Bubble chamber and Cerenkov Detector

Bubble chamber

To detect very energetic particles the cloud chamber is not very suitable sincwe the interaction of energetic particles cannot be completely observed in the chamber .To overcome this disadvantage it is necessary to use a chamber filled a substance having a much greater stopping power .This is has led to the devlopment of the bubble chamber.In 1952,D.A.Glasar,at the Univesity 0f Michigan,desined and made the first bublle chamber and this has now become one of the same time one of the costliest type of particle detector in use.

Principle

Normally ,the liquid boils with the evolution of vapour at the boiling point.The boiling point can be raised by increasing pressure on it’s surface.If the liquid is heated under a high pressure to a temperature well aboveit’s normal boiling point,a sudden release of pressure will leave the liquid in the track of a particle act as condensation centres for the formation of vapour bubbles that grow quickly in size.The bubble chamber track thus formed can be phtographed under strong illummination.Thus,in a bubble chamber a vapour bubble chamber forms in a superheaded liquid wheas liquid drop forms in a super –saturated vspour .

Construction and working

A schematic diagram of a hydrogen bubble chamber,opertating at a temperature of 27 K and developed by Alvarez and his coworkers is illustrated in the figure.It consist of a vessel made of stainless steel with glass ports at the top for the viewing cameras.A box of thick glass walls is filled with liquid hydrogen and connected to the expansion pressure system.To maintain the chamber at costant tempreture it is surrrounding by liquid nitrogen and liquid hydrogen shields.From the side window W the high energy partices were allowed to enter the camera takes a steroscope view of the chamber,The light flash is triffered by the delayed pulse from a counter which is activated by the same ionizing particle that traverses the bubble chamber.From the curvature and length of the track when the chamber is operated in a magnetic field,the momentum,energy and sign of the ionizing particle is determined.

Figure

After the pressure over liquid is released,the bubble chamber remains sensitive for a short interval of time ready to record an ionizing event before the suoerheaded liquid starts boiling.This interval is called sensitive time of the bubble chamber.

Advantages

(1) The density of the liquid is very large as compared to that of a gas even at a high pressure.Therefor,the chances of collision of high energy particles with liquid molecules are very much greater .As a result, there is much higher chance of their track being recorded .

(2) The sensitive time of bubble chamber is very short,the backgrpund due to unwanted tracks is, therefore,considerably reduced.

(3) The bubbble chamber is sensitive even to particles of low ionizing power.

Drawbacks

(1) It has a lage recovery time.

(2) It is not continously sensitive.

CERENKOV COUNTER

when a charge particle moves thgrough a transparent dieelectric medium with a velocity greter than the velocity of light in that medium,weak elecro-magnetic radiation is emitted.These radiations are named Cerenkov radiation,after the discoverer P.A.Cerenove, a Russian physicst.The radiations are emitted mainly in the direction of the charged particle and has a continous spectrum from red to ultra-violet at least.The properties of Cerenkov radiations discussed in the previous chapter have been extensively utilized in Cerenkov detectors which can be used not only to detect swiftly moving charged particles but also to determine their velocities .These detectors posses the following advantages:

(1) The duration of light pulse at any point with in the Cerenkov cone is vanishingly short\((\sim\leq 10^{-10})\)

(2) High efficiency and high countig rate.

(3) Diretional emission of light,with an angle dependent on velocity.

Cerenkov radiation:

When a charge particle moves through a trasparent dielectric medium with a velocity of the light greater than the velocity light in that medium,weak electromagnetic radiation is emitted called Cereknov radiatio.The direction of the radiation emitted depends on the velocity of the dielectric .

In dielectric medium of refrative idex \( \mu\) ,the photon move with a velocity \(\frac{c}{\mu}\).Consider a charged particle moving with a velocity v through the dielectric medium .The condition for Cerenkov emission is given by \(v>\frac{c}{\mu}\) .The radiation is an isotropic radiation and is emitted in a direction making an angie \(\theta\) With incident particle in forward cone of half angle \(\theta\).If a particle goes from A to C and the radiation travel from A to B in the medium under consideration in time t,then,$$sin\theta=\frac{\frac{c}{\mu}t}{vt}=\frac{c}{nv}$$Where the angle \(\theta\) to the the cone of radiation depends on the speed v of the particle.Thus the speed of the particle can be determined by measuring the angle \(\theta\)

Constrution and Working

A Cerenkov detector designed by Marshall is shwon in figure .A collimated beam of pions is allowed to pass along the axis of a large hemisphere perpex lens.The Cerenkov radiation radiated in this lens is reflected by cylidrical mirrors \(M_1\),on to one of the piane mirrors \(M_2,M_3\) and coincidence.Thus,Cerenkov radiation coming out of the medium can be detected with the help of and measure their velocities with the help of Cerenkov counters.From,the direction of the radiation,it is possible to the path along which the particle travels.

Application:

The Cerenkov detector is used for the detection of high energy particles.Is has efficiency and a high counting rate.Cerenkov radiation havee been put too many practical applications in nuclear phycics .When a reactor of swimming pool type becomes critical,the hot fuel elements start producing an intense glow of bluish colour in the surrounding water which is used as coolant.This is because of the high enery electrons mostly produced by \(\gamma\)-ray scattering which are emitted from highly excited fission fragments.These radiations can be used to control the thermal activity and of a nuclear reactor.

References:

Reviews of Modern Physics. Lancaster, P.A.: Published for the American Physical Society by the

American Institute of Physics, 1952. Print.

Wehr, M. Russell, and James A. Richards. Physics of the Atom. Reading, MA: Addison-Wesley

Pub., 1984. Print.

Young, Hugh D., and Roger A. Freedman. University Physics. Boston, MA: Pearson Custom,

2008. Print.

Adhikari, P.B. A Textbook of Physics. 2070 ed. Vol. II. Kathmandu: Sukunda Publication, 2070.

Print.