popinga ha scritto:
> Un momento. Mi sta bene che, storicamente, la terminologia si basi sui
> meccanismi che dici. Ma ora pensa all'osservazione astrofisica: la
> definizione di "raggio gamma", qualunque essa sia, deve applicarsi a
> prescindere dai meccanismi di produzione. Quelli li possiamo solo
> ipotizzare. Ad esempio un fotone X puo' venire energizzato (attraverso
> urti Compton) ed entrare nella banda gamma prima di essere rivelato.
> Quello e' un raggio gamma. Oppure pensa alla CMB: solo "adesso" e'
> microonde.
Tutto vero, ma la questione e' stabilire quale sia l'uso corrente.
Ti riporto per es. quello che si legge in
http://en.wikipedia.org/wiki/Gamma_ray
"In the past, the distinction between X-rays and gamma rays was based
on energy (or equivalently frequency or wavelength), the latter being
considered a higher-energy version of the former. However, high-energy
X-rays produced by linear accelerators ("linacs") and astrophysical
processes now often have higher energy than gamma rays produced by
radioactive gamma decay. (In fact, one of the most common gamma-ray
emitting isotopes used in nuclear medicine, technetium-99m, produces
gamma radiation of about the same energy (140 KeV) as produced by a
diagnostic X-ray machine, and significantly lower energy than the
therapeutic treatment X-rays produced by linac machines in cancer
radiotherapy.) Because of this overlap in energy ranges, the two types
of electromagnetic radiation are now usually defined by their origin:
X-rays are emitted by electrons outside the nucleus, while gamma rays
are specifically emitted by the nucleus (that is, produced by gamma
decay). There is no lower limit to the energy of photons produced by
nuclear reactions, and thus ultraviolet and even lower energy photons
produced by these processes would also be defined as "gamma
rays".[citation needed] In certain fields such as astronomy, gamma
rays and X-rays are still sometimes defined by energy, as the
processes which produce them may be uncertain."
--
Elio Fabri
Received on Tue Jan 19 2010 - 20:52:16 CET