history part

There are 2 type of cloud chambers : the Wilson expansion cloud chamber (1912) and the Langsdorf diffusion cloud chamber (1939). The Wilson chamber did most of the discovery of particles which originated in cosmic rays, until 1950. The diffusion chamber didn’t make such discovery because it only worked in an horizontal plane. As the cosmic particles come « from above », this chamber was not suitable for it. This one was principally used with accelerators, as the incoming particle are produced in an horizontal plan. But diffusion chamber was later superseded in 1952 with a new type of detector, the bubble chamber. With the advent of modern particle detectors, cloud chambers disappeared. 

table discovery cloud chamber2

 

To understand how the cloud chambers contributed to the particles physics, we have to remember what were the available particles detector in the last century.


 

Particles detector available between 1900-1950

1896 : Photographic plates

Glass photographic plate invented in 1851 where used to capture energetic radiations such as X-rays or α,β particles.

Discovery of X-ray. On the evening of 8 November 1895, while working in a carefully darkened room, Rontgen noticed that a piece of a cardboard coated with barium platinocyanide showed a faint, flickering, greenish light (fluorescence) when electrical discharges took place in a Hittorf-Crookes tube near the screen. The tube itself was carefully covered with a black shield impervious to any visible light. Rontgen verified that the tube was the source of a new kind of radiation, which was invisible but which revealed its existence when hitting the luminescent screen. Rontgen subsequently performed many careful experiments on the radiation which he named X-rays. His first important step was to replace the fluorescent screen by a photographic plate- this was susceptible to X-rays and thus provided a tool for recording X-ray pictures.

 roentgen1

Discovery of radioactivity. February 26, 1896, was an overcast day in Paris — and that presented a problem for French physicist Antoine Henri Becquerel. Becquerel was hoping to demonstrate a link between minerals that glow when exposed to strong light and a new type of electromagnetic radiation called X-rays. The weather thwarted this experiment — but that failure inadvertently produced an entirely new discovery: natural radioactivity.

Becquerel was interested in the phenomenon of fluorescence, in which some materials glow when exposed to sunlight. Physicist Wilhelm Röntgen had recently discovered X-rays; Becquerel thought the two phenomena might be connected, and had designed an experiment of his own. He planned to expose a fluorescing material to the sun, and then place it and a metal object over an unexposed photographic plate. If the developed plate showed the image of the object, he concluded, that would suggest that fluorescing materials are actually emitting X-rays.

But the next day was cloudy as well, and Becquerel was forced to postpone his experiment. He wrapped his fluorescing crystals — a uranium compound called potassium uranyl sulfate — in a black cloth, along with the photographic plate and a copper Maltese cross, and waited for a sunnier day.

Several days later, when Becquerel finally removed the plate from the drawer, he discovered to his surprise that a distinct image of the cross appeared on the plate — although it had never been exposed to sunlight (image below, at left). The only conclusion was that the crystals themselves were emitting radiation. Excited by this prospect, Becquerel decided to repeat the conditions of his unintentional experiment: He again placed a crystal of uranium salt on a photographic plate; he also experimented with putting a crystal on a photographic plate with a sheet of aluminum between, and with a sheet of glass.

After being placed in the dark for several hours, all three plates were blackened by radiation (the crystal in direct contact with the plate showed the strongest blackening). “I am now convinced that uranium salts produce invisible radiation, even when they have been kept in the dark,” he wrote in his diary of his experiments.

 becquerel1

You can make you own photograph of radioactive source using a Polaroid film. The above, right picture show the radioactive print of a Thorium mantle and some autunite mineral, after an exposition of several days in a dark place.

 

1896 : Electrometer and Ionisation chamber

An electrometer is an electrical instrument for measuring electric charge. The first electrometer come from 1787 (Gold-leaf electroscope). Basically, it’s a voltmeter : the deflection of a needle is proportionnal to a difference of potential applicated to the electrometer. It wasn’t until the development of the Dolezalek electrometer in 1896 that a device was available with the sensitivity needed to measure the very small currents (picoamps) associated with the ionization chambers used to measure radioactive samples. This electrometer used a suspended moving vane (or needle) inside, but not in physical contact with, a metal “pill box” shaped device consisting of four quadrants. Each quadrant was electrically connected to the quadrant diagonally opposite it so that they had the same charge.  One pair of quadrants had a positive charge and the other pair had a negative charge.  The electrical charge on the vane caused it to take up a particular orientation within the quadrants. If the potential difference between the vane and the quadrants changed, the vane and the mirror would rotate. 

electrometer2

To determine the position of the vane, a beam of light was shone through the window in the electrometer case so that it reflected off the mirror onto a scale usually positioned one meter away.  As the current from the ion chamber changed the potential difference between the vane and the quadrants, the vane rotated and the reflected beam of light moved across the scale.  The time requited to move across a specified number of divisions on the scale could be related to the activity of the sample by calibration with a known source. 

A device was needed to charge the needle of the electrometer : a current source, coming from an ionisation chamber, submitted at the effect of a radioactive source.

Within a couple of months of Roentgen’s discovery of X-rays, J.J Thomson demonstrated that X rays could make a normally insulating material, such as air, conductive. With Rutherford in 1896,  they made an experiment in which they exposed a gas to X-Rays. The irradiated gas was blowing to an electrically charged electrode, connected to a quadrant electrometer. They show that the charge of the electrode was leaking and concluded that X-rays can ionize gas (as ions conduct electricity, the charged wire was discharged to the ground by the ionized gas). Plus, the rate of leak of charge from the initially charged central electrode could serve as a measure of the X-rays intensity. At first their ion chamber was employed to measure the intensity of X-rays beams, but Rutherford soon extended this technique to the analysis of uranium and other radioactive materials (1899).

rutherford thomson

Rutherford used an electrometer to measure an electric current created by the radiation of Uranium in an ionization chamber. A uniform layer of powered uranium compound was spread on plate A and the rays allowed to ionize the gas between plates A and B. The amount of ionization is measured by the « saturation current » received at B when the potential difference between A and B is great enough to pull all the ions to the plates before they are able to recombine.

Rutherford then proceeded to cover the uranium with aluminum sheets of various thicknesses and measure the current, using the electrometer and find that there were at least two different « rays » being emitted by the uranium. He called them α and β.

 

1903: Spinthariscope

 The first instrument that was able to detect individual rays was the spinthariscope, invented by Crookes in 1903. While observing the apparently uniform fluorescence on a zinc sulfide screen created by the radioactive emissions (mostly alpha radiation) of a sample of radium bromide, he spilled some of the sample, and, owing to its extreme rarity and cost, he was eager to find and recover it. Upon inspecting the zinc sulfide screen under a microscope, he noticed separate flashes of light created by individual alpha particle collisions with the screen. Crookes took his discovery a step further and invented a device specifically intended to view these scintillations. It consisted of a small screen coated with zinc sulfide affixed to the end of a tube, with a tiny amount of radium salt suspended a short distance from the screen and a lens on the other end of the tube for viewing the screen. Crookes named his device from Greek « spark ». You can build a spinthariscope using an Am241 source and a ZnS:Ag screen available on ebay.

spinthariscope

Thus a very simple method for counting individual alpha particles was available (but it imposed considerable strain and fatigue on the observers). The method was refined and used most profitably by Rutherford and his students and was extensively used for many years until the 1930s. However Rutherford said about this visual counting method :

In considering a possible method of counting the number of a-particles, their well-known property of producing scintillations in a preparation of phosphorescent zinc sulphide at once suggests itself. With the aid of a microscope, it is not very difficult to count the number of scintillations appearing per second on a screen of known area when exposed to a source of a-rays. The doubt, however, at once arises whether every a-particle produces a scintillation, for it is difficult to be certain that the zinc sulphide is homogeneous throughout.

So in 1908 Rutherford & Geiger found an alternate method for counting single alpha particle, without the fatigue of observation and possible losses due to the quality of the scintillating screen. 

 

 1908:  Rutherford-Geiger Counter

Until 1908 the intensity of radiation was measured with an ionisation chamber coupled to an electrometer. This device was insensitive to single rays and can only detect « bulk » radiations. Rutherford wanted to measure the current induced by a single alpha particle (in order to estimate the number of alpha particles produced by a radioactive compound, and thus, calculating it’s mass activity).  The detection of a single particle producing a small direct electrical effect in the ionisation chamber, would require a very sensitive electrometer, isolated from any disturbance. As the current is very low, the movement of the electrometer’s needle is very thin (about 0.3 mm) thus the repeatability and  the fiability of this counting method is not demonstrated.

Rutherford and Geiger in 1908 produced a remarkable article about a new electrical method of counting. This method automatically magnified several thousand times the electrical effect due to a single α-particle which gave easily measurable movement of the needle of an ordinary electrometer. They still used an ionisation chamber but this time the electric field between the electrode was very high, just below the dielectric breakdown of air and the pressure inside the chamber was few mm of mercury (previous ionization chamber were at atmospheric pressure, 760 mm Hg). This apparatus follow the research of J.S Towsend who found the Avalanche Effect in 1902.

dfgdg

The Rutherford-Geiger counter of 1908

This counter was the first approach of the final version of the Geiger-Muller counter of 1928 as a practical instrument. The apparatus of 1908 was not widely used as a counter because of it’s unexplained spurious discharges in absence of alphas. This was interpreted as an instability of the instrument as it’s was not clear if a discharge came from a ‘real’ counting events of α- or β-radiation or if it’s was due by the instrument itself. In 1928 it was discovered by Muller, who isolated the device from the exterior, that these unexpected discharges were triggered by cosmic rays (discovered in 1912). So in 1928, Geiger-Muller improved the apparatus to a stable experimental setup with reliable and reproducible results.

 

 1911 :  Wilson Expansion Cloud Chamber

 In 1895 C. T. R. Wilson wanted to reproduce in the Cavendish laboratory the rainbow halo that he saw during a trip on the summit of Ben Nevis (picture below). He made some experiments for this purpose – making clouds by expansion of moist air after the manner of Aitken. He found that after dust particles are removed, cloud formation will not occur until the volume expansion ratio equals 1.25. If the expansion ratio was beyond 1.25, dense clouds were formed in the dust-free air. The number of drops in the shower showed no diminution however often the process of producing the shower and allowing the drops to fall was repeated. It was evident then that the nuclei were always being regenerated in the air. Note : we know nowadays that theses « nuclei » are in fact ions which act as cloud condensation nuclei (like dusts and aerosols). These ions are produced continuously in the atmosphere with the cosmic rays, and are able to discharge electroscope, a mystery of early 1900 which was solved in 1912 with the discovery of cosmic rays.

wilson inspiration

In early 1896, few months after the discovery of X-rays and using a primitive form of X-rays tube, he exposed X-rays to his cloud chamber : this greatly increased the number of droplets during an expansion. The X-rays produced more « nuclei » of the same kind as were always being produced in very small numbers in the air within the cloud chamber. In 1899, he demonstrated that the condensation nuclei produced by X-rays were shown to be ions by their behavior in an electric field. He also noticed that the least expansion required (1,25) to condense water in ionized air had all been concerned with the negative ion; to catch the positive ions the expansion ratio had to exceed a limit of about 1.31. Wilson then studied the conductivity of air until 1904 and did some research about atmospheric electricity and thunderstorms. It was not until 1910 that he again worked actively at condensation phenomena which led to his discovery of the Wilson Cloud chamber. As the corpuscular nature of α and β radiations was established, he had in view the possibility that the track of an ionizing particle might be made visible and photographed by condensing water on the ions which it liberated.

Much time was spent in making tests of the most suitable form of expansion apparatus and in finding an efficient means of instantaneous illumination of the cloud particles for the purpose of photographing them. In the spring of 1911 tests were still incomplete, but it occurred to me one day to try whether some indication of the tracks might not be made visible with the rough apparatus already constructed. The first test was made with X-rays, with little expectation of success, and in making an expansion of the proper magnitude for condensation on the ions while the air was exposed to the rays I was delighted to see the cloud chamber filled with little wisps and threads of clouds – the tracks of the electrons ejected by the action of the rays. The radium-tipped metal tongue of a spinthariscope was then placed inside the cloud chamber and the very beautiful sight of the clouds condensed along the tracks of the α-particles was seen for the first time.

In April 1911, he published some rough photographs of his find. He improved the apparatus until early 1912 and published a paper in June 1912 showing beautiful and precise photographs of alpha and beta radiations.

  wilson 1911 1912

cambridge wilson cloud chamber

In the middle, a replica of the Wilson chamber of 1912. The wood apparatus in left was one of the illuminating source. The camera was placed above the surface (perpendicular to the illuminating source) or in a horizontal position. At right Rochester‘s expansion cloud chamber (1946). Exposition du Palais de La Découverte sur le LHC, 11/2014.

Pictures from the original paper of Wilson :

wilson interestinf picture cloud

  

During the last few years many physicists have been using the method; in some cases with refinements which made it possible to attain an accuracy much exceeding that arrived at in my experiments. It would take too long even to enumerate these investigations. But I should like to mention as examples of the applications of the method: the work of Blackett on collisions of α-particles with atomic nuclei and on atomic disintegrations thus produced, on the ranges of individual α-particles by Mlle Curie and by Miss Meitner, on &rays by Chadwick and Emeleus, on the mobility of radioactive ions by Dee, on X-rays and the ,&rays associated with them by Bothe, by Auger, by Nuttall and Williams, and by Compton and his collaborators; and on the study of the wavelengths of γ -radiations, by measurement of the ranges of the Compton recoil electrons, by Skobelzyn.

He used the cloud chambers to study the processes of ionisation and ejection of electrons by X-rays. By 1923 he had perfected his chamber and introduced stereoscopic photography. The photographs he obtained established the reality of the Compton effect by showing the existence of Compton recoil electrons.

The cloud chamber was further developed by Blackett who made many important discoveries with it, notably the demonstration of the creation and annihilation of electron-positron pairs.

 

This kind of chamber is called a pulsed chamber because the conditions for operation are not continuously maintained.

 

 

1928: Geiger-Müller Counter

Later, Geiger and Walther Müller turned Geiger’s early crude particle counter into a very useful detection device, and we see above a Geiger-Müller tube of 1932 (second image).

1930: Nuclear Emulsion, M. Blau

1939: Diffusion Cloud Chamber

1940-1950: Scintillator, Photomultiplier

1952 : Bubble Chamber

1962 : Spark Chamber

 

 For the a chronology of particles physics, you should read this document.

 blackett automated cloud f nobel lecture

 

 

Chambre à brouillard à expansion de C.T.R Wilson

250px-Wilson

En 1912, Charles Wilson inventa la chambre à expansion (ci-contre) qui fut améliorée jusqu’en 1950 pour l’étude du rayonnement cosmique. Les chambres à expansion peuvent fonctionner suivant un plan d’observation vertical, elles étaient donc parfaites pour étudier le passage des rayonnement cosmiques dont le flux vertical est 1000 fois plus intense par rapport à l’horizontal.  Dans une chambre à expansion la sursaturation est obtenue par l’expansion d’un volume de vapeur d’eau à l’aide d’un piston (détente du gaz). De par son principe de fonctionnement une chambre à expansion ne permettait de voir les tracés des particules que pendant une fraction de seconde par minute (juste après l’expansion du gaz). Avant de refaire une nouvelle expansion (et donc une nouvelle observation), il était nécessaire d’attendre que la vapeur soit en équilibre thermique avec la phase liquide.

La chambre à brouillard à diffusion mis au point par Langsdorf en 1939 permet de s’affranchir du fonctionnement « pulsé » des chambres à expansion en créant la couche sursaturée de manière permanente en utilisant de très basses températures : les particules deviennent alors visibles de manière continue dans le temps ce qui est idéal pour étudier la radioactivité émis par des sources. L’inconvénient des chambres à diffusion c’est qu’elles ne peuvent fonctionner que sur un plan horizontal (la gravité stabilisant la couche sursaturée) et donc ne peuvent être utilisées pour étudier le rayonnement cosmique (qui vient très majoritairement perpendiculairement au plan de sensibilité de la chambre). Diffusion cloud chambers were used with accelerators, as the incoming particle are produced in a horizontal plan. But cloud chambers had their limitations for research purposes. They were too small for use at large accelerators. The liquid density wasn’t sufficient to interact with a large number of highly energetic particles . It also had a slow cycle (for the expansion chamber) : the process of reactivating the cloud chamber took too long compared to the accelerator cycles. In 1952, D. Glaser build a bubble chamber which solved these problem.

This chamber had significant advantages of stability and building simpleness over the expansion ones. It had, though, various drawbacks, among which Langsdorf highlighted the following: 1. It required the working area to be horizontal, an inconvenience for the study of cosmic rays, which mostly hit vertically . 2. The large horizontal dimensions of the chamber complicated the application of strong and uniform magnetic fields for track deflection. 3. The limited supply of vapour made the use of strong ionization sources difficult. 4. Because of the continuous supersaturation, any radioactive source placed inside the chamber would constantly 

 

Bubble chambers used an overheated liquid instead of a supersaturated gas for track formation, therefore giving more stability and allowing easy resets and triggering that reduced the background signal

Unlike the Cloud Chamber, the Bubble Chamber could not be triggered, i.e. the bubble chamber had to be already in the superheated state when the particle was entering. It was therefore not useful for Cosmic Ray Physics, but as in the 50ies particle physics moved to accelerators it was possible to synchronize the chamber compression with the arrival of the beam. 

 

 

 

 

2) Principe de fonctionnement d’une chambre à brouillard à diffusion (Chambre de Langsdorf)

cloudylabs diffusion cloud

Les chambres à brouillard à diffusion sont pédagogiquement plus intéressante pour étudier la radioactivité car la visualisation des rayonnements se fait de façon continu et sur de grandes surfaces sensible.

Le principe de fonctionnement à des température négative impose que le solvant soit de l’alcool car sa température de solidification est bien plus basse que l’eau (-114°C).

Dans une enceinte semi-étanche à l’air ambiant (schéma ci-dessous), une surface noire est portée à une température de -30°C . En haut de la chambre des supports permettent de contenir de l’éthanol liquide. Une partie de l’alcool s’évapore naturellement grâce à sa pression de vapeur et lorsque les vapeurs d’alcool entrent en contact avec le bas de la chambre porté à une très basse température, elles se condensent sous forme de gouttelettes créant un brouillard, l’air contenant toujours une quantité appréciable de poussières.

principe

Toutefois, une petite fraction des vapeurs d’alcool refroidies par l’intermédiaire de la surface froide ne se condensent pas et flottent au dessus de la surface formant un volume sursaturé en vapeur instable. Il suffira d’une perturbation dans ce volume de gaz instable pour que ces vapeurs retournent à un état plus stable (l’état liquide). L’épaisseur du volume de gaz sensible est de quelques millimètres, situé juste au dessus de la surface de la chambre.

La transition d’un état à un autre (ici la condensation de l’alcool gazeux) est facilitée lorsque le milieu contient des impuretés (poussières) à l’exemple de la neige qui peut se former dans l’atmosphère que si il existe des sites de nucléation permettant aux cristaux de germer. Lorsqu’une particule nucléaire chargée traverse la matière, elle perd de l’énergie en ionisant sur son passage les atomes qu’elle rencontre. Les ions résultants deviennent des « impuretés » où le gaz peut se condenser.

Vue rapprochée de deux particules alpha en fin de parcours. Les tracés des particules, traversant le volume de gaz sensible dans la chambre à brouillard, sont constitués de milliers de gouttelettes microscopiques d’alcool.

Les vapeurs instables vont passer à l’état liquide en se condensant en gouttelettes là où les ions ont été crées : les ions « semés » tout au long du parcours de la particule vont matérialiser le tracé de la particule dans la matière sous la forme de milliers de gouttelettes d’alcool. Un éclairage suffisamment fort permettra ensuite de mettre en évidence les tracés (la plaque noire du fond permet de maximiser le contraste).

Seules des particules chargées peuvent créer au cours de leurs trajectoires des ions dans la matière. Ainsi, les particules observables dans une chambre à brouillard seront les électrons (e-), les positons (e+), les protons (p+), les alphas (He2+) et les muons (μ+/-) . D’autres particules sont observables en altitude.

Les particules neutres (neutron, gamma, rayon X) seront détectables indirectement par les particules chargées qu’elles créeront dans la matière suite à leur interaction avec celle-ci (spallation, effet photoélectrique, Compton..).