Semiconductor device and linear accelarator

Semiconductor device:

A semiconductor detector in ionizing radiation detection physics is a device that use a semiconductor (usually silicon a germanium) to measure the effect of incident charge particles or photons.

Semiconductor detectors have found broad application during recent decades, in particular for gamma and X-ray spectrometry and as particle detectors.

Detection mechanism:

In semiconductor detectors, ionizing radiation is measured by the number of charge carriers set free in the detector material which is arrange between two electrodes, by the radiation. Ionizing radiation produced free electrons and holes. The number of electrons –holes pair is proportional to the energy of the radiation to the semiconductor. As a result, a number of electrons are transferred from the valence band to the conduction band, an equal number of holes are created in the valence band under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measure in outer circuit, as described by the Shockley –Ramo theorem the holes travels in the opposite direction and can also be measured. A s the amount of energy required to create an electron –holes pair is known, and is independent of the energy of the incident radiation, measuring the number of electron-hole pairs allows the intensity of the incident radiation to be determined.

The energy required to produce electron-hole-pairs is very low compared to the energy required to produce pair ions in a gas detector. Consequently, in semiconductor detectors the statistical variation of the pulse height is smaller and energy resolution in higher as the electrons travel fast, the time resolution is also very good, and is dependent upon right time. Compare with gaseous ionization detectors, the density of a semiconductors detector is very high, and charge particles of high energy can give off their energy in a semiconductor of relatively small dimensions.

Detectors types:

Silicon detector

Most silicon particle detectors or, in principle, by doping narrow(usually around 100 micrometer wide) strips of silicon to turn them into diodes, which are then reverse biased. As charged particles pass through these strips they cause small ionization current that can be detected and measured. Arranging thousands of these detectors around a collision point in a particle accelerator can yield an accurate picture of what paths take. Silicon detectors have a much higher resolution in tracking charged particles than older technologies such as cloud chambers or wire chambers. The drawback is that silicon detectors are much more expensive than these older technologies and require sophisticated cooling to reduce leakage currents (noise source).They also suffers degradation over time from radiation.

Diamond detector

Diamond detectors have many similarities with silicon detectors, but are expected to offer significant advantages, in particular a high radiation hardness and very low drift currents. At present they are much more expensive and more difficult to manufacture.

Germanium detector

Germenium detectors are mostly used for gamma spectroscopy in nuclear physics, as well as X –ray spectroscopy.

Cadmium (Zinc) Telluride detectors

Cadmium telluride and cadmium zinc telluride detectors have been developed for use in X-ray spectroscopy and gamma spectroscopy.

Accelerator

It is a device to increase the kinetic energy of charge particle or elementary particle. Depending upon the motion of particle there are two types of accelerator;

• Linear accelerator
• Cyclic accelerator

Linear accelerator

It is a device in which the motion of charged particle is along a straight line and charged ion is accelerated between successive gap between two hollow metallic cylinder.

The metallic cylinder represented by odd number (1, 3, 5…) are connected to one terminal of radio frequency generator .And the even number cylinders (2, 4, 6…) are connected to the another terminal. The electric field intensity inside cylinder is reverse in time t=T/2 (half time period of R.F. generator) .The electric generator field intensity inside cylinder is zero due to Gauss law and electric potential is constant. The speed of charge particle inside cylinder remains constant.

Consider a charge particle (ion) of mass ‘m’ and charge ‘q’ is emitted from source (A).The kinetic energy of charge particle due to potential difference between source and first cylinder is,

$$\frac{1}{2}mv_1^2=qv\dotsm(1)$$ $$v_1=\sqrt\frac{2qv}{m}\dotsm(2)$$Where v=potential difference between source A and first cylinder

\(v_1\)=speed of charge particle as it just enters first cylinder or leaves it

Let,\(V_2\) be speed of charge particle as it just enters the second cylinder. The kinetic energy of charge particle is,

$$\frac{1}{2} m v_2^2=2qv\dotsm(3)$$

$$v_2^2=\frac{4qv}{m}$$ $$v_2=\sqrt{\frac{4qv}{m}}=\sqrt 2 \times\sqrt{\frac{2qv}{m}}=\sqrt2.v_1\dotsm(4)$$Similarly,speed of change particulars as it enters third, fourth, fifth,……,\(n^{th}\) cylinders given by,$$v_3=\sqrt 3 v_1,v_4=\sqrt 4 v_1, v_5=\sqrt5 v_1,……,v_n=\sqrt n v_1 $$The maximum kinetic energy of charge particle after \(n^{th}\) cylinders,

$$ (K.E)_{max}= \frac{1}{2} mv_{n}^{2}=\frac{1}{2}m(\sqrt n v_1)^2=\biggl(\frac{1}{2}mv_1^2\biggr).n$$

$$ (K.E)_{max}=nqv\dotsm(5)$$Hence, kinetic energy of charge particle is directly proportional to number of gap or number of cylinders.

The length of cylinder is so adjust that the charge particle travels during \(t=\frac{T}{2}\) inside cylinder.

$$ s=ut$$

$$ t=\frac{s}{u}=\frac{l_1}{v_1}=\frac{l_2}{v_2}=\frac{l_3}{v_3}=\dotsm=\farc{l_n}{v_n}$$

$$t=\frac{l_1}{v_1}=\frac{l_2}{\sqrt 2v_1}=\frac{l_3}{\sqrt 3 v_1}=\dotsm=\frac{l_n}{\sqrt n v_n}$$

$$\therefore l_1:l_2:l_3:\dotsm:l_n=1:\sqrt 2:\sqrt 3:\dotsm:\sqrt n$$Hence, length of successive cylinder must be proportional of square root of integers.

Advantages :

(1)Since,the motion of charge particle is along straight line the energy loss by radiation is negligible.

(2) The injection extraction of charge particle is quit simple.

Disadvantage:

(1)It is very difficult to mention pressure inside accelerator due to its large size.

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.