Ionising Radiation

Alpha rays are doubly ionised helium nuclei, which, because of their large mass and double charge interact strongly with any matter through which they pass. They are able to travel a mere few centimetres in air and a fraction of a millimetre in solid matter.

Shielding material with the highest density is able to absorb the greatest amount of gamma photons. Lead has the highest density, so it would reduce the count rate the most.

Absorbed radiation dose is measured in “gray” (=joule/kg). When the different types of radiation are weighted by their biological effect (so that a gamma ray is “equivalent” to an alpha particle say), the gray becomes the sievert.

X-rays are photons, so they have no mass or charge. Hence they interact only weakly with the matter through which they pass. In addition, 5 MeV are very energetic x-rays, so they are more penetrating than gamma ray photons with less energy.

A radionuclide with a short half-life must be emitting a lot of radiation in that time as half of the nuclei present will have emitted their radiation. Hence it may be termed “highly radioactive”, albeit for a short time.

Diagnostic x-rays are not associated with radioactivity.

Mammography uses x-rays which ARE ionising radiation, albeit giving the patient a small dose of radiation.

To produce an ion, one or more electrons must be removed from an atom.

This is the best answer. Half-life refers to the atoms of an isotope of the one element. The daughter nuclei may themselves also be radioactive and so contribute to the radioactivity of the sample now containing a mixture of original isotope atoms and daughter atoms.

As radiation leaves a point source, it diverges and spreads equally in all directions over an ever-expanding spherical surface. A spherical surface area is proportional to the radius squared, so exposure to radiation diminishes as it spreads more thinly in space.