Phosphor screen

Details Phosphor screen Phosphor screen

2000-08-28

2002-08-27

Yamnitzky, Marie (Department: 1774)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S691000, C428S323000, C250S484400, C250S488100, C250S582000

Reexamination Certificate

active

06440587

ABSTRACT:

BACKGROUND OF THE INVENTION
It is well known to use photoluminescent storage phosphor screens (hereinafter referred to as a “phosphor screen”) for various purposes, including computed radiography. Such phosphor screens may be created by applying a phosphor layer onto a substrate which may be formed of a polymeric material. The phosphor screens include materials capable of trapping electrons when exposed to ionizing radiation energy.
The phosphor screens typically include a thin, flexible substrate which can be coated with a layer of phosphor powder. A schematic representation of a typical phosphor screen
9
in shown in FIG.
1
. In
FIG. 1
, a phosphor layer
10
is situated on top of a substrate
12
. A protective layer
14
covers the top of the phosphor layer
10
. Phosphor grains or particles
16
can be found throughout the phosphor layer
10
. Such phosphor screens, when exposed to radiation photons, are capable of storing an image, or spatially varying energy pattern, by trapped electrons. The screens undergo a reversible alteration of the electronic state of the screen when they are exposed to the radiation photons. The state is reversed by mildly exposing the screen to infrared photons, which is accompanied by emission of more photons within the wavelength range of the visible spectrum. Thus, the phosphor screen can absorb the radiation pattern, store the information as trapped electrons, and later be read optically by converting the stored radiation pattern to a visible pattern.
Most phosphor screens include a phosphor composition which uses a base material such as strontium sulfide (SrS) crystalline material. One such screen is available from Liberty Technologies, Inc. of Conshohocken, Pennsylvania. The crystalline material is doped with trace amounts of rare earth ions, for example, as in the Liberty Technologies' composition, cerium ions (Ce
3+
) and samarium ions (Sm
3+
). The strontium sulfide, when doped with the rare earth ions, generates new energy levels within the crystalline lattice. The function of the ions in the crystal lattice will now be described in further detail.
The ions consist of a nucleus of protons and neutrons, surrounded by outer electrons. The electrons surrounding the nucleus can only occupy certain energy levels which can each accommodate a fixed number of electrons. Electrons can undergo transition between levels if the levels are only partially filled. Transition of an electron from a lower energy level to a higher energy level requires an absorption of energy by the electron. Transition of an electron from a higher energy level to a lower energy level requires an emission of energy by the electron. With respect to the rare earth ions, the 4f level is only partially filled, but is surrounded by electrons in higher energy levels. As such, the electrons can undergo transition, for example, the 4f electrons can move to the higher 5d level. The energy difference between the 4f and the 5d levels is similar to visible light energy such that the 4f electrons can be excited to the 5d level by absorption of visible light. As a further example, the 5d electrons can move to the 4f level accompanied by the emission of light. These transitions are shown in
FIG. 2
in which the nucleus N is shown with respect to the corresponding energy levels L
1
, L
2
, L
3
, L
4
, and L
5
. The energy E provided to the 4f level in the form of visible light causes the shift of an electron to the 5d level, and the emission of photons P causes the shift of an electron from the 5d to the 4f level.
When the rare earth ions are introduced within the crystalline lattice, the energy level configurations change due to interaction between the ions' electron energy levels with the electron energy levels of the strontium sulfide crystal. Further, the electrons of the rare earth ion energy levels may interact with each other. Examples of such interactions are shown in FIG.
3
. As shown, when the crystal is exposed to ionizing radiation, electrons from the valence band are excited to the conduction band. The movement of the electron leaves behind a net positive charge, or “hole”. The electron and hole are referred to as an “electron-hole pair”. Electron-hole pairs are movable within the lattice, however, due to the potential barriers, the pair generally remains bound as it travels through the lattice. The bound pair is known as an “exciton”.
Excitons are long-lived in strontium sulfide and can migrate through the lattice for some time before recombining and neutralizing each other. Such excitons preferentially recombine at distortions such as at the occurrence of a cerium ion within the lattice. The energy generated from the recombined pair is transferred to the cerium ion which results in excitation of the cerium ions' ground level 4f electron to a 5d level. Once in the 5d level, it can either move back to 4f, or tunnel to a neighboring samarium ion. The probability that this will happen increases with the number of available samarium ion sites near the cerium ions. Once the exchange of electrons takes place, the cerium ion (Ce
3+
) becomes Ce
4+
, and the samarium ion (Sm
3+
) becomes Sm
2+
. This process is referred to as “electron trapping”. The cerium is the luminescent center, and the samarium is the “trap”. By creating a population of trapped electrons in the phosphor screen, a latent image is created.
The trapping process is reversed by stimulating electrons trapped at Sm
2+
sites with external energy as shown in FIG.
3
. The energy to move the trapped electron to an excited state is about 1 eV which is equivalent to about a 1 &mgr;m wavelength photon. The optical stimulation wavelength range for a strontium sulfide crystalline lattice doped with cerium and samarium is shown in
FIG. 5
which shows the peak sensitivity at 1 &mgr;m in the near-infrared (NIR) region.
Once in the excited state, the electron can tunnel back to its Ce
4+
neighbor and drop its energy level to create luminescence, properly referred to as “photostimulated luminescence” or “PSL”. The intensity of the PSL is directly proportional to the number of trapped electrons which is proportional to the amount of radiation energy absorbed by the phosphor screen.
In the absence of a neighboring samarium ion, the cerium electron from the recombined excited pair in the 5d level would likely move back to the 4f level, generating visible photons. This process is known as prompt luminescence or “fluorescence”. The luminescence spectrum for the cerium ion is shown in FIG.
4
.
The rate at which electron trapping occurs depends upon the rate at which the various trapping steps take place, including exciton generation; exciton recombination at cerium ion sites; transition between the 4f and 5d energy levels of the cerium ion; tunnelling between cerium and samarium ions; and electron movement from the Sm
2+
excited state to the ground state. The rate at which excitons are generated, and therefore, the number of excitons, is proportional to the rate of radiation energy absorption, i.e., the dose rate. Most of the trapping steps occur very fast in comparison with the rates of exciton generation and recombination. As such, the rate equations which best express the rate of electron trapping are as follows:
ⅆ
n
e
ⅆ
t
=
f
-
n
e
⁡
(
N
-
n
)
⁢
A
(
I
)
ⅆ
n
ⅆ
t
=
n
c
⁡
(
N
-
n
)
⁢
A
(
II
)
wherein, f is the exciton generation rate, n
e
is the number of excitons generated, N is the number of available trapping sites, n is the number of trapped electrons and A is the transition coefficient for trapping, i.e., A provides the probability with which trapping may occur. Equations (I) and (II) are solved to yield Equation III below for the build up of trapped electrons:
n
(t)
=N
[1−exp(−ft/
N
)]  (III)
wherein ft=&ggr;D, D is the radiation dose and &ggr; is the proportionality factor. By plotting the number of trapped electrons, n, against exposure time (dose, D), it can be seen that the number o

Affiliated with

Also associated with

Rating

Phosphor screen does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Phosphor screen, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Phosphor screen will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2920760