21 senttimetrin säteily

21 cm säteilyn syntymekanismi: elektoronin spin-vektorin suunta muuttuu.

21 senttimetrin säteily on sähköisesti neutraalin vedyn energiatason muuttumisessa vapautuvaa sähkömagneettista säteilyä, jonka taajuus on noin 1 420,40575 MHz ja aallonpituus noin 21 senttimetriä. Säteilyä hyödynnetään runsaasti erityisesti radiotähtitieteessä, sillä se on radiosäteilyä.

Säteilyn syntyminen

Sähköisesti neutraali vety koostuu yhdestä protonista, jota kiertää yksi elektroni. Kummallakin on spin-niminen ominaisuus. Spinin klassinen analogia on pyörimisliike (kuten Maan pyöriminen akselinsa ympäri sen kiertäessä Auringon ympäri) mutta koska kyseessä on hiukkasmaailman ilmiö, on spinin merkitys hieman toinen. Protonin ja elektronin spinit voivat saada kaksi arvoa analogiana myötä- ja vastapäivään tietyn akselin ympäri. Protonin ja elektronin spinit voivat olla saman- tai erisuuntaiset.

Hiukkasten magneettisten vuorovaikutusten vuoksi vetyatomilla, jonka protonin ja elektronin spinit ovat samansuuntaiset, on hiukan enemmän energiaa kuin atomissa, jossa spinit ovat vastakkain. Siirtymä pienemmälle energiatasolle on hyvin epätodennäköistä, 2,9 · 10−15 s−1. Tämän vuoksi sitä tuskin havaitaan laboratoriossa, mutta koska vetyatomeja on niin runsaasti avaruudessa, havaitaan säteily helposti radioteleskoopeilla. Lisäksi toisten atomien kanssa törmäämisen ja kosmisen taustasäteilyn vaikutuksesta tapahtuma nopeutuu.

Säteilyn löytäminen

Hendrik van de Hulst ennusti vuonna 1944 kylmän tähtienvälisessä aineessa olevan atomaarisen vetykaasupilven emissioivan tietyn taajuista radioaaltoa.[1] Ensimmäisen kerran Harold Irving Ewen ja Edward Mills Purcell havaitsivat säteilyn Harvardin yliopistossa vuonna 1951. Havainnon vahvistivat pian Jan Oort ja A. C. Müller. Löydön pohjalta pystyttiin myös päättelemään Linnunradan spiraalimainen muoto.

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