Order-of-magnitude estimate of the target thickness.

We have about 5000 counts of 40Ca(p,d)39Ca(g.s.) in an angular range of 10-30 degrees.
The total beam flux is calculated as follows.

2 hours x 1.5 x 10^5 pps = 1 x 10^9 particles
About half is on target -> 5 x 10^8 particles

By assuming the 5-mb cross section (integrated with respect to the cm angle),
and ignoring the Jacobian and the detector coverage, we get

thickness = 5000 counts / (5 mb x 5 x 10^8 particles) = 2 x 10^21 cm^-2

c.f. 1-mm thick solid hydrogen 4 x 10^21 cm^-2