IAS Course Notes: PHYSICS 27. Nuclear Physics

DIPONT Educational Resource - Science

1

27. Nuclear Physics__________________________ Content 27.1 The nucleus [AS] 27.2 Isotopes [AS] 27.3 Nuclear processes [AS]

Learning outcomes_____________________________________ Candidates should be able to:

(a) infer from the results of the α-particle scattering experiment the existence and small size of the nucleus

Evidence…

1. Geiger-Marsden experiment 2. Very small number repelled by nucleus 3. Most pass straight through 4. Some repelled by positive nucleus.

Source of alpha particles

Beam of alpha particles

Gold Foil (10-8m thick)

~1:8000 repelled back

Most pass straight through

Some are deviated through large angles

DetectorGeiger and Marsden’s Experiment

~1:8000 repelled back

Some are deviated through large angles(the +ve alpha particles are deviated by the +ve nucleus)

Gold Atom

+ve nucleus

IAS Course Notes: PHYSICS 27. Nuclear Physics

DIPONT Educational Resource - Science

2

(b) describe a simple model for the nuclear atom to include protons, neutrons and orbital electrons

Protons Neutrons Electrons Relative Mass 1 1 Negligible Charge +1 Neutral -1

(c) distinguish between nucleon number and proton number

Nucleon number A: The number of nucleons (Protons and Neutrons) Proton number Z: The number of protons in the nucleus Neutron number N: The number of neutrons in the nucleus

(d) show an understanding that an element can exist in various isotopic forms,

each with a different number of neutrons

Nuclide: An atom with a particular number of protons and neutrons Isotope: Isotopes are nuclides that contain the same number of protons, but

different numbers of neutrons. Nucleon: Component of the nucleus = Protons and Neutrons

IAS Course Notes: PHYSICS 27. Nuclear Physics

DIPONT Educational Resource - Science

3

(e) use the usual notation for the representation of nuclides

XA

Z

Chemical Symbol

Nucleon number

Proton number

C12

6Carbon

12 nucleons

6 protons

NOTATION

EXAMPLE

(f) appreciate that nucleon number, proton number, and mass-energy are all conserved in nuclear processes In nuclear processes:

• Nucleon number, proton number and mass-energy are all conserved (g) represent simple nuclear reactions by nuclear equations of the form 14

7N + 42He --- 178O + 11

H

Alpha decay…. Example…. α4

242 +→ −

− YX AZ

AZ α4

223793

24195 +→ NpAm

Beta decay νβ ++→ −+

011YX A

ZAZ νβ ++→ −

01

9039

9038 YSr

Gamma decay γ0

0* +→ XX A

ZAZ

(h) show an appreciation of the spontaneous and random nature of nuclear decay

Natural radioactive decay is the RANDOM process where an atom in a sample of material emits alpha ( α ), beta (β ) or gamma ( γ ) radiation, as the atom is unstable. The process cannot be predicted or affected by, for example, temperature.

IAS Course Notes: PHYSICS 27. Nuclear Physics

DIPONT Educational Resource - Science

4

(i) show an understanding of the nature and properties of α-, β- and γ- radiations (β+ is not included: β- radiation will be taken to refer to β–)

Property Alpha Beta Gamma Description Helium nuclei Electron from

the nucleus Electromagnetic radiation

~Ionising power High Medium Low Penetration (absorbed by)

Low (paper) Medium (5mm Al)

High (Thick lead)

Charge +ve -ve None (j) infer the random nature of radioactive decay from the fluctuations in count rate The random nature of radioactive decay can be seen in the random fluctuations in count rate (e.g. using a Geiger counter to measure radioactivity)