Chapter 5 RADIOACTIVITY5.2 ANALYSING RADIOACTIVE DECAYA student is able tostate what radioactivity isname common detectors for radioactive emissionscompare the!inds of radioactive emissions in terms of their naturee"plain what radioactive decay isuse e#uations to represent chan$es in the composition of the nucleus when particles are emittede"plain half%lifedetermine half%life from a decay curvesolve problems involvin$ half%life5.2 ANALYSING RADIOACTIVE DECAYRADIOACTIVE DECAYRadioactivity is the spontaneous disintegration of an unstable nucleus into a more stable nucleus with the emission of ener$etic particle or photons&5.2 ANALYSING RADIOACTIVE DECAYRADIOACTIVE DECAYRadioactivity is a random and spontaneous process&It is said to be a random process because there is no way to tell which nucleus will decay' nor is there any way to predict when it is $oin$ to decay&A spontaneous process means the process is not tri$$ered by an e"ternal factor&5.2 ANALYSING RADIOACTIVE DECAYRADIOACTIVE DECAYA nucleus is unstable if it is too big&It is found that all nuclei with ( ) * or A ) +,- are unstable&5.2 ANALYSING RADIOACTIVE DECAYCHARACTERISTICS OF RADIOACTIVE EMISSIONThere are three main types of nuclear radiation emitteda& Alpha particles' b& .eta particles' c& /amma rays' 0ore than one type of radiation can be emitted at any one time durin$ a radioactive decay&5.2 ANALYSING RADIOACTIVE DECAYCHARACTERISTICS OF RADIOACTIVE EMISSIONTable below $ive the nature and fundamental properties of the three types of nuclear radiation&5.2 ANALYSING RADIOACTIVE DECAYe01 CHARACTERISTICS OF RADIOACTIVE EMISSIONTable below $ive the nature and fundamental properties of the three types of nuclear radiation&5.2 ANALYSING RADIOACTIVE DECAYCHARACTERISTICS OF RADIOACTIVE EMISSIONTable below $ive the nature and fundamental properties of the three types of nuclear radiation&5.2 ANALYSING RADIOACTIVE DECAY5.2 ANALYSING RADIOACTIVE DECAYCHARACTERISTICS OF RADIOACTIVE EMISSIONTable below $ive the nature and fundamental properties of the three types of nuclear radiation&5.2 ANALYSING RADIOACTIVE DECAYCHARACTERISTICS OF RADIOACTIVE EMISSIONTable below $ive the nature and fundamental properties of the three types of nuclear radiation&5.2 ANALYSING RADIOACTIVE DECAY1esser mass /reater massCHARACTERISTICS OF RADIOACTIVE EMISSIONAlpha dea! % 2hen alpha decay occurs' the radioactive parent nucleus losses two protons an two ne!trons which carry away ener$y&The $eneral e#uation for alpha decay35.2 ANALYSING RADIOACTIVE DECAYHe Y XAZAZ4242+ CHARACTERISTICS OF RADIOACTIVE EMISSION5.2 ANALYSING RADIOACTIVE DECAYHe U Pu422369224094+ Alpha dea! CHARACTERISTICS OF RADIOACTIVE EMISSION"eta dea! % 2hen beta decay occurs' a neutron disinte$rates into a proton and an electron accordin$ to the e#uationThe proton stays in the nucleus but the electron is shot out of the nucleus at hi$h speed&5.2 ANALYSING RADIOACTIVE DECAYe p n011110+ CHARACTERISTICS OF RADIOACTIVE EMISSIONThe $eneral e#uation for beta decay45.2 ANALYSING RADIOACTIVE DECAYe Pa Th012349123490 + e Y XAZAZ01 1 ++ CHARACTERISTICS OF RADIOACTIVE EMISSION#amma dea! % /amma ray is a type of ele"tro#agneti" raiation produced durin$ radioactive decays& /amma radiation is often emitted durin$ an alpha or beta decay&There is no chan$e in the proton number and nucleon number for a nuclide that emits a $amma ray& It is 5ust ma!es the nucleus more stable&5.2 ANALYSING RADIOACTIVE DECAYCHARACTERISTICS OF RADIOACTIVE EMISSION6"ample of $amma decay45.2 ANALYSING RADIOACTIVE DECAY + Dy Dy1526615266 + + He Pb Po422068221084 + + e Hg Au011988019879DETECTOR OF RADIATIONThe radioactive emission can be detected usin$ detector of radiation&5.2 ANALYSING RADIOACTIVE DECAYDETECTOR OF RADIATIONTable below summarises the detectors of radiation and the type of radioactive emission that can be detected&5.2 ANALYSING RADIOACTIVE DECAYDETECTOR OF RADIATION/ei$er%0uller 7/08 Tube5.2 ANALYSING RADIOACTIVE DECAYDETECTOR OF RADIATION/ei$er%0uller 7/08 Tube5.2 ANALYSING RADIOACTIVE DECAY9& The radioactive emission enters the tube throu$h the mica window and ionises the ar$on $as&DETECTOR OF RADIATION/ei$er%0uller 7/08 Tube5.2 ANALYSING RADIOACTIVE DECAY+& The electrons and positive ions are attracted towards the anode and cathode respectively&DETECTOR OF RADIATION/ei$er%0uller 7/08 Tube5.2 ANALYSING RADIOACTIVE DECAY& 2hen the electrons are collected by the anode' a pulse of current is produced&DETECTOR OF RADIATION/ei$er%0uller 7/08 Tube5.2 ANALYSING RADIOACTIVE DECAY:& The pulses of current are counted by a rate meter&5& The rate meter $ives the count rate in counts per second or counts per minute&DETECTOR OF RADIATION/ei$er%0uller 7/08 Tube5.2 ANALYSING RADIOACTIVE DECAY;& Initially the /0 tube is switched on without the presence of any radioactive substance& The readin$ displayed by the rate meter is !nown as the bac!$round count rate&DETECTOR OF RADIATION/ei$er%0uller 7/08 Tube5.2 ANALYSING RADIOACTIVE DECAY$uration is achieved&A radioactive decay series can be displayed on a $raph of nucleon number A' a$ainst proton number' (' or a $raph of number of neutrons' ?' a$ainst proton number' (&5.2 ANALYSING RADIOACTIVE DECAYRADIOACTIVE DECAY SERIESThe decay series above can also be represented as follows45.2 ANALYSING RADIOACTIVE DECAYRADIOACTIVE DECAY SERIES.elow are two e"amples of radioactive decay series&5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFERadioactive decay is a random and spontaneous process& This means that all the unstable nuclei do not decay at the same time& It is not possible to predict when a particular nucleus is $oin$ to decay&The nucleus will decay at di@erent times' some decay earlier while others will decay at a much later time&5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFETherefore the number of unstable nucleus that have not decayed decreases with time&5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFEThe half%life' T9=+ of a radioactive substance is the time for the number of undecayed nuclei to be reduced to half of its ori$inal number&5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFEThallium%+9, has a half%life of 9& minutes& This means that 9; $ of thallium ta!es 9& minutes to decay to * $ of thallium& The time ta!en for further decay to : $ of thallium is also 9& minutes&Aome radioactive nuclides have very short half%lives while others have very lon$ half%lives&5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFE5.2 ANALYSING RADIOACTIVE DECAYIt is possible to >nd out the half%life of a radioactive substance from a $raph of the "o!nt rate against ti#e& The $raph shows the decay curve for a radioactive substance&HA$F%$IFE5.2 ANALYSING RADIOACTIVE DECAYThe count rate drops from *, to :, counts a minute in two days' so the half%life is two days& In the ne"t two days' it drops from :, to +, % it halves& In the two days after that' it drops from +, to 9, % it halves a$ain % and so on&HA$F%$IFE6"ample 9Aodium%+: has a half%life of 95 hours& The ori$inal mass of the sample containin$ sodium%+: is ;: $&7a8 2hat is the mass of the sodium%+: in the sample after :5 hoursB5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFE6"ample +A radioactive substance has an initial activity of -;, counts per second& 2hat is the half%life of the substance if its activity becomes 9+, counts per second after 9;* sB5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFE6"ample A sample of iodine%99 has a half%life of * days and an initial activity of *,, counts per second& 2hat is the activity of iodine%99 after 9; daysB5.2 ANALYSING RADIOACTIVE DECAYHA$F%$IFE6"ample :A scientist measures the count rate from a radioactive substance over a period of +, minutes& The table shows the results of his measurements&a& Crom the table' estimate the half%life of the substance&5.2 ANALYSING RADIOACTIVE DECAY5 minutesHA$F%$IFE6"ample :A scientist measures the count rate from a radioactive substance over a period of +, minutes& The table shows the results of his measurements&b& Dow many half%lives elapsed durin$ the +, minute e"perimentB5.2 ANALYSING RADIOACTIVE DECAY4 half-lifeHA$F%$IFE6"ample :A scientist measures the count rate from a radioactive substance over a period of +, minutes& The table shows the results of his measurements&c& If the scientist had ta!en readin$s for +5 minutes' what mi$ht the count rate have been at the end of his e"perimentB5.2 ANALYSING RADIOACTIVE DECAY25 counts/sHA$F%$IFE6"ample 5The >$ure shows the decay curve for a radioactive sample& a& 2hat is the half%life of the sampleB 5.2 ANALYSING RADIOACTIVE DECAY25 minutesHA$F%$IFE6"ample 5The >$ure shows the decay curve for a radioactive sample& b& Atate the value of T& 5.2 ANALYSING RADIOACTIVE DECAYTE75