BETA DECAY VISUAL PHYSICS ONLINE

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1 VISUAL PHYSICS ONLINE BETA DECAY Suppos now that a nuclus xists which has ithr too many or too fw nutrons rlativ to th numbr of protons prsnt for stability. Stability can b achivd by th convrsion insid th nuclus of a proton into a nutron or a nutron into a proton. In this transmutation: Charg is consrvd a bta particl ( + or - ) is mittd from th nuclus Enrgy and momntum ar consrvd a particl calld a nutrino ( or ) must also b mittd from th nuclus. Grk lttr nu ( )

2 Rduction of a nutrons & incras in a proton insid nuclus N / Z too larg n p N N 1 Z Z + 1 A A A P Z A D Z C 6 14 N Th lctron - mittd in dcay is not an orbital lctron, th lctron is cratd in th nuclus itslf. W us th trm particl for th lctron to show its origin, nonthlss it is indistinguishabl from an orbital lctron. Th symbol rprsnts th particl calld th lctron antinutrino (antiparticl of th lctron nutrino). A fr nutron can also dcay but not a fr proton (on avrag a fr nutron at rst livs for ~15 minuts) n p

3 Rduction of a proton & incras in a nutron insid nuclus N / Z too small p n N N + 1 Z Z 1 A A A P Z A D Z N F To consrv charg, th bta particl mittd is th positron +. Th positron is idntical to an lctron xcpt that its charg is positiv and is th antiparticl of th lctron. In this transmutation, an lctron nutrino is also mittd.

4 Elctron captur Bsids - and + missions, thr is a third procss calld lctron captur and occurs whn a nuclus capturs (absorbs) on of its orbiting lctrons N N + 1 Z Z 1 A A A P Z + - A D Z B Li 3 + Bta rays ar mor pntrating than alpha rays and mov at a vry high spd.

5 no. particls dtctd THE NEUTRINO In bta dcay, a mor stabl nuclus is producd and hnc in th procss nrgy is libratd as kintic nrgy of th products. It was first nvisagd that th products of bta dcay wr th only th daughtr nuclus and an lctron. Sinc daughtr nuclus has a mass much largr than that of an lctron it would rcoil only vry slowly with small nrgy whras th lctron would gain most of th nrgy libratd. Hnc, it was xpctd that th mittd lctrons in bta dcay would hav a fixd kintic nrgy in ach bta dcay transmutation. But it was found xprimntally that th kintic nrgy of th mittd lctron could hav any valu from zro up to th maximum valu. It was if th law of consrvation of nrgy was violatd. Carful masurmnts indicatd that linar and angular momntum wr also not consrvd. Figur (1) shows th nrgy spctrum for th dcay of bismuth into polonium. 210 Bi Po KE of particls (MV) Fig. 1. Bta dcay of bismuth-210 into polonium-210 whr a nutron changs into a proton.

6 Th nd to account for th nrgy distribution of lctrons mittd in bta dcay (figur 1) and to satisfy th laws of consrvation of nrgy, linar and angular momntum, Austrian physicist Wolfgang Pauli in 1930 proposd that a nutral particl was mittd along with th particl. This particl would hav no charg and zro rst mass (hnc, travl at th spd of light) but would possss spin, nrgy and momntum. For ach bta mission, th total nrgy carrid away from th dcaying nuclus would b shard btwn th bta particl and th nutral particl mittd with it - it would b xpctd that th bta particls mittd would hav a rang of nrgis dpnding on th nrgis of th nutral particls mittd with thm. In 1934, Italian physicist Enrico Frmi ( ) namd Pauli s particl th nutrino ( ), maning littl nutral on in Italian, and formulatd a thory of dcay using this particl. Frmi s thory succssfully xplaind all xprimntal obsrvations. For instanc, th shap of th nrgy curv shown in figur (1) for Bi-210 can b prdictd from th Frmi Thory of bta dcay. Dspit svral ingnious attmpts, th nutrino was not xprimntally obsrvd until In that yar, two Amrican Physicists, Cowan and Rins succssfully idntifid th nutrino by dtcting th products of a raction that could only hav bn initiatd by th nutrino.

7 It was Frmi who, in this thory postulatd th xistnc of th fourth forc in natur th wak nuclar forc. In bta dcay, it is th wak nuclar forc that plays th crucial rol. dcay is oftn rfrrd to as th wak intraction bcaus it is tims wakr than th strong nuclar forc that holds th nuclus togthr. Th nutrino is uniqu in that it intracts with mattr only via th wak nuclar forc, which is why it is so hard to dtct. VISUAL PHYSICS ONLINE If you hav any fdback, commnts, suggstions or corrctions plas mail: Ian Coopr School of Physics Univrsity of Sydny