X-ray Tube

Question 1

How are X-rays produced in an X-ray tube?

Answer 1

X-rays are produced by energy conversion when a fast moving stream of electrons is suddenly decelerated in the “target” anode of an X-ray tube.

Question 2

What material is the X-ray tube made of?

Answer 2

The X-ray tube is made of Pyrex glass.

Question 3

What does the vacuum inside the X-ray tube contain?

Answer 3

The vacuum inside the X-ray tube contains two electrodes (diode).

Question 4

What is the role of the cathode in an X-ray tube?

Answer 4

The cathode (negative electrode) produces electrons that can be accelerated towards the anode (positive electrode).

Question 5

Why is it important to seal the two electrodes in a vacuum in an X-ray tube?

Answer 5

It prevents issues that were present in early x-ray tubes called ‘gas x-ray tubes’.

Question 6

What must be sealed inside the X-ray tube besides the electrodes?

Answer 6

The connecting wires must be sealed inside the X-ray tube.

Question 7

Why are special alloys used in X-ray tubes?

Answer 7

Special alloys are used to prevent damage to the tube and loss of vacuum, as they have approximately the same linear expansion coefficient as Pyrex glass.

Question 8

What is the cathode also referred to as in X-ray tubes?

Answer 8

The terms cathode and filament are often used interchangeably in reference to X-ray tubes.

Question 9

What are the three elements of the cathode?

Answer 9

The filament, the connecting wire, and the focusing cup.

Question 10

What material is the filament in an X-ray tube made of?

Answer 10

The filament is made of tungsten wire.

Question 11

What happens when tungsten is heated to at least 2200 Celsius?

Answer 11

The Edison Effect occurs (thermionic emission).

Question 12

Why is tungsten chosen for the filament in X-ray tubes?

Answer 12

Tungsten can be drawn into a thin wire that is quite strong, has a high melting point of 3370 Celsius, and has little tendency to vaporize, giving the filament a longer life.

Question 13

What happens when electrons are emitted from the filament?

Answer 13

The electrons form a cloud around the filament, known as the space charge effect.

Question 14

What is the role of the focusing cup in an X-ray tube?

Answer 14

The focusing cup keeps the electrons in line with the target spot by using the principle of repulsion.

Question 15

What is the focusing cup in an X-ray tube usually made of?

Answer 15

The focusing cup is usually made of nickel.

Question 16

What is the Line Focus Principle in an X-ray tube?

Answer 16

The Line Focus Principle was created to allow for a larger focal spot for heat dissipation while maintaining a small focal area to improve radiographic detail.

Question 17

What is the range of the anode angle in an X-ray tube?

Answer 17

The anode angle may vary from 6 to 20 degrees.

Question 18

Why not make all anodes with a 6 degree angle?

Answer 18

A 16.5 degree anode gives a smaller apparent focal spot, which is desirable for certain applications.

Question 19

What are the two types of anodes in X-ray tubes?

Answer 19

The two types of anodes are stationary and rotating.

Question 20

What is a stationary anode in an X-ray tube composed of?

Answer 20

A stationary anode consists of a tungsten target of 2 or 3 mm thick embedded in a large mass of copper.

Question 21

What is the purpose of the rotating anode in an X-ray tube?

Answer 21

The purpose of the rotating anode is to spread the heat produced during an exposure over a large area of the anode.

Question 22

What is the typical rpm for a rotating anode in an X-ray tube?

Answer 22

Due to mechanical imperfections, 3000 rpm is usually the rpm for a rotating anode.

Question 23

What material is used to house the target and separate it from the rotor in an X-ray tube?

Answer 23

Molybdenum is used to house the target and separate it from the rotor.

Question 24

What is the composition of most anodes in modern X-ray tubes?

Answer 24

Most anodes today are built with 90% tungsten and 10% rhenium.