W. C. Rontgen conducted various experi-ments to study the discharge of electricity through gases and accidently discovered X-rays.
He used a glass tube fitted with two electrodes cathode and anode. An exhaust pump was connected through a side tube and a high electric potential of about 25 kV was applied across the electrodes. Rontgen made the following important observations:
(a) When air inside the tube was at atmospheric pressure there was no discharge of electricity through it.
(b) When pressure inside the tube was reduced to few cm of mercury, a feeble discharge of electricity occurred between the electrodes. At the same time air inside the tube started giving visible light.
(c) When air pressure was reduced to 10–3 mm of mercury, air lumininescence gets disappeared but the glass walls of the tubes starts glowing. Rontgen concluded that this glow was due to some invisible penetrating radiations from anode that were falling on the glass and producing fluorescence. Rontgen did not know much about the nature of these rays and therefore, he called them X-rays.
(B) Explanations : Rontgen explained the production of these X-rays due to the bombardment of high velocity electrons on anode. Due to low pressure (10–3 mm) inside the tube, the air gets ionised producing a few electrons and the positive ions.
The positive ions are highly accelerated towards the cathode, these ions knock out electrons from the cathode. The released electrons move with tremendous speed towards highly positive anode. They strike the anode surface and produce X-rays. On his experimental basis, Rontgen observed that in order to produce X-rays, the following three things are required.
(a) The source of electron
(b) Means of accelerating these electrons to high speeds.
(c) Anode or target on which these high speed electrons should strike to produce X-rays.
Origin of X-ray:
X-rays are produced by bombarding high speed electrons on some heavy elements (e.g. tungsten) known as target.
A big fraction of the kinetic energy of the majority of striking electrons is spent in undergoing collisions with the atoms of the target and consequently the temperature of target material is considerably increased. It is found that nearly 99% of the incident energy of electron is used up in heating the target.
However, some fraction of the K.E. of the bombarding electrons is used up to produce X-rays in the following two ways :
CONTINUOUS X-RAYS OR BRAKING X-RAYS :
The bombarding electrons may be slowed down by the nucleus as they pass close to it , thus releasing the loss of energy in the form of X-rays.
The high speed electrons go into the interior of the atoms of the target material and are attracted by the positive charge on their Nuclei. As an electron passes lose to the positive nucleus of an atom in the target the electron is deflected from its path as shown in fig. This result in deceleration of the electron.
The loss in energy of the electron during deceleration is emitted in the form of X-rays. X-rays produced in this way are called Braking or Bremsst Rahluung X-rays as they are produced due to the braking or slowing down of the bombarding electrons by the atoms of the target.
Properties Of X-ray:
X-rays are produced when fast moving electrons (or cathode rays) strike any heavy element and have the following properties.
(1) X-rays are electromagnetic waves of very short wave length (0.03 A° to 30 A° ). Hence they carry high energy. It is to be noted here that wave length of light rays ranges from 4000 to 7500 A° . This is the only difference between X-rays and light rays.
(2) X-rays travel in straight lines like light.
(3) X-rays travel with speed of light.
(4) Just like light rays, X-rays also show the phenomenon of reflection, referaction, interference, diffraction and polarisation.
(5) X-rays are not deflected by magnetic or electric fields. Hence they are not charged particles.
(6) X-rays produce flourescence (i.e., give out light) when they fall on certain materials like zinc sulphide, glass, rock salt etc.
(7) X-rays produce continuous spectrum just like light rays.
(8) X-rays penetrate through different substant e.g. thin metal sheets, flesh etc. depending upon their frequency or energy. They do not pass through heavy metals and bones. If such objects are placed in their path, they cast their shadow.
(9) X-rays blacken the photography plate when they fall in it.
(10) X-ray ionise the gases they pass through.
(11) X-rays produce photo electric and compton effects when they fall on matter.