1. 1) T. KATO and K. MATUDA: 1st International DH-Vacuum Conference, (1962) Riechtenstein
2. 3) N. MORITAMA and H. TANAKA: 6th International DH-Vacuum Conference, (1973) Stresa
3. 5) H. ISO and H. TANAKA: 5th International DH-Vacuum Conference, (1970) Morgantown
4. 6) G. ZAHS: 4th International DH-Vacuum Conference (1968) Bürgenstock
5. 7) T. KATO and K. MATUDA: 2nd International DH-Vacuum Conference, (1964) Puerto Rico