My first scientific paper was inspired in 1965 by Willard Libby who suggested that I look into reasons for C-14 variations. This led to Rich Lingenfelter who had already done C-14 production calculations based on his neutron production work. Our scientific collaboration, which is still ongoing, thus has lasted for 35 years, I believe a real record.

In 1966 Rich was invited to give a talk on nuclear reactions in solar flares at the University of Pennsylvania meeting on High Energy Nuclear Reactions in Astrophysics. Rich asked me to work with him on this paper in which we did quite good calculations on gamma ray line emission from solar flares. The paper, published in 1967, was sufficiently impressive for Benz Kozlovsky to initiate a collaboration that started in 1972 and is also still ongoing. Benz brought to the collaboration his expertise in nuclear physics which has remained an essential ingredient of our work to this day.

In 1972 Ed Chupp and his team observed for the first time gamma ray lines from solar flares. This topic thus became suitable for PhD dissertations. My first graduate student, H.T. Wang, did a nice dissertation on neutron capture in the photosphere. In a joint paper we showed that observations of the 2.223 MeV neutron capture line can provide information on the photospheric He-3 abundance. Here I must give credit to Hubert Reeves who told me about the very large neutron capture cross section on He-3, an essential ingredient in this endeavor.

In 1974 Benz and I showed that observable gamma ray lines are produced in the interactions of accelerated alpha particles with ambient He. These lines were subsequently observed with SMM, launched in 1980, and remain an important observational goal for HESSI. Just prior to SMM launch, graduate student Taeil Bai did important work on Compton backscatter of X-rays from the photosphere, a topic that will have to be taken into account in the analysis of HESSI data.

Xin Min Hua, working with Rich at UCSD, improved on Wang's calculations. Recently, with the availability of much more neutron capture line data, Natalie Mandzhavidze and I showed that the gamma ray determined photospheric He-3 abundance could be an important ingredient in Galactic chemical evolution. Natalie started to work with me in 1990, but her previous work in the Soviet Union with Grant Kocharov was significantly influenced by our work on solar flare gamma rays and neutrons.

The physics of positron production and annihilation was always of great interest to me. I collaborated on this topic with Carol Crannell in 1976, with graduate student Roger Bussard and Dick Drachman in 1978 and with Nidhal Guessoum in 1991. Positron astrophysics led me to interesting and valuable interactions with Marvin Leventhal. My last graduate student, Jeff Skibo, did much work on the Galactic distribution of positron annihilation radiation.

SMM operated between 1980 and 1989 and produced a great wealth of solar flare gamma ray data. The analysis of these data in terms of the computational tools that we developed was the dissertation of Ron Murphy who is continuing this work to this day. SMM data are still being analyzed yielding interesting results, mostly due to the efforts of Gerry Share. Ron and I collaborated with Chuck Dermer on a variety of high energy processes in solar flares. My collaboration with Ron, Natalie and Benz on high energy solar physics is ongoing. In addition to SMM, gamma ray lines have been observed also with Phebus on Granat, OSSE and Yohkoh. Natalie and I collaborated with Gerald Trottet on Phebus data, finding heavy ion abundance variations on relatively short time scales. I learned a lot about particle acceleration from Miriam Forman, Ellen Zweibel and Don Ellison. Jim Miller, as a graduate student, took up the topic, and by now has become an authority on particle acceleration in solar flares.

In 1993 I spent 3 wonderful months in Japan, at the invitation of Y. Muraki, which led to a collaboration with S. Enome and H. Nakajima on millimeter wave emission from solar flares.

In 1976 Bonnard Teegarden was preparing a proposal for a Ge spectrometer, probably the predecessor of GRIS and INTEGRAL. He asked me to help with the theory. With Rich we found that the 1.809 MeV line from Al-26 decay is a good candidate for detection. The line was subsequently observed with HEAO-C, SMM and most recently COMPTEL.

My interest in cosmic rays dates back to my PhD dissertation at UCLA under Gordon MacDonald. While working on my thesis I met Frank McDonald at a small cosmic ray gathering in California. This meeting resulted in the job that I still hold. The collaboration with Len Fisk and Benz Kozlovsky on the anomalous cosmic rays is a highlight of my early cosmic ray activity. More recently Rich, Benz and I got interested in the origin of Li, Be and B, light elements made mostly by cosmic rays. Our conclusion, based on Be observations in old metal poor stars, that the cosmic rays are accelerated from relatively fresh nucleosynthetic material still awaits definite proof. It hopefully will be provided by future cosmic ray measurements of actinides. The other conclusion, that superbubbles are the acceleration site has been worked out convincingly by Jim Higdon and Rich. The work on light element origin led us to studies of Galactic chemical evolution, and to interesting discussions with Elisabeth Vangioni-Flam and Michel Cassé. Interesting interactions are ongoing with another French scientist, Vincent Tatischeff, who is currently measuring nuclear cross sections at Orsay, relevant for future HESSI observations.