Authors: Pertti Makela (CUA), Nat Gopalswamy(NASA/GSFC), Sachiko Akiyama (CUA), Hong Xie (CUA), Seiji Yashiro (CUA)

Sustained gamma-ray emission (SGRE) events are solar eruptions that show prolonged >100 MeV gamma-ray emission after the end of the impulsive phase of the associated gamma-ray flare. The >100 MeV SGRE is produced by >300 MeV protons precipitating down into solar chromosphere. Based on the list of SGRE events reported by Ajello et al. (2021), we divided metric type II associated (shock-driving) halo coronal mass ejections (CMEs) in solar cycle 24 into those with and without an SGRE event.  We compared the average acceleration and space speed of the two halo CME (HCME) groups between the HCME populations having or lacking major solar energetic particle (SEP) events and decameter-hectometric (DH) type II radio bursts. We used GOES and STEREO/IMPACT particle data together with Wind and STEREO WAVES radio observations. We find that the SGRE-associated HCMEs have higher average acceleration than those without an SGRE event. The SGRE-associated HCMEs that produce a major SEP event and/or an DH type II radio burst have the highest average acceleration and space speed. The HCMEs without an SGRE event and without an SEP event or DH type bursts have the lowest space speed, whereas the SGRE-associated HCMEs without a major SEP event and the SEP or DH type II -associated HCMEs without an SGRE event have similar average space speeds. We also looked at a third group of HCMEs only associated with DH type II radio bursts and found that HCMES without an SGRE event accelerate very slowly as expected when the shock forms at the higher distance. The results indicate that the SGRE-associated HCMEs are like CMEs associated ground level enhancements that have enhanced fluxes of >300 MeV proton. Therefore, our results provide support for the suggestion that CME-driven shocks are the source of the SGRE-producing >300 MeV protons.