Nuclear Physics (Decay)

Question 1

What is the definition of radioactive decay?

Answer 1

Spontaneous disintegration of a nucleus to form a more stable nucleus

Resulting in the emission of alpha/beta/gamma rays

Question 2

How can the random nature of radioactive decay be proved?

Answer 2

Observing count rate of Geiger muller tube

When near radioactive source counts are irregular == cannot be predicted

(Note: the count rate is a measure of the decay of unstable nucleus)

Question 3

Describe how the closest approach can be used to estimate the radius of a atomic nucleus (long)

Answer 3

During Rutherfords experiment alpha particles are fired at a thin gold foil

These particles have an initial kinetic energy as moves towards positive nucleus

At the point of closest approach the electrostatic force reduces speed of the alpha particle to zero immediately

At this point kinetic energy is zero and electrical potential energy is equal to the inital kinetic energy

== Calculate using equation given

Question 4

Describe the forces/energy at the point of closest approach

Answer 4

Maximum electromagnetic NOT STATIC force of repulsion is felt

Minimum kinetic energy

Maximum potential energy

Maximum acceleration

Question 5

Advantages of closest approach method?

Answer 5

• Good estimate of upper limit of nuclear radius
• Simple mathematics
• Alpha particles are scattered only by protons

Question 6

Disadvantages of closest approach?

Answer 6

• This method doesn’t give accurate result (always over estimate since measures distance not radius)
• Alpha particles are hadrons therefore are affected by strong nuclear force
• Alpha particles in beam must all have the exact same inital kinetic energy

Question 7

Why is electron diffraction preferred over closest approach?

Answer 7

Because electrons are leptons therefore they will not interact via strong nuclear force

But alpha particles will since they are hadrons

Therefore electron diffraction far more accurate

Question 8

During electron diffraction explain why the electrons must have high speed

Answer 8

Electrons must have a high speed so that there de broglie wavelength is around 10^-15

Gap = wavelength gives most diffraction

Question 9

What are electrons diffracted through during electron diffraction?

Answer 9

Crystal lattice

Question 10

Describe the pattern given by electron diffraction

Answer 10

Concentric circles with a central bright spot

This gets dimmer as you move away from the centre

Question 11

Describe the graph of intensity against angle

Answer 11

• Peak at maximum in centre of graph
• Sharply decreases for other maximums

Question 12

Where is the first minimum on the graph of intensity against angle?

Answer 12

The first dip closest to max

Question 13

What is the general radius of a nucleus

Answer 13

1x10^-15

Question 14

How can you calculate the radius of any nucleus

Answer 14

Radius = (1.4x10^-15) x (Nucleon number)^1/3

Question 15

As the nucleon number increases, what happens to the nucleon number

Answer 15

Increases

Question 16

Since nuclear density is constant, what is its general calculated value

Answer 16

1.45 x 10 ^ 17

(revise derivation of equation for nuclear density btw)

Question 17

Describe Rutherford’s apparatus

Answer 17

• Alpha source and gold foil in an evacuated chamber
• Covered by a fluorescent coating

(The fluorescent coating is so you can see where the alpha particles hit inside the chamber)

-Microscope moved around outside chamber to observe path of alpha particles

Question 18

What were the conclusions from the Rutherford experiment?

Answer 18

• Most alpha particles passed straight through the foil with no deflection
  = Suggested atom is mostly empty space
  (Not uniform density as suggest by thompson model)

A small amount of particles were deflected by a large angle - this suggested that the
centre of the atom is positively charged, since repelled

● Very few particles were deflected back by more than 90° - this suggested that the that the centre of the atom was very dense as it could deflect fast moving alpha particles but also very small as only small amount deflected

Question 19

General conclusion of atom following Rutherford experiment

Answer 19

Atom has a small, dense, positively charged nucleus at its centre

Question 20

3 uses of gamma radiation

Answer 20

• Detector = radioactive source with a short half life to reduce exposure = injected into patient detected using gamma cameras
• Sterilise surgical equipment
• Radiation therapy = kill cancerous cells in targetted area however damages healthy cells

Question 21

Why is alpha radiation so dangerous

Answer 21

It is highly ionising

= If ingested it can ionise body tissue

Question 22

Explain why Technetium 99 is used as a suitable source of radiation in medical diagnosis

Answer 22

• Only emits gamma rays
• Which are weakly ionising
• Therefore will not cause harm to tissues in body
• Short half life
• Will not remain active in body after use
• However remain active long enough during diagnosis

Question 23

Why was Rutherford experiment in a evacuated chamber (vacuum)?

Answer 23

• To ensure that any collisions the alpha particles undertook where due to the gold foil and not any other substance such as air