section 18.3 radioactive decay

8/14/2019 Section 18.3 Radioactive Decay

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Section 18.3Section 18.3

Radioactive decayRadioactive decay

• The cause of radioactivityThe cause of radioactivity

• Random nature of decayRandom nature of decay• Decay curveDecay curve

• Half-lifeHalf-life




The cause of radioactivity

electrostatic repulsion

strong attractive nuclear force

For stable nucleus

18.3 Radioactive decay (SB p. 20)

proton neutron

proton proton




The cause of radioactivity

Radioactive decay occurs in unstable nuclides

For a stable nucleus,

N (neutron no.)

Z (proton no.)511 .≤≤

Z

N


Otherwise,nucleus → unstable

→ breaks up

→ emits radiation → stable




Random nature of decay

— no. of disintegrations / s

1 Becquerel (Bq)

= 1 disintegration per second


activity




Throwing a dice

Decay process similar to throwing a dice


probability of getting any one face = 1/6




Dice decay analogue

Expt 18D Dice

decay analogue

undecayed nucleus

decayed nucleus


http://movie/Expt%2018D/18D.avi




activity ∝ no. of undecayed nuclei


Dice decay analogue

Radioactive decay — random process

W h e n ?

W h i c h ? unpredictable




Decay curve

decay curve

count rate:

no. of counts recorded / s


corrected count rate:

measured count rate− background count rate




Decay curve

A typical decay curve

corrected count rate falls

exponentially with time

Time / s 0 10 20 30 40

Corrected count rate / s−1 800 400 200 100 50





Half-life

— time for half the sample nuclei decay


e.g., half-life of randon = 3.8 days

2 000 000 / 2 1 000 000 / 2 500 000 / 2

half-life




Half-life


CAL

Workshop 2

Decay curve

and half-life

http://cal/E-SimPhy_502.exe




Half-life

shorter half-life → decays faster





Example 4:Example 4:

The following graph shows the decay curve of

protactinium-234.

(a) What is the background count rate? Solut

ionThe background count rate is 2 counts per second.





Example 4: (Cont)Example 4: (Cont)

(b) Determine the half-life of protactinium-234.

The graph of corrected count rate against time is shown below.

From the graph, the half-life of protactinium-234 is

about 56 s.


Solut

ion




Half-lives of some typical radioactive substancesRadioactive substance Half-life

Polonium-214 0.000 164 second

Radon-222 3.82 daysCobalt-60 5.3 years

Radium-226 1 600 years

Carbon-14 5 600 yearsUranium-238 4.5 × 109 years


Half-life




Larger sample more dangerous

• short half-lives havehigh initial count rates


Half-life

• very long half-lives not

very radioactive BUTprolonged effect

Are the radioactive substances hazardous




Class Practice 4:Class Practice 4:

A student uses a GM counter to measure the radiationemitted by actinium-228 nuclei. He does so by recording

the count rate of actinium-228 at every 30-minute interval.

The background count rate is found to be 5 counts per

second. The following table shows the results.

Time / min 0 30 60 90 120 150

Count rate / count s−1 410 299 215 160 125 90





Class Practice 4: (Cont)Class Practice 4: (Cont)

(a) Complete the following table.

Time / min 0 30 60 90 120 150

Corrected count rate /count s−1

Ans

wer

405 294 210 155 120 85






(b) Plot a graph showing the corrected count rate due to

actinium-228 against time. Determine the half-life of

actinium-228.

From the graph,

the half-life of actinium-228 is

____________ minutes.

Ans

wer

65


Ans

wer





(c) Explain briefly why not all the points lie on the decay

curve. Ans

wer

Since radioactive decay is a random process,

there are fluctuations in the number of decayed

nuclei. Thus, not all the points lie on

the decay curve.





To section 18.4

http://e-ch18_04.ppt/

http://e-ch18_04.ppt/

section 18.3 radioactive decay

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