## Abstract

Absolute G values for chemical change when various aqueous solutions are irradiated with $\alpha$-particles from an external polonium source have been determined for different fractions (x) of the $\alpha$-particle track spent within the solution. G(Fe$^{3+}$) for an aerated solution containing 1 mM ferrous ions and 0$\cdot$1 N sulphuric acid decreases from 5$\cdot$0 at x = 0$\cdot$02 through a minimum value 3$\cdot$65 at x = 0$\cdot$11 x 0$\cdot$01 and then increases to 5$\cdot$94 at x = 0$\cdot$525. The local value G(Fe$^{3+}$), defined as G(Fe$^{3+}$) for an element of track length a distance xR from the end of the track, where R is the range of the whole track in water, shows a similar but more pronounced dependence on x which is strongly reminiscent of the inverted Bragg curve for l.e.t. plotted against x in this medium, having a minimum of 3$\cdot$2$_5 \pm$ 0$\cdot$1$_5$ at x = 0$\cdot$08$_5\pm$ 0$\cdot$01$_5$. As x is increased G(Ce$^{3+}$) for an aerated 200 $\mu$M solution of ceric sulphate in 0.1 N sulphuric acid shows no minimum but increases from 1$\cdot$0 at x = 0$\cdot$04 to 3$\cdot$0 at x = 0$\cdot$25 and 4$\cdot$05 at x = 0$\cdot$525. Thallous ions have no effect on G(Ce$^{3+}$) for x $\leqslant$ 0$\cdot$45. G(H$_2$O$_2$) for aerated water was found to be 1$\cdot 22 \pm 0 \cdot$06. It is concluded that very few hydroxyl radicals are available and that the intra-track reactions between H$_2$O$_2$ and H as well as OH are very important and vary in extent as x is varied.