Gray (enhet)

En gray (1975) er en måleenhet for absorbert dose, som brukes i ioniserende stråling.[1][2]

Når stråling går gjennom materie avgir den energi. Det er bare den strålingsenergien som blir absorbert i cellene, som kan ha noen virkning og gjøre skade på levende organismer. Mengden av absorbert energi per masseenhet kalles absorbert dose (stråledose), som altså måles i gray. Det har symbolet Gy og ble definert av SI-systemet i 1975.

Den er oppkalt etter den engelske fysikeren Louis Harold Gray (1905–65) som forsket på hvordan radioaktivitet påvirket biologiske ting.

Definisjon

Når vi måler 1 Gy betyr det at 1 kg av det bestrålte stoffet har absorbert 1 J (joule) stråleenergi. En gray er altså en joule per kg (J/kg) og dermed en SI-enhet avledet fra to av de syv grunnenhetene.

Ved å gange stråledose med en virkningsfaktor får man den biologiske effekt målt i sievert.

Det gamle "rad"

Før 1975 hadde man i det "gamle" CGS-systemet brukt måleenheten rad 1 rad (engelsk: Radiation Absorbed Dose). Den ble i 1953 definert som hvor mye dose som trengs for å få 1 gram materie til å absorbere 100 erg med energi. Altså er 1 rad = 100 erg per gram. Siden 1 erg = 10-7 joule er 1 rad = 10-2 J/kg. Derfor er da 1 Gy = 100 rad.

Referanser

  1. ^ Gray i Store norske leksikon.
  2. ^ Thormod Henriksen (1995). Stråling og helse. Oslo: Fysisk institutt, Universitetet i Oslo. ISBN 8299207320. 

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