward Pressure Vessel VME Background due to atmospheric gamma 100mCrab (detected) proton energy 100 keV 20 keV cos(theta)