Ion losses in a plane-parallel ionisation chamber irradiated with a pulsed electron beam

Saturation curves for a plane-parallel ionisation chamber have been studied in a pulsed electron beam of 5.2 MeV initial energy as a function of chamber voltage, chamber depth and charge density liberated per pulse. It is shown that, if ion losses by diffusion and by initial recombination are appropriately considered, the remaining ion losses by volume recombination can be described by Boag's theory. The constant mu in this theory has been determined to be (2.7 +/- 0.1) x 10(10) V m C-1. This value is consistent with data on charge collection characteristics obtained using the same ionisation chamber exposed to continuous radiation from beta-sources.

The perturbation correction factor of ionisation chambers in beta-radiation fields

In determining the absorbed dose in a solid medium by means of gas-filled ionisation chambers, the perturbation of the radiation field by the chamber needs to be taken into account. So far, an appropriate correction factor has neither been calculated nor measured for beta-radiation. This work describes its experimental determination for an extrapolation chamber and beta-radiation fields of 147Pm, 204Tl, and 90Sr + 90Y. The results show that the correction factor may be assumed to be the product of a shield factor and a scatter factor the magnitudes of which depend on the chamber geometry and the radiation field. The change of the perturbation correction factor with phantom depth is important for the measurement of depth dose curves. This is demonstrated by an example.

Studies on protein-phosphorylation reactions in isolated chromatin

The endogenous protein-phosphorylating activity of isolated chromatin was tested. We have found that a group of high-molecular-weight proteins (Mr greater than 50 000) was preferentially phosphorylated when chromatin from mouse ascites cells or from bovine lymphocytes was incubated in the presence of ATP. After disintegration of chromatin by nuclease treatment or by high salt concentration, a larger spectrum of chromatin proteins becomes accessible for phosphorylation by the chromatin-bound protein kinase. Some observations described in this communication may help to partially explain this result. The protein kinase was not found in nucleosomal subunits, indicating a non-random distribution of the enzyme in chromatin. This suggests that enzyme and substrate have to be in close spatial contact for the phosphorylation reaction to occur. Furthermore, we have shown for one protein, histone H1, that phosphorylation sites for the endogenous protein kinase are available on the free but not on the DNA-bound protein, suggesting that phosphate-accepting sites in chromatin proteins may be blocked by protein-DNA or by protein-protein interactions. We also discuss the possibility that chromatin protein kinase occurs in stable complexes with its phosphate-accepting substrates, as has been suggested by the findings of other [Kish, V.M. & Kleinsmith, L.J. (1974) J. Biol. Chem. 249, 750-760].

Saturation corrections for plane-parallel ionization chambers

Experiments described in the literature lead to different formulae for saturation corrections in ionization chambers. To elucidate the differences, saturation curves of an extrapolation chamber irradiated with beta particles from 90Sr+90Y sources have been studied experimentally. The results could be described by one formula which was a combination of known formulae for charge collection losses due to volume recombination, initial recombination and diffusion.