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1 This contnt has bn downloadd from IOPscinc. Plas scroll down to s th full txt. Download dtails: IP Addrss: This contnt was downloadd on 09/03/2018 at 05:30 Plas not that trms and conditions aly. You may also b intrstd in: Th Catastrohic Univrs: An Essay in th Philosohy of Cosmology Gordon Millr Som Intrsting Faturs for Extrnal Rgion of ShricalSymmtric Mass in Nw Thory of Gravitation VGM Qian Shang-Wu Linar aroximation in a nw thory of gravity R B Mann and J W Moffat Th nw univrs around th nxt cornr L Smolin Origin of th U(1) fild mass in surconductors Hiroyasu Koizumi Thortical and xrimntal dvlomnts in th dtrmination of lctron nrgy loss functions and inlastic man fr aths in lmntal solids and binary comounds Jay D Bourk and Christohr T Chantlr Physics nws on th Intrnt (basd on lctronic rrints) Yurii N Eroshnko Gnralisd non-uniform magntic fild mthod for th convntional two-sin intraction Ising frromagnt O Mitran

2 IOP Concis Physics A Tour of th Subatomic Zoo A guid to articl hysics Cindy Schwarz Chatr 1 Mattr in th arly 20th cntury In this first chatr w xamin viws of mattr that xistd in th arly art of th 20th cntury. Whn rsults of xrimnts could not b xlaind by known thoris, ithr mor xrimnts wr don or nw thoris wr roosd. Nw thoris ld to diffrnt idas about what th world around us was mad of. Through a look at ths xrimnts and thoris, w will larn about som articls that you hav robably hard of and som that you may not hav hard much about. Aftr comlting this chatr you will: know what rotons, nutrons, lctrons, and nutrinos ar. know th constitunts of th atom and th nuclus. b abl to nam thr tys of radiation. know about consrvation ruls for nrgy, lctric charg, and momntum. 1.1 Parts of th atom Ruthrford s xrimnts: th nuclus is uncovrd W bgin with xrimnts by Ernst Ruthrford and his collagus in Grat Britain around Thy usd a ty of radiation, alha articls, to bombard atoms in an attmt to uncovr thir innr arts. Thy did not undrstand fully what alha articls wr, but thy wr abl to us thm. Figur 1.1 shows what thir xrimnt was lik. Th main oints of th xrimnt wr as follows: olonium, which is a radioactiv substanc, was usd as a sourc of alha articls. Alha articls wr mittd from th olonium in all dirctions, but Ruthrford was only concrnd with th articls that hit th gold foil targt. a movabl scrn aintd with a matrial calld a scintillator was usd to dtct th alha articls that mrgd from th targt. Scintillator matrial givs off a flash of light whn struck by an alha articl. Ruthrford was thrfor abl to study th osition of th alhas aftr thy assd through th targt. doi: / ch1 1-1 ª Morgan & Clayool Publishrs 2016

3 A Tour of th Subatomic Zoo Figur 1.1. Ruthrford s xrimnt. Bam of alha articls Figur 1.2. Plum udding. Th rsults of ths xrimnts wr startling. At th tim, a oular modl of th atom was J J Thomson s lum udding : a shrically shad mass of ositiv charg in which th ngativly chargd lctrons wr mbddd. (Elctrons had bn discovrd just rior to th bginning of th 20th cntury by Thomson.) If this modl wr corrct, th rsults of Ruthrford s xrimnts should hav bn similar to what is shown in figur 1.2; th havy alha articls would soar through th atom, dflctd slightly by thir tri through th ositiv udding. To th scintists surris, howvr, som of th alha articls cam right back at thm, as if thy had bouncd off somthing vry massiv, and that was inconsistnt with th lum udding modl of th atom. 1-2

4 A Tour of th Subatomic Zoo Figur 1.3. Discovry of th nuclus. alhas nitrogn hydrogn alhas Figur 1.4. Alha articls at nitrogn. Ths rsults, howvr, wr consistnt with a nw modl, sn in figur 1.3. In this modl, th atom containd: a solid nuclus in which all th atom s ositiv lctric charg and almost all its mass wr concntratd. Th alha articls bouncd off this dns nuclus. light lctrons xisting somwhr in th mty rgion outsid th nuclus. Th lctrons had a ngativ lctric charg to balanc th ositiv charg of th nuclus. Th alha articls brushd ast th lctrons dflctd only slightly. But this was not th last of th scintists xrimnts. To larn mor about th nuclus, Ruthrford and Jams Chadwick continud to us alha articls. In on sris of xrimnts thy shot th alha articls at nitrogn nucli and obsrvd th rsults. As thy xctd, alha articls cam out, but so did hydrogn nucli (s figur 1.4). Wll, if hydrogn nucli could b jctd from nitrogn nucli, thn rhas nitrogn nucli wr comosd of hydrogn nucli. In fact, rhas all nucli wr mad of hydrogn nucli. A roton is th nam givn to a hydrogn nuclus. Protons hav on unit of lctric charg, qual but oosit to th lctron charg. Protons also hav mass, and for simlicity w quot all masss in trms of th roton mass (1 unit). In ths units, th mass of an lctron is about 1/1800. Ths xrimnts showd that nucli had som sort of intrnal structur. Thy too wr comosd of arts, or rotons. Th mor ositiv charg a nuclus was found to hav, th mor rotons it must hav containd. 1-3

5 A Tour of th Subatomic Zoo Modls of th nuclus: rotons and nutrons Th nuclus contains rotons. But this is not th comlt ictur. Th simlst modl of th nuclus, comosd solly of rotons and lctrons, was suggstd in 1914 (s figur 1.5). Lt s s how this modl works for th nitrogn nuclus. Blow is a tabl with rortis of th nitrogn nuclus, th roton, and th lctron. Objct Charg Mass roton +1 1 lctron 1 0 nitrogn nuclus nuclus Figur 1.5. Early modl of th nitrogn nuclus. Th only way to combin rotons and lctrons into a nitrogn nuclus is shown in figur 1.6. Rmmbr, ths articls ar in th nuclus, thr will still b svn lctrons outsid th nuclus. Or, in trms of th articls involvd, on can s how th charg and mass of th constitunts add u to th charg and mass of th nuclus as a whol = nitrogn nuclus charg 14 7 = 7 mass 14 0 = 14 Thr is a roblm with this modl, howvr, in that th sin is not corrct. Sin is a rorty that a articl can hav, just as it can hav charg and mass. Th classical analogy that is most oftn givn for sin is that of a to sinning on its axis. You may think of sin this way, but lctrons and rotons and all othr articls with sin do not actually sin lik a to. Sin is an intrnal rorty of a articl that can b calculatd or masurd, just lik mass can. Elctrons and 1-4

6 A Tour of th Subatomic Zoo nuclus Figur 1.6. Early nuclar modl. rotons ach hav a sin of 1/2 unit. Th sin of th lctron or roton is said to b u or down, and ths ar th only ossibilitis. Now, our modl has 21 articls in th nuclus, ach with a sin of 1/2. Th nitrogn nuclus has a sin masurd at 1 unit. It is not ossibl for an odd numbr of articls with 1/2 unit of sin ach (21 in our cas) to hav a combind sin qual to an intgr (1 in our cas). This oint is illustratd in figur 1.7 for som simlr cass with thr or four articls, ach with a sin of 1/2. By 1930, many hysicists ralizd th inadquacy of th siml modl of rotons and lctrons as th constitunts of th nuclus. If w rquir th nitrogn nuclus to hav th corrct sin as wll as charg and mass, thn thr must b an vn numbr of objcts in th nuclus. So, if w combin 7 of th 14 ositivly chargd rotons with th 7 ngativly chargd lctrons in th nuclus, w gt 7 nutral objcts with = 3/2 = 1/2 = = 1 Figur 1.7. Examls of sin combinations. (Sin: htts:// 1-5

7 A Tour of th Subatomic Zoo masss ssntially qual to th roton mass. Figur 1.7 shows how this combination might b viwd. As arly as 1920, thr wr susicions of a nutral objct (with th sam mass as th roton) in th nuclus. By 1932, th nutron had bn discovrd. Svn rotons and svn nutrons in th nitrogn nuclus giv th corrct charg, mass, and sin. Go ahad and vrify it. Th nutron was discovrd by Chadwick in anothr xrimnt involving alha articls. A sktch of th basic stu is shown in figur 1.8. Th main oints of this xrimnt wr: alha articls wr aimd at a bryllium targt. Chadwick obsrvd somthing coming out of th bryllium that did not hav lctric charg (w us th trm nutral articls or nutral radiation). this radiation hit a araffin targt. rotons wr knockd out of th araffin targt. Chadwick concludd that th nutral radiation was indd th nutron, as it had to hav a mass clos to th roton to knock on out of th araffin. 1.2 Radiation Figur 1.8. Discovry of th nutron. Unstabl nucli sontanously dcay by mitting articls. This rocss is calld radioactivity. Th thr tys of radiation ar calld: alha (α). bta (β). gamma (γ). W hav alrady sn that alha articls wr usful in Ruthrford s xrimnts on th atom. Alha (α) radiation, or alha articls ar just hlium nucli (two rotons and two nutrons). An xaml of a rocss that roducs alha articls is a radium nuclus dcaying into a radon nuclus and an alha articl, as shown in figur 1.9. You can s that th total numbr of rotons and th total numbr of nutrons rmain constant. radium nuclus radon nuclus + alha Gamma (γ) radiation is high-nrgy lctromagntic radiation. Whn a nuclus gts into an xcitd stat, it can mit th xtra nrgy it has by gamma radiation. Just to ut th nrgy in rsctiv, gamma rays rvalnt in high-nrgy hysics rocsss 1-6

8 A Tour of th Subatomic Zoo n Radium nuclus n Radon nuclus 2 2n Alha Figur 1.9. Examl of alha radiation. Figur Gamma radiation as scondary rocss. hav an nrgy about 10 billion tims that of microwavs. Many diffrnt tys of nucli can bcom xcitd and mit gamma rays. Th xaml shown in (figur 1.10) shows first bta radiation (nxt sction) and thn th Ni-60 is in an xcitd stat so it mits gamma radiation. xcitd nuclus unxcitd (sam) nuclus + γ Bta (β) radiation involvs th mission of an lctron. But this lctron is not on of th lctrons from outsid th nuclus. It is an lctron that is cratd within th nuclus. An xaml, as shown in figur calcium otassium + lctron Unlik th rocss that roducs alha radiation, in this rocss th total numbr of rotons and th total numbr of nutrons ar not saratly consrvd. W hav gaind a roton and lost a nutron. Th undrlying rocss hr is a nutron dcaying into a roton and an lctron. Studying this rocss of bta dcay ld hysicists to roos a nw articl calld th nutrino (as sn in figur 1.11). Th nutrino was ostulatd whn nutron bta dcay did not follow som imortant consrvation rincils of hysics. Th nutrino will b discussd in gratr dtail aftr w discuss ths consrvation rincils. 1-7

9 A Tour of th Subatomic Zoo 1.3 Som consrvation laws Figur Examl of bta radiation. Consrvation rincils ar imortant in hysics, and thy can hl us to xlain why som things han and othrs don t. Thr consrvation ruls that must b obyd in all hysical rocsss ar: consrvation of lctric charg consrvation of momntum consrvation of total nrgy. W will not driv ths ruls, but w will us thm to dtrmin whthr crtain articl ractions can or cannot occur Charg consrvation Th total lctric charg of a systm of articls must rmain th sam. That is, if you add u all th chargs of th articls on on sid of a raction, thy must xactly qual th sum of all th chargs of th articls on th othr sid of th raction. For xaml, both th following ractions oby th rincil of charg consrvation. Not that ths ar not th only ossibilitis, but just two xamls of ractions In both xamls, two chargd objcts, on with a ositiv charg and th othr with a ngativ charg, ar th initial articls (thos to th lft of th arrow). In th first cas, th final articls (thos to th right of th arrow) both hav no lctric charg. In th scond cas, thr ar thr articls: on ositiv, on ngativ, and on nutral. In ithr cas, th total charg of th final articls is zro. 1-8

10 A Tour of th Subatomic Zoo Enrgy consrvation Th total nrgy of th articls bfor a dcay or raction (two articls in th initial stat) must b qual to th total nrgy of th articls aftrward. Now, hr w sak of th total nrgy, bcaus th nrgy that concrns us coms in two forms: kintic nrgy, or nrgy of motion, which dnds on th vlocity of th articl mass nrgy, which ariss from Einstin s famous quation E = mc 2 In this quation, th nrgy (E) is qual to th roduct of th mass of th articl and th sd of light (which is a constant) squard. Th gratr th mass of th articl, th mor mass nrgy it has. For dcays, our critrion for satisfying nrgy consrvation will b that th mass (or mass nrgy) of th dcaying articl b gratr than or qual to th sum of th masss of th nd roducts. How dos this lad to nrgy consrvation? Enrgy consrvation for articl A initially at rst givs us mass of A = mass of B + kintic nrgy of B + mass of C + kintic nrgy of C and sinc all nrgis ar ositiv (mass of A) > (mass of B) + (mass of C) For ractions, th two articls in th initial stat can hav a total mass lss than th total mass of th final articls bcaus thy can bring kintic nrgy into th raction to nsur that nrgy is consrvd Momntum consrvation Th total momntum of th articls must rmain th sam in any hysical rocss whn no xtrnal forcs ar involvd. For vlocitis that ar vry low comard to th sd of light, th momntum of a articl is th roduct of its mass and its vlocity. In a articl raction, if th initial total momntum (th sum of th momnta of th individual articls involvd) is zro thn th final total momntum must b zro as wll. For articl dcays, such as on articl bcoming two or mor articls, this consrvation of momntum manifsts itslf in an intrsting visual way. In articular, if on articl dcays into xactly two articls thn ths articls must mrg from th raction in xactly oosit dirctions for momntum to b consrvd. This is illustratd in figur In this xaml A B + C M, th mass of articl B, is largr than m, th mass of articl C. Thrfor, th vlocity of articl C must b largr than th vlocity of articl B (as indicatd by th lngth of th arrows). Equivalntly, th momntum of articl B (Mv) is th sam as th momntum of articl C (mv). 1-9

11 A Tour of th Subatomic Zoo BEFORE A AFTER B C Mv mv 1.4 Nutrinos Figur Consrvation of momntum. A raction that violats on of ths consrvation rincils cannot occur. Th rocss of nuclar bta dcay was studid carfully in th 1920s. On of th things that scintists found was that nrgy in bta dcay of th nutron was not consrvd. Physicists at this oint had a tough choic: Thy could abandon th rincil of nrgy consrvation, or thy could acct a nw hyothsis ut forth by Wolfgang Pauli. Pauli roosd that a nw articl, which could not b dtctd, ran off with th missing nrgy. This articl was to hav no lctric charg, littl (or no) mass, and th sam sin as rotons and lctrons. It was givn th nam nutrino, which mans littl nutral on in Italian. Existnc of this articl was wll acctd by hysicists by th 1950s. By th lat 1950s, it was also sn that momntum was not consrvd in bta dcay of th nutron unlss th nutrino was considrd a art of th rocss. If th nutron dcayd into only two articls, thn th diagram on th lft in figur 1.13 shows what on must s to guarant consrvation of momntum. Th roton and th nutron must mrg from th dcay back-to-back. BEFORE nutron AFTER AFTER Proton () Elctron () Proton () Elctron () Figur Nutron dcay ossibilitis. 1-10

12 A Tour of th Subatomic Zoo But that is not what hysicists obsrvd in cloud chambr icturs. Instad, th mrging articls lookd somthing lik thos shown in th diagram on th right in figur Th ida of th nutrino fixd nutron bta dcay so that it obyd all th consrvation ruls. Howvr, xrimntal vrification was ndd bfor vryon would bliv in th nutrino. In 1956, about 25 yars aftr it was initially roosd, th nutrino was discovrd outsid a nuclar ractor. At th Savannah Rivr (Ga.) ractor, th numbr of nutrinos mrging r scond was xtrmly high, and hysicists waitd with thir dtctor until thy vntually dtctd som. So th thory of bta dcay must b modifid to includ th nutrino. Th symbol for th nutrino is th Grk lttr ν, and it has no lctric charg and littl (or no) mass. Currnt xrimnts ar lacing limits on th mass of th nutrino, but for our uross it will b takn as zro. This may b th first articl that you hav not hard of. To this oint w hav dalt with th traditional vryday atom, stabl or not, comosd of rotons, nutrons, and lctrons. Now w ncountr th nutrino, whos xistnc was ostulatd whn nutron bta dcay did not follow imortant consrvation rincils. A diagram of th rocss is shown in figur 1.14, with th four subatomic articls all linkd togthr in bta dcay. In this rocss it is actually an antinutrino (but mor about that latr). Radioactiv dcay is rsonsibl for my xistnc today. Som lik to say th wak forc brought m into this world today, but I rfr to think my mothr gav hr lif for m. Mom was a nutron. Aftr a rlativly short lif of fiftn minuts, sh dcayd into hr thr childrn. I m a roton and my two littl sistrs ar an lctron and a nutrino. Mom was a modl articl. Sh didn t caus much troubl and stayd out of othr articls businss bcaus sh was so nutral. Tagan D Goddard v n + + v Figur Nutron dcay. 1-11

13 A Tour of th Subatomic Zoo Summary In this chatr: W saw how th xrimnts don in th arly art of th cntury ld to: th modl of th atom: a dns nuclus containing most of th mass and all of th ositiv charg, with lctrons somwhr outsid th nuclus th modl of th nuclus: rotons and nutrons, with rotons having ositiv lctric charg and nutrons having non th discovry of th nutron. W larnd about thr tys of radiation (α, β, γ) and that th study of on of th tys, bta radiation, ld to th thory of th nutrino. W introducd thr consrvation laws for: charg momntum nrgy. 1-12

14 A Tour of th Subatomic Zoo Slf-tst 1 For multil choic, chck all that aly. 1. Ruthrford usd which kind of radiation to study atomic and nuclar structur? a. alha b. bta c. gamma 2. Match th itms in th lft column with thos in th right. alha bta gamma a nutron changs into a roton high-nrgy lctromagntic radiation a hlium nuclus 3. Nutrinos wr roosd by Pauli: a. bcaus thy wr sn at a nuclar ractor. b. bcaus of missing nrgy in bta dcay. c. to guarant charg consrvation in bta dcay. 4. Nam th thr consrvd quantitis you hav larnd about so far. 5. Fill in th chart objct symbol mass charg sin roton nutron lctron nutrino 6. Tst ach raction for charg consrvation and conclud whthr it asss or fails. + n + n + v + n + n + n 7. Draw a ictur of a hlium atom as sn by a hysicist in

15 A Tour of th Subatomic Zoo Answrs to slf-tst 1 1. Ruthrford usd which kind of radiation to study atomic and nuclar structur? a. alha b. bta c. gamma 2. Match th itms in th lft column with thos in th right. 3. Nutrinos wr roosd by Pauli: a. bcaus thy wr sn at a nuclar ractor. b. bcaus of missing nrgy in bta dcay. c. to guarant charg consrvation in bta dcay. 4. Nam th thr consrvd quantitis you hav larnd about so far. momntum nrgy charg 5. Chart objct symbol mass charg sin roton /2 nutron n 1 0 1/2 lctron 1/ /2 nutrino ν 0 0 1/2 6. Tst ach raction for charg consrvation. + n + n consrvd + v + n consrvd + n + n not consrvd 7. A hlium atom as sn by a hysicist in Hlium has 2 rotons, 2 nutrons, and 2 lctrons. This and all othr drawings in this book ar not 1-14

$A Tour of th Subatomic Zoo to scal. Th nuclus taks u an xtrmly small fraction of th total siz of an atom. Th atom has a diamtr about 10 000 to 100 000 tims as big as th diamtr of th nuclus.$

16 A Tour of th Subatomic Zoo to scal. Th nuclus taks u an xtrmly small fraction of th total siz of an atom. Th atom has a diamtr about to tims as big as th diamtr of th nuclus. Oh, this ndlss sinning. Th monotony is ovrwhlming. I could hav bn a roton; I could hav bn a nutrino, but no. I m an lctron; a ngativly chargd, crazily sinning articl, and I don t vn gt to choos my ath. I m stuck on this highway and it s on big circl (sort of). I start in th sam lac, and I nd u in th sam lac, and thn I start all ovr again. Basically, I am lashd onto this nuclus, and I can t gt away until som bttr rosct coms along or until I m sit out. I just k going around and around. I v got this srious attraction to my nuclus. Thr s somthing so ositiv about it, so lctrically alluring Danill Otis 1-15

BETA DECAY VISUAL PHYSICS ONLINE

BETA DECAY VISUAL PHYSICS ONLINE 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