state. Calculation of the hydrogen density and analysis of the errors was carried out as described elsewhere. 3 Knowing the evaporation rate from the target, the rate of rise of bubbles and the total volume of the target, an upper limit to the instability in density due to boiling can be calculated. Assuming that the vapour bubbles form on the Lawsonite surface of the cylinder, this amounts to 0.1%, The amount of extraneous matter in the path of the beam is made up of 2 x 270 At Lawsonite on the vacuum jacket windows, 2 x 10 x 12/a multilayer Lawsonite insulation and 2 x 150 At Lawsonite on the ends of the target, totalling 0.15 g cm "2. The main contribution comes from the 3 mm thick aluminium vacuum jacket.
The rates of evaporation are 0.55-0.6 1 h "1 liquid hydrogen and 0.5 1 h "1 liquid nitrogen. The nitrogen is refilled once a day and the hydrogen once in three days. Transfer-ring hydrogen from the target into the reservoir and back takes 3 minutes. Transfer is achieved by opening or closing the valve of a flow meter.
In conclusion the authors wish to thank Yu. D. Prokoshkin, S. B. Nurushev and V. N. Bolotov for initiating the work and for their part in preparing the target, and A. G. Zel'dovich, L. B. Golovanov and N. I. Balandikov for much valuable advice.