7.7 Particle Detectors

Question 1

What is the basic principle behind most particle detectors?

Answer 1

The fact that charged particles at high energies cause ionisation through materials which they pass

Question 2

What happens during ionisation?

Answer 2

Electrons are knocked out of the atoms due to high energies of changed particles

Question 3

What are the two methods of particle detection that I have to know about?

Answer 3

Cloud chambers

Bubble chambers

Question 4

How do cloud chambers work?

Answer 4

These make use of super cooled vapour, a gas below its usual condensation temperature. The ions created make the vapour condense so we get vapour trails.

Question 5

How do bubble chambers work?

Answer 5

Bubble chamber has liquid hydrogen above boiling point by putting it under pressure. Ions suddenly reduce the pressure causing bubbles of gas along the trail of ions.

Question 6

How can we make the liquid hydrogen go back to original form after detection?

Answer 6

By increasing the pressure again the bubbles will collapse

Question 7

Do bubble chambers use a magnetic field ? Why

Answer 7

Yes, so that charges, velocity and hence momentum can be determined

Question 8

Why would an alpha particle leave thicker heavier, but shorter ion tracks than a beta particle ?

Answer 8

Alpha is more ionizing due to greater energy transfer

Question 9

Why do beta particles have a range of different curvatures show up on the particle detectors?

Answer 9

As they can take on a range of energies when emitted due to the fact it is a part of a three body state hence energy can take on a range of values

Question 10

In particle detectors why will paths of differently charged particles be in opposite directions?

Answer 10

Due to flemings left hand rule

Question 11

Why do charged particles often leave a spiral path?

Answer 11

Energy is lost when ionising the materials, hence the radius of circular motion decreases. So Ek is proportional to the radius.

Question 12

Another type of detector?

Answer 12

A spark chamber

Question 13

What does the spark chamber rely on?

Answer 13

Detecting sparks produced between the layers of metal foils.

Question 14

Why do particles usually travel in spirals in particle detectors?

Answer 14

• each ionisation requires energy to be transferred from the kinetic energy store
• the radius of spiral decreases when looking at r = p/bq as electron path is proportional momentum and as velocity decreases so does the momentum

Question 15

Where is the deflection of charged particles by magnetic fields used in real life?

Answer 15

Mass spectrometers

Question 16

What is the spark chamber a forerunner of?

Answer 16

A drift chamber

Question 17

How can we work out the direction of magnetic field, charge of particle or the direction of centripetal force in a particle detector ?

Answer 17

using flemings left hand rule

Question 18

What are mass spectrometers used for? and how

Answer 18

Analysis of chemical samples,
The process:
- ions enter a magnetic field with the same velocity and are deflected by the magnetic field. The radius of the deflection varies based on their m/q ratio(from r = p/bq), as v and B are kept constant.
- by measuring detection and ratio mentioned above the identity can be determined

Question 19

How to decays usually show up on particle detectors?

Answer 19

As a V shape out of nowhere.

Question 20

Why is it a V shape?

Answer 20

To conserve momentum (which is high for both emitted particles/elements)

Question 21

Does it have to be out of nowhere?

Answer 21

No, as alpha particles can also decay

Question 22

Except momentum in nuclear interactions what else has to be conserved?

Answer 22

The mass-energy interchange

Question 23

When a neutral element is emitted with high energy so that it is to decay again, the distance from emission to decay point depends on what? (this distance doesn’t show up on particle detectors due to no charge on element)

Answer 23

It depends on the half life. The longer the half life the further the particles are likely to travel.

Question 24

What is the however with the above question?

Answer 24

If the emitted particles or nuclei travel close to speed of light they’re likely to experience relativistic time dilation.

Question 25

What does this mean?

Answer 25

That time seems to run more slowly for particle than for stationary observer and so they seem to survive for much longer.

Question 26

What do we assume when looking at bubble chamber/cloud chamber photographs?

Answer 26

• Straight lines that go straight through the photograph will not be used in analysis
• Little spirals coming off straight lines, shows knock off electrons(from liquid hydrogen). These can tell us which way the particle is going and hence the direction of magnetic field
• Several curved tracks coming from a point means a reaction happens there

Question 27

What are some modern particle detectors?

Answer 27

Drift chambers
Scintillation counters
Solid State Detectors

Question 28

What do these modern detectors do?

Answer 28

Send electrical signals to computer

Question 29

If in a photograph of bubble chamber we have two similar paths curving the same way what can we assume?

Answer 29

they have the same sign of charge, they can be equally charged and hence would have equal momentum. If not equally charged then different momentums. (for example if they have equal charge but the radius of one is half radius of other then the one with half radius will have double the momentum )

Question 30

if a photograph of bubble chaaber had a straight line barely deflecting what can we assume?

Answer 30

The momentum is very high

Question 31

After a collision if we only have one charged partcile coming off in a photograph of bubble chamber what do we assume?

Answer 31

A neutral particle has been emitted