Tuesday, December 7, 2010

What is an alpha particle?.

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Investigating the nature of alpha radiation took a Lot of careful experimental design involving electric and magnetic fields, electromagnetic forces and fine spectra.

It is more than likely that you know the alpha particle is a helium atom that has lost both its electrons. It took a lot I of experiments and inspiration to discover that knowledge, 100 years ago this year.

Beginnings--the Greek a, b, c

Radioactivity was discovered accidentally in 1896 by Henri Becquerel, working in France. He was studying the properties of the rays now known as X-rays, which had recently been discovered by Roentgen. He thought the emission of X-rays was connected to the phosphorescent glow observed on the side of the glass X-ray tube and so he studied phosphorescence, which is the ability of a substance to glow in the dark after exposure to light (like the numbers on an alarm clock dial). Uranium salts were known to possess this ability. Becquerel found that these salts, even if they had not been exposed to light, could darken covered photographic plates.

While other researchers concentrated on studying the newly discovered X-rays, Becquerel was almost the only scientist working on radioactivity before Ernest Rutherford took up the subject. Rutherford was a young New Zealander who had originally gone to Cambridge to work with J. J. Thomson (the discoverer of the electron) on extending the range of radio signals.

Rutherford discovered that there were two sorts of radiation emitted from the uranium salt he was studying, one component that was easily stopped, e.g. by a sheet of paper, and another that was more penetrating. He called these components respectively a (alpha) and [beta] (beta), the first two letters of the Greek alphabet. Rutherford thought that the two radiations might be two types of the X-rays, which were all the rage at the time, but soon decided this could not be the case as they carried an electric charge.

Paul Villard in Paris, also working on uranium, discovered an even more penetrating radiation which he named [gamma] (gamma) rays, following Rutherford's example of using successive letters of the Greek alphabet.

Electrons and ions

In 1897, J. J. Thomson measured the specific charge (charge per unit mass, e/m) of the electron by using crossed electric and magnetic fields. Around the same time, electrolysis experiments using water led to measurements of the charge on a hydrogen ion. Using these measurements, Thomson was able to suggest that the hydrogen ion and electron carried equal and opposite charge, but the ion had about 1800 times the mass of the electron. (Later, in a series of oil-drop experiments begun in 1909, Millikan confirmed Thomson's suggestion. He measured the electron charge e and so the mass m of the electron was determined as nearly [10.sup.-30] kg, now known to be 9.11 x [10.sup.-31] kg.)

What are [alpha] particles?

From 1898 to 1907, Rutherford worked at McGill University in Montreal, Canada. In 1907 he moved to Manchester University where he worked until he was invited to return to Cambridge in 1919. He remained there until his death in 1937.

In 1902 and in the years following, Rutherford investigated the mass and charge of alpha particles in an attempt to find out what they were. Like Thomson, he used electric and magnetic fields, but this time separately.

The equipment used in an experiment at McGill is shown in Figure 1. A thin layer of radium was placed below a vertical stack of thin, equally spaced brass plates, which ensured that only alpha particles travelling vertically emerged into a charged gold leaf electroscope. The radiation creates ions when it collides with the gas molecules and so allows the charge on the electroscope to leak away.



Before entering the electroscope chamber, the alphas emerging from the slits passed through a very thin aluminium foil. Because these particles are stopped by a sheet of paper, the foil had to be just 3 x [10.sup.-3]mm thick, thin enough to allow them through and thin enough to be porous to gases. This foil is necessary because, as radium decays, it emits the radioactive gas radon (or 'radium emanation' as it was then called) as well as alphas. This gas, if it entered the electroscope, would discharge it very quickly and hide the effect of the original alphas. To sweep the radon out of the electroscope, dry hydrogen gas is made to flow through the electroscope, through the porous foil and over the radium, carrying the radon away. Another reason for using hydrogen is that the alpha particles travel further and generate more ions in hydrogen than in air. An added effect is that, in hydrogen, beta or gamma rays generate fewer ions--so using hydrogen is a much better idea than just using air.

Effect of a magnetic field

Applying a big enough magnetic field perpendicular to the plane of the diagram, parallel to the slit system openings, deflects all the alphas so that they collide with the brass plates, which prevents them emerging into the electroscope and so stops the leaves collapsing (see Box 1). The percentage of particles deviated away from the electroscope is plotted as a function of the magnetic field strength.

Box 1 Magnetic deflection

A particle with charge q and moving at a velocity v at right angles
to a magnetic field B experiences a force Fat right angles to the
field direction and its velocity.


Figure 1.1 A stream of alphas moves in the arc of a circle when
in a perpendicular magnetic field.

This force will deflect it into an arc of a circle of radius r.

F = Bqv


Bqv = [mv.sup.2]/r


r = mv/Bq

By measuring how big B has to be to stop the particles getting
through the narrow path between the brass sheets, the
geometry of the slits will give q/m, the specific charge of the
Rutherford did all the subsidiary experiments you would expect. He measured the background rate of collapse of the leaves without any radioactive material present. He covered the radium with a sheet of mica thick enough to stop the alphas emerging but thin enough to allow the beta and gamma rays through and subtracted their effect. He added a metal plate on top of the slits (Figure 3), which covered half the opening.


Reversing the field direction alters the deflection of the beam, so instead of hitting A the rays emerge through B. This should produce a big change in the proportion of the rays that do not make their way through the openings.

Rutherford was thus able to show that a magnetic field deflects the rays in the opposite direction to that expected for cathode rays (made up of electrons)--that is, alpha radiation carries a positive charge.

Watch the sparks!

Electric fields could be applied by connecting together alternate plates of the slit assembly and connecting a voltage source across them. With 600 volts between the pairs of plates a difference of 7% in the rate of discharge was seen compared to that observed without the electric field. Rutherford could not apply a bigger voltage to give a greater effect because sparks jumped across the gap between the plates, which were now just 0.55 mm apart. In later experiments the plates making the slits were much closer but better insulated, and the emerging rays were detected by their effect on a photographic plate. This revised experiment allowed more accurate results to be obtained.


Results but not an answer

Boxes 1 and 2 show how Rutherford was able to calculate the speed of the particles making up the rays and also their specific charge, q/m. He measured a speed of 2.5 x [10.sup.7]m[s.sup.-1], about 10% of the speed of light. Just to make sure the alpha radiation from other radioactive material was the same as that from radium, similar experiments were carried out, using thorium and actinium as the sources.

The charge on the alpha particle had been measured by this stage--the source becomes steadily more negatively charged if it sits on an electrical insulator--because it emits alphas. Counting the particles emitted and measuring the charge left behind on the source allows this to be done. Knowing q/m and the charge q, you can get the mass.

The results implied that the alpha particle was either 'a molecule of hydrogen, an atom of helium or a helium molecule carrying twice the ionic charge', because q/m was about half that known for the hydrogen ion (obtained from electrolysis experiments) but it was 'not at present possible to decide definitely between these possibilities'.

Rutherford seems to have decided that doubly charged helium ions were the most likely candidates. Helium had recently been discovered in the Sun's spectrum. He had noted that helium was only found on Earth locked up in thorium or uranium ores and guessed it was there because trapped alpha particles had lost their electric charge and, as helium atoms, had become trapped. Final proof came in 1908.

Thin glass

To clinch the argument, Rutherford needed to show that the alpha particles, kept separate from whatever is producing them, form helium gas when they pick up electrons and become electrically neutral. While in Manchester, he set himself this task.

Most successful research work depends on good technical help. In Manchester, the university had an expert glassblower, Baumbach, who, after some trials, was able to make glass tubes with walls so thin (about [10.sup.-2] mm) that alpha particles could get through, yet strong enough to withstand atmospheric pressure. The alpha particles from Rutherford's source had three different energies and hence three ranges in air, 43 mm, 48 mm and 70 mm. The tube walls were so thin they reduced the range by only 20mm, so all three sets of alphas got through to the other side without the radioactive gas itself emerging.

Rutherford compressed the radioactive gas (radon) coming from 170 mg of radium into one of Baumbach's tubes, about 15 mm long (Figure 5). This tube was surrounded by a second tube, evacuated this time, 15 mm in diameter and 75 mm long. By raising the open reservoir of mercury, the level of mercury in the enclosed tube was forced to rise, so that any gas emerging from the thin-walled tube could be compressed into a small tube that was to be used as the light source for a spectrometer.


An electrical discharge was used to excite the collected gas. Rutherford was able to call on the assistance of Thomas Royds, who later became an eminent spectroscopist, to help him recognise the collected gas in the outer tube by its spectral lines (Figure 6). After 2 days, the yellow line of helium became visible when an electrical discharge was applied. After 4 more days all the stronger lines of helium could be seen in the spectrum. If air had leaked into the system the red neon lines would have been seen but none were, so there were no leaks.


Just to make sure

Rutherford was concerned that maybe helium from the radioactive source had diffused through the thin walls, so he replaced the radon with about 10 times its volume of helium and also changed the outer glassware. No helium was detected after 10 days, so diffusion could be ruled out. The helium was then pumped out and replaced by the radioactive source and helium was detected by its spectrum as before. The delay in seeing the spectrum worried the experimenters, but subsidiary experiments showed that the helium became trapped in the glass and escaped only slowly. Freshly distilled mercury was used to ensure there was no helium lurking there.


At last

Rutherford's portrait for the Royal Society (Figure 7) shows him standing proudly in front of this elaborate piece of glassware, Baumbach's pride and joy. The detection of helium in the outer tube allowed Rutherford and Royds to say: 'We can conclude with certainty from these experiments that the [alpha] particle after losing its charge is a helium atom. Other evidence indicates that the charge is twice the unit charge carried by the hydrogen atom set free in the electrolysis of water' (Philosophical Magazine, February 1909). Fittingly, while writing up the experiment with Royds, Rutherford received news that he had just won the Nobel prize--for chemistry, not physics, to his amusement.

Box 2 Electric deflection


Figure 2.1 Alpha particles are deflected by an electric field.

Applying a voltage V between two plates d apart, will produce an
electric field E acting at right angles to the plates.

E = V/D

A particle of charge q will experience a force F in the same
direction as the field, where

F = qE = qV/d

A positively charged particle will accelerate towards the
negative plate. The acceleration is given by

a = F/m = qV/dm

As the particle is travelling at velocity v between the plates,
the time t over which the force will act is

t = L/v

where L is the length of the plate. The small sideways velocity u
acquired in that time is therefore

u = at
= qVL / mvD

The sideways velocity caused by the voltage across the plates
will deflect the particles away from their original direction by an
angle [theta], where

tan[theta] = u/v

(see Figure 2.1). By measuring the voltage across the plates
needed to stop the particles emerging from the narrow gap
between the plates, another value of q/m, the specific charge,
can be calculated, because V, v, d, L etc. are known.
Derek Jacobs is an Honorary Visiting Fellow in the Department of Physics at the University of York and a member of the Editorial Board of PHYSICS REVIEW.

Source Citation
Jacobs, Derek. "What is an alpha particle?" Physics Review Feb. 2008: 18+. General OneFile. Web. 7 Dec. 2010.
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