Roentgen (unit)

The roentgen (R, also röntgen) is a legacy unit of measurement for the exposure of X-rays and gamma rays up to several megaelectronvolts. It is a measure of the ionization produced in air by X-rays or gamma radiation and it is used because air ionization can be measured directly. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays. Originating in 1908, this unit has been redefined and renamed over the years. It was last defined by the US National Institute of Standards and Technology (NIST) in 1998 as 2.58×10⁻⁴C/kg, (i.e. 1 C/kg = 3876 R) with a recommendation that the definition be given in every document where the roentgen is used. One roentgen of air kerma (kinetic energy released per unit mass) deposits 0.00877 grays (0.877 rads) of absorbed dose in dry air, or 0.0096 Gy (0.96 rad) in soft tissue. One roentgen (air kerma) of X-rays may deposit anywhere from 0.01 to 0.04 Gy (1.0 to 4.0 rad) in bone depending on the beam energy. This tissue-dependent conversion from kerma to absorbed dose is called the F-factor in radiotherapy contexts. The conversion depends on the ionizing energy of a reference medium, which is ambiguous in the latest NIST definition. Even where the reference medium is fully defined, the ionizing energy of the calibration and target mediums are often not precisely known.

Röntgen (crater)

Röntgen is a relatively large lunar impact crater that lies along the northwestern limb of the Moon. Its northwestern outer rim is partly overlain by the crater Nernst. Both Nernst and Röntgen overlie the eastern rim of the much larger walled plain Lorentz. The smaller crater Aston is separated from the eastern edge of Röntgen by only a few kilometers of terrain. To the south-southeast is Voskresenskiy.

The outer rim of Röntgen has been heavily eroded by subsequent impacts, and it now forms an uneven, jumbled ring of ridges in the surface. There is a relatively fresh, cup-shaped crater along the common rim between Röntgen and Nernst. The interior floor of Röntgen is nearly level, with only a few patches of uneven terrain near the edges and a low ridge near the midpoint. The surface of the floor is marked by a few small and several tiny craterlets.

Satellite craters

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Röntgen.