ProtonAcceleration

Particle Acceleration

We have previously spoke of the Cathode Ray Tube, the device that make oscilliscope and ECG machines (and older Televisions) work. In that tube, electrons are accelerated towards a screen to convey a signal sent. This is a very basic form of acceleration, we can see a similiarity in the the electron and Proton accelerators work.

It is very easy to 'liberate' electrons, we can do this using thermionic emission.

The proton

So how do we get a proton, where do they come from?

Well all atoms are made up of a nucleus, in here, we find protons and neutrons. If we want to isolate a proton on its own, which element do you think would be the easiest to get on from?

Hydrogen is usually represented in the following way

You should know what the numbers mean at this stage. The top one is the mass number (= the number of protons + the number of neutrons). The number at the bottom is the atomic number the number of protons. Therefore the Hydrogen atom has just one proton (usually), and therefore one electron too.

If we can strip the electron (ionise) from the hydrogen atom we will be left with just a proton.

energy approximately 1 MeV:

outline of experiment.

First artificial splitting of nucleus.

Ernest Walton was one of the legendary pioneers who made 1932 the annus mirabilis of experimental nuclear physics. In that year James Chadwick discovered the neutron; Carl Anderson discovered the positron; Fermi articulated his theory of radioactive decay; and Ernest Walton and John Cockcroft split the nucleus by artificial means. In their pioneering experiment Cockcroft and Walton bombarded lithium nuclei with high-energy protons linearly accelerated across a high potential difference (c. 700 kV). The subsequent disintegration of each lithium nucleus yielded two helium nuclei and energy. Their work gained them the Nobel Prize in 1951.

Cockcroft and Walton – proton

(i) Draw a labelled diagram to show how Cockcroft and Walton accelerated the protons. (6)

The Disharge tube has plenty

this figure is taken from

http://www.fnal.gov/pub/news03/update_archive/update_10-20_10-22.html

http://en.wikipedia.org/wiki/Ernest_Rutherford

What is more this apparatus is stil used in the Fermilab particle laboratories to preaccelerate and separate particles. But the accelerator was not the only significant achievement; the high voltage multiplier was just as important.

A picture of the original Voltage Multiplier http://en.wikipedia.org/wiki/File:Cockcroft%E2%80%93Walton_generator.jpg

• It uses Capacitors and Diodes,
• the Silver rings on the legs house rather large capicitors,
• the rolling pin brown diagonal stutts are diodes. Going from +ive @ base → -ive @ top
• The electricity input is A.C. the output is a constant D.C.
• Each Cascade doubles the Voltage that comes into it.
• Large voltages can be achieved with little cost.

Draw out the circuit diagram for this setup

What is the velocity of a proton when it is accelerated from rest through a potential difference of 700 kV? (12)

First transmutation using artificially accelerated particles.

Appropriate calculations.

How much energy is given off ?

Calculate the masses of the reactants and then of the products. Are these masses the same? If not then the energy given off can be found by using Einstein's equation

The following table is many of the constant values you will require.

charge on electron = 1.6022 × 10^–19 C;

mass of proton = 1.6726 × 10^–27 kg;

mass of lithium nucleus = 1.1646 × 10^–26 kg;

mass of helium nucleus = 6.6443 × 10^–27 kg;

mass of alpha particle = 6.6447 × 10^–27 kg

speed of light = 2.9979 × 10⁸ m s^–1,

Planck constant = 6.626 × 10^–34 J s;

The law of conservation of momentum holds at the subatomic level too.

The Law states the momentum of all the involved particles before an interaction is equal to their momentum after.

This did not seem to hold true. The velocities of the particles were not usually adding up to the speeds predicted by the conservation of momentum. Wolfgang Pauli predicted there were some very small neutral particles that are also emitted. This would account for the differences in momentum. It was very difficult to detect a small and neutral particle. It was named the Neutrino, after Chadwick had stolen its original name, the neutron for the large neutral particle.

A transmutation is the changing of a nucleus of one atom into a nucleus with a different atomic number (i.e. the changing of an atom of one element into an atom of another element)

Irish Nobel laureate for physics,

Professor E. T. S. Walton (1951).

http://understandingscience.ucc.ie/pages/sci_ernestwalton.htm

Following the link you can hear Albert Einstein pay respect to Cockcroft and Walton for their work.

http://www.pbs.org/wgbh/nova/einstein/lega-audio.html

Other more modern accelerators

Check out this Linear Accelerator at Stanford,

http://maps.google.ie/maps?q=Stanford+Linear+Accelerator+Center,+San+Mateo,+CA,+United+States&hl=en&ll=37.414243,-122.209682&spn=0.026928,0.038581&sll=53.344104,-6.267494&sspn=0.323848,0.617294&oq=stanford+linear+acc&hq=Stanford+Linear+Accelerator+Center,+San+Mateo,+CA,+United+States&t=h&z=15

• Opened in 1962
• built the world’s longest particle accelerator,
• discovered some of the fundamental building blocks of matter
• created the first website in North America
• accelerate electrons to nearly the speed of light

The more modern and more powerful choice for accelerators is the Circular Accelerator or the Synchrotron

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