Experimental investigation of dynamic pressure in a cryosorbing beam tube exposed to synchrotron radiation

摘要

Results of photodesorption experiments on a 4.2-K beam tube irradiated with synchrotron radiation from the VEPP-2M storage ring are being reported. The experiments have been performed on SSC1 and SSC2 beamlines. Synchrotron radiation parameters of the SSC1 beamline are the same as the SSCL 20 TeV proton collider; critical energy = 284 eV, photon intensity 1(center dot)10(sup 16) photons/m/s. Photon intensity of the SSC2 beamline is eight times higher than intensity of the SSC1 beamline. We have used two experimental configurations to observe the density increase due to: (1) photodesorption of tightly bound molecules not previously desorbed and (2) photodesorption of weakly bound cryosorbed molecules. The two configurations used were a simple 4.2-K beam tube and a 4.2-K tube with a coaxial perforated liner. The photo-desorption coefficient of tightly bound H(sup 2) measured on the SSC1 beamline was observed to decrease monotonically with photon exposure, reaching (eta)4(center dot)10(sup (minus)4) molecules per photon at the end of exposure ((approximately)1(center dot)10(sup 22) photons/m). The same experiment on the SSC2 beamline gave a similar result at photon dose 3.5(center dot)10(sup 22) photons/m. The photodesorption coefficient of cryosorbed H(sub 2) increased with increasing H(sup 2) surface density, reaching (eta)'(sigma)(sub w)(approximately)7 molecules/photon at one monolayer surface density (s(sub m)(approximately)3(center dot)10(sup 15) H(sub 2)/cm(sup 2)), where (sigma)(sub w) is the sticking coefficient. The liner was shown to effectively shield cryosorbed molecules from synchrotron radiation.