Xeroradiography is the production of visible image utilizing the charged surface of a photoconductor (amorphous selenium) as the detecting medium, partially dissipating the charge by exposure to X rays to form a latent image and making the latent image visible by xerographic processing.

1. Dr. SHEFALI B. MESHRAM

2.  Xeroradiography is the production of visible image
utilizing the charged surface of a photoconductor
(amorphous selenium) as the detecting medium,
partially dissipating the charge by exposure to X rays
to form a latent image and making the latent image
visible by xerographic processing.

3. PHOTOCONDUCTION
 The valance band is the highest energy band in which
electrons normally exist at a very low temperature.
 The next highest permissible band is called the
conduction band.
 These two bands are separated by a forbidden gap.

4. ENERGY
CONDUCTION
BAND
VALENCE BAND
FORBIDDEN
ENERGY GAP

5.  Solids that have many electrons in the conduction
level are known as conductors. Conductors do not have
forbidden region.
 In insulators the forbidden gap is so large that
electrons seldom absorb enough energy to bridge the
gap.
 A semiconductor is a solid that contains a small
forbidden gap. Addition of appropriate quantity of
energy will allow some electrons to bridge the
forbidden gap and enter the conduction band.

6.  In photoconductor, this energy is provided by
absorption of visible electromagnetic radiation (light).
 Photoconductors are a special type of semiconductors.
 In Xeroradiography, the photoconductor used is
selenium.

7.  When electron leaves the valence band, the area of
absent negative charge is known as positive hole. Thus
a electron- hole pair is created. When they are
subjected to an electric field, the two members of the
pair migrate in opposite direction.

8. XERORADIOGRAPHIC PLATE
 It is a sheet of aluminum approximately 9 ½ x 14
inches in size on which a layer of amorphous selenium
has been deposited. There is an interface layer
between the selenium and the aluminum and an over
coating protecting the selenium layer.

9. XERORADIOGRAPHIC PLATE
0.1 µ OVERCOATING
150 or 320 µ SELENIUM
0.1 µ INTERFACE
2 mm ALUMINUM

10. ALUMINIUM SUBSTRATE
 The aluminum plate on which selenium is deposited is
made up of meticulously cleaned aluminum with an
exceedingly smooth surface.

11. INTERFACE LAYER
 Heat treatment of aluminum substrate serves to form a
thin layer of aluminum oxide.
 Although aluminum is a conductor, aluminum oxide is
an insulator.
 The purpose of interface is to prevent negative charges
induced in the aluminum from migrating into
selenium and dissipating the positive charge induced
on the selenium surface.

12. SELENIUM COATING
 Highly purified selenium in the amorphous (vitreous
form) is deposited onto the aluminum substrate by
condensation of vaporized liquid selenium in a high
vacuum.
 The thickness of selenium layer is 150µ for powder
toner development while it is 320µ for liquid toner
development plates.

13. PROTECTIVE OVERCOATING
 Cellulose acetate is used . It is applied by dip- coating
from solution to form about 0.1µ thick layer.
 It bonds well with selenium.
 It has high resistance enough to prevent lateral
conduction of charges, which would degrade the
electrostatic latent image.
 It extends the life of Xeroradiographic plate by a factor
of about 10.

14. Steps involved in Xeroradiographic
imaging process.
 A uniform charge is deposited onto the surface of
selenium. This sensitizes the plate before exposure to
x-rays.
 This charged plate is placed in a light tight cassette
and is exposed to x rays just like a film screen cassette.
The x rays reaching the plate cause the
photoconductor layer to lose its charge in an amount
corresponding to the intensity of x ray beam. The
uniform charge is thus dissipated, and the remaining
charge pattern forms the latent electrostatic image.

15.  This latent electrostatic image is then developed by
exposing the surface of the plate to fine charged
particles called toner that are attracted to the plate
surface in proportion to the intensity of the remaining
charge.
 The toner image is transferred to a receiver sheet by
an electrostatic process. It is fixed to the sheet to make
a permanent record.
 The plate is then cleaned of all the remaining toner
and prepared for reuse.

16. PHOTOCONDUCTIVE LAYER
 Three properties required by the photoconductor to be
used in Xeroradiography are.
1. The electrical conductivity in the dark must be that
of a good insulator so that a charge pattern present
on the surface will be retained long enough to
complete the steps of development.
2. The material must become electrically conducting
during exposure to X rays so that an electrostatic
image can be formed on its surface by the exposure .
3. It must have mechanical properties of durability and
ease of fabrication

17.  The most stable form of selenium is its crystalline
form. It is not used in Xeroradiography because it has
relatively high electrical conductivity.
 The amorphous form is a super cooled form of liquid
selenium. It is formed by cooling the liquid suddenly
so that crystals do not have time to form. It is
deposited onto the aluminum plate by condensation of
vaporized liquid selenium in a high vacuum.

18.  Pure amorphous selenium is mandatory as the
presence of any impurity increases the dark decay rate.
Dark decay rate is the reduction in plate voltage while
it remains in darkness.

19. PLATE CHARGING
 The first step in the Xeroradiographic process is to
sensitize the photoconductor by applying a uniform
electrostatic charge to its surface in the dark.
 Because selenium is an insulator in the dark, during
charging, the xeroradiograhic plate can be considered
to be a parallel plate capacitor in which the outer layer
of selenium surface and the aluminum backing
(substrate) acts as parallel plates and the selenium
itself acts as dielectric.

20.  A capacitor or condenser consists of two sheets of
conducting material with a sheet of insulating
material between them . One plate is charged positive
and the other is negative. So a potential difference or
voltage exists between the two plates. The insulator
material in between is known as dielectric.

22.  If a positive charge is deposited on the surface of
selenium, the remainder of the selenium atoms are
distorted into the configuration of induced dipoles.
The negative pole of each atom are attracted towards
the positive charge, causing the atomic layer adjacent
to the aluminum substrate to present a positive charge
at the selenium- aluminum interface.

23.  Polarization of the selenium atoms will attract
negative charges in the aluminum substrate towards
the back of selenium layer. In other words negative
charge has been induced in the aluminum substrate by
the presence of positive charge on the surface of
selenium.
 The interface layer reduces the negative charge leakage
from the substrate into the selenium.

24. DIELECTRIC

25.  The device used to produce a charge on the surface of the
selenium operates on the principle of corona discharge.
 If a sufficiently high potential difference called corona
threshold voltage is applied between a fine wire and
ground, the air near the wire becomes ionized.
 If the voltage is positive, the free electrons of the gas near
the wire will move towards the wire. These electrons will be
of high energy and instead of going straight to the wire,
they will interact with many molecules or atoms in the air
and create many additional ions.

26.  The positive ions thus created will move away from
the wire.
 This movement of ions is called corona current.
 When such a wire is placed near the surface of
Xeroradiographic plate, some positive ions
repelled from the wire will be deposited on the
surface of the plate.

27. EXPOSURE OF CHARGED PLATE
 After the plate is sensitized by corona charging it must
be enclosed in a cassette that is light tight and rigid
enough to provide mechanical protection for the
fragile plate.
Exposure to Xray.
electron- hole pairs.
electrons migrate to plate surface and discharge
the positive charge originally laid down.

28. the positive holes migrate through the selenium
toward the substrate, where they are
neutralized by the induced negative charge.
The amount of discharge of the positive charge is
proportional to the intensity of Xrays that
penetrate the patient.
The remaining charge pattern on the plate
surface is called electrostatic latent image.

29. CONDUCTIVITY INDUCED BY THE
XRAY
 The energy gap between valence band and conduction
band of amorphous selenium is 2.3 eV which corresponds
to a wavelength of about 5400 A ( visible light in the range
of green- yellow).

30.  Photoconductivity induced by Xrays and that by
light differ in two important ways,.
1. The Xray photons can penetrate further into the
selenium and their absorption may be uniform
throughout the selenium layer.
2. The energy of Xray photon is transferred to a
photoelectron or a recoil electron and each of
these electrons will usually have enough kinetic
energy to produce many more charged carriers
(electron- hole pairs) along its tract.

31. ELECTRIC FIELD DISTRIBUTION
ABOVE LATENT IMAGE
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34. XERORADIOGRAPHIC
UNDERCUTTING
 Caused by ionisation of the air in the space between
the selenium surface and the lid of the cassette.
 Ionisation may be caused by interaction of Xrays
directly with air molecules or it may be caused by high
speed electrons being able to escape from the
selenium and enter the air space.

35. XERORADIOGRAPHIC
UNDERCUTTING

36.  To avoid this, a DC voltage is applied between the
cassette lid and the aluminum backing.
 If the cassette lid is made positive, negative air ions
can be attracted away from the plate surface.

37. POWDER DEVELOPMENT
 Development consists of attracting small charged
particles called toner to the electrostatic latent image.
 The toner used is a pigmented thermoplastic material
of average diameter 4 µ.
 The powder cloud development or the aerosol
development is fast, simple and it provides edge
enhancement.
 The exposed plate is placed on top of a dark box into
which an aerosol of charged toner particles is sprayed
through a nozzle.

39.  Aerosol is created when a pressurized gas ( usually
nitrogen) is used to force toner through small bore tube.
 An aerosol is a suspension of small liquid or solid particles
in a gaseous medium. They have random movements.
 Agglomeration of toner particles into large particles is
prevented as turbulent flow is produced when aerosol is
passed through a small bore nozzle.
 The electric charge on the toner particle is produced by
friction between the toner and the nozzle wall.
 Both positive and negative charges are produced.
 POSITIVE DEVELOPMENT involves attracting negatively
charged toner particles to the remaining positive charge on
the surface of selenium.

40.  To get positive development, positive voltage is applied to
the aluminum backing to attract only negatively charged
particles.
 There are now two electrostatic forces influencing the
motion of the particles; the uniform electric field of the
back-bias voltage and the non-uniform field created by
latent image.
 For typical positive development, the back-bias potential is
in the range of 2000V DC.
 With positive development, unexposed or minimally
exposed areas of the plate will be dark blue. ( calcification
will appear as blue).

41. POSITIVE DEVELOPMENT.

42. NEGATIVE DEVELOPMENT.

43. LIQUID DEVELOPMENT
 Liquid toner particles are black.
 The particle size of is 1.7 µ (blue powder toner – 4µ).
 Also they have smaller charge per particle ( 150 elementary
charge per particle, while in powder toner it is 1000
elementary charge per particle).
 Smaller particle plus less charge on each particle results in
more toner particles being deposited per image on the
imaging plate.
 Cancelling all the charge on the plate may mean that toner
particles have to be stacked several layers deep on the plate.
 Toner particles stacked on top of each other will increase
optical density in the final image recorded.

46. EDGE ENHANCEMENT AND
DELETION

47. There is a relationship between edge
enhancement, plate voltage, back-bias voltage
and size of deletions.
CONTRAST DELETION
WIDTH
HIGH BACK
BIAS
LOW SMALL
HIGH PLATE
VOLTAGE
HIGH LARGE

48. IMAGE TRANSFER AND FIXING
 The image on the surface of Xeroradiographic plate is
then transferred to paper and fixed to form a
permanent image.
 A electrostatic transfer process is used.
 The paper is coated with a slightly deformable layer of
plastic, such as a low molecular weight polyethylene
material.
 When this paper is pushed against the powder image
under relatively high pressure, the toner particles get
embedded into it.

49.  After the image is transferred, the paper is peeled off
the plate and the loosely held powder image is made
into a permanent bonded image by heating the paper
to about 475˚ F.
 The heat softens the plastic coating on the paper and
allows the toner particles to sink into and become
bonded to the plastic.

50. PLATE CLEANING
 All toner must be removed before the plate is to be
reused .
 The plate is exposed to a light source
(electroluminescent strip) that reduces the bond
holding the residual toner to the plate.
 A preclean corotron exposes the plate to an
alternating current that serves to neutralize the
electrostatic forces holding toner to the plate.
 A cleaning brush then mechanically brushes the
residual toner from the plate.

51. RELAXATION
 To prevent faint ghost images.
 Absorption of X-rays in the selenium produces an
alteration in the physical property of selenium that
causes some photoconductivity to persist for as long as
several hours after exposure.
 If the plate were charged for a new exposure without
allowing it to rest, a ghost image of the previous
exposure will appear on development.
 This rest period can be reduced to 2-3 min if the plate
is relaxed by heating it to 140˚F for 150 sec.

52. STORAGE
 The cleaned and relaxed plate is then held in the
storage compartment at 89˚ F until needed for
another exposure.

53. ADVANTAGES:
 Pronounced edge enhancement.
 Choice of positive and negative display.
 Good detail.
 No need of silver halide coated film.
DISADVANTAGE:
 High radiation exposure.

56.  THANK YOU.