Industrial Applications of Rare Earth Elements

12 Rare Earth X-Ray Phosphors for Medical Radiography JACOB G. RABATIN

Downloaded by UNIV LAVAL on November 3, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch012

Quartz and Chemical Products Department, General Electric Company, 1099 Ivanhoe Road, Cleveland, O H 44110

When a uniform flux of x-ray photons passes through an object, a radiologic image is formed of the object parts. In order to be viewed, this radiologic image must be converted into an optical image by means of inorganic crystalline phosphors which are disperesed in polymeric screens. The optical image can be viewed directly by means of fluoroscopic screens. However, in general radiological practices, the optical images are recorded on films which are subsequently viewed by radiologist. Without these x-ray intensifying screens, modern radiology would be impossible. The general physical aspects of diagnostic radiology have been thoroughly covered by M. Ter-Pogossian (I). For many years, most intensifying screens contained CaWO phosphors. After numerous improvements in CaWO efficiencies and particle shapes, a plateau was reached for the speed of these screens in recording satisfactory radiologic images. In recent years, several new phosphors were discovered which contain rare earth elements and are considerably more efficient under x-ray excitations. T h e s e n e w r a r e e a r t h p h o s p h o r s 4

include: L a 0 S : T b (2), G d 0 S : T b (2,3), BaFCIiEu (4,5), LaOBr:Tm (7). Several physical properties of these x - r a y p h o s p h o r s h a v e i m p o r t a n t effects o n t h e final image q u a l i t y as viewed b y the radiologist. These physical properties include x - r a y a b s o r p t i o n , c o n v e r s i o n efficiency, emission c h a r a c t e r i s t i c s , absolute d e n s i t y , particle size a n d shape a n d r e f r a c t i v e i n d e x . T h e p u r p o s e o f this p a p e r is to d e s c r i b e a n d c o m p a r e t h e s e p r o p e r t i e s f o r s e v e r a l p h o s p h o r s a n d to indicate d e s i r e a b l e p r o p e r t i e s n e e d e d i n f u t u r e more ideal x - r a y p h o s p h o r s . 2

2

2

2

Experimental Materials. A l l phosphors used in this study were p r e p a r e d b y solid state methods p r e v i o u s l y r e p o r t e d . LaOBr:Tb a n d L a O B r : T m p h o s p h o r s were p r e p a r e d b y a molten K B r flux r e c r y s t a l l i z a t i o n m e t h o d (8) w h i c h g i v e s c l e a r , s i n g l e c r y s t a l

0097-6156/81/0164-0203$05.00/0 © 1981 American Chemical Society

In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

204

RARE E A R T H

particles

that have

activated

BaFCI

pheres can

(4).

The

resulting

be p r e p a r e d

La 0 S:Tb 2

and

2

method

as plates

crystal

particles when

Gd 0 S:Tb 2

involving

stallization

plate-like

habits.

phosphors were prepared

media.

polysulfides,

europium

reducing

irregular

an alkali

in

atmosshape

chloride flux

were prepared

2

alkali

are

Divalent

in

by

Na C0 2

a high and

3

These phosphor crystals

is

but

used.

temperature

sulfer

have

ELEMENTS

as

recry-

polyhedral

shapes. Screen fying

Preparations.

screens were prepared

techniques.

The

final

ings were d r i e d .


were prepared

In

100 m i c r o n t h i c k using standard

phosphor volume

most

instances,

using polyvinyl

butyral

to

care

t a k e n to a v o i d c o n v e c t i o n cell

section of the completed thick,

a

the

screen construction

50 m i c r o n t h i c k

micron

thick

acetate

butyrate

(Ti0

phosphor layer, top

is s h o w n

coat-

viscosities

(9).

and

A

in F i g u r e

(Mylar)

cross

I.

The

base about

reflector

a 10 m i c r o n t h i c k

protective

with

blade operation

formation

(rutile)

2

coating

the

phosphor suspensions

doctor

screens consist of polyester

intensi-

blade

50% w h e n

binders

adjusted was

2000 c e n t i p o i s e f o r

was

the

x-ray

doctor

layer, clear

a

10

mil.

100

cellulose

layer.

Measurements. Emission S p e c t r a . were obtained phosphor The tus

using cathode

powders

were uniformly

samples were excited w i t h 10 k i l o v o l t s

demountable meter

for

and

apparatus

shown

based on found

in

the

in t h e

fundamental

emission

absorption i/io

on conductive cathode

energetic

Data.

= e -

y

The

p

Israel for

x

ray

glass. appara-

electrons.

The

spectrophoto-

x-ray

absorption

using a computer

coefficients,

equation

14

spectra

purpose,

spectra.

2 were obtained

of Storm a n d

emission

For this

was c o u p l e d to a C a r y

mass a b s o r p t i o n paper

settled

7 microampere

Absorption

Figure

resolution

excitation.

in a demountable

r e c o r d i n g of the X-Ray

data

High

ray

(I0J)

x-rays

y,

program

total e n e r g y ,

and

using

as

the

(II).

(D

where l is the i n c i d e n t x - r a y b e a n , I is the t r a n s m i t t e d beam, p is the p h o s p h o r d e n s i t y a n d x is its t h i c k n e s s . T h e computer p r o g r a m i n c l u d e s a p p r o p r i a t e c h a n g e s in the e n e r g i e s o f t h e x r a y s p e c t r a a f t e r p a s s a g e o f 80 K V p e a k x - r a y s t h r o u g h a 10 inch human body equivalent absorption before impinging on the x-ray screens. T h e t h i c k n e s s x w a s s e t e q u a l t o 100 m i c r o n s (typical of x - r a y s c r e e n s ) . Not included are c o r r e c t i o n s for the e s c a p e o f some s e c o n d a r y r a d i a t i o n . T h e s e corrections are d i f f i c u l t to make a c c u r a t e l y (12). 0


RABATIN

X-Ray

Phosphors

for Medical

Radiology

205

SCREEN BASE-

PHOSPHOR

LAYER-

CLEAR LAYERFILM EMULSIONFILM BASE -


FILM EMULSIONCLEAR LAYERPHOSPHOR LAYERREFLECTOR SCREEN BASE-

Figure 1.

Cross section of x-ray screens and film assembly

Figure 2. Relative x-ray absorptions of 100-micron thick x-ray screens using a filtered 80 KV peak x-ray beam


206

RARE E A R T H

Relative surements a Faxitron

x-ray

DuPont

one u s i n g green Co.

Par

CaWOi*

green

3.

It

other


about

blue

hand,

480

for

noted

inch

body

using

aluminum

equivalent

speeds were

to set

commerequal

For phosphors Ortho

typical

G green

medical

films

blue emitting

or

blue

in medical

no effect

on

3M

film. are

shown

phosphors

sensitive blue

to

with

were used including

Co.

that

green

mea-

films.

emitting p h o s p h o r s with emissions

practically

used

One

film.

Kodak

curves

speed

settings

were compared

medical

either

green

nm h a v e

most commonly

exposures.

s e n s i t i v e films

should be

screen beam

a 10 i n c h h u m a n

film a n d

will e x p o s e e f f i c i e n t l y the

All

x-ray

screens whose

BB-54

log sensitivity

Figure

peak

for

simulate

emissions, green XM

KV

speed measurements

Kodak

brand

Relative in

All

80

apparatus

f i l t e r s w e r e u s e d to absorption. cial

Screen Speeds.

w e r e m a d e at

ELEMENTS

On above

sensitive

films

radiography.

Image Q u a l i t y . Resolution of screens were meas u r e d a t 50 K V p e a k x - r a y e x p o s u r e s a t f i l m d e n s i t i e s o f 1.0 u s i n g s t a n d a r d lead resolution g r i d s with sets of etched lines a b o u t I l i n e p a i r p e r mm u p t o 15 l i n e p a i r s p e r m m . The results are p e r mm.

reported

Other were measured measurements

by

as the

maximum

Measurements.

the

Coulter

w e r e made

set o f line

Particle

Counter

u s i n g the

pairs

size

method.

well k n o w n

resolved

distributions Absolute

density

pycnometer

method. Results and

form

Discussions

A t f i r s t g l a n c e , x - r a y i n t e n s i f y i n g s c r e e n s a p p e a r to p e r a simple f u n c t i o n o f c o n v e r t i n g x - r a y p h o t o n s to l i g h t

p h o t o n s w h i c h e x p o s e t h e film s a n d w i c h e d between two s c r e e n s ( S e e F i g u r e I). H o w e v e r , x - r a y s c r e e n s s e r v e a m u c h more important f u n c t i o n of faithfully c o n v e r t i n g radiologic images into optical images w h i c h a r e s u b s e q u e n t l y r e c o r d e d as p h o t o g r a p h i c images. T h e s e several complex processes are graphically illus t r a t e d in F i g u r e I a n d c a n be d e s c r i b e d as follows. After p a s s i n g t h r o u g h a b o d y p a r t , a beam o f x - r a y p h o t o n s c o n t a i n s useful information. A small f r a c t i o n , n , o f t h i s beam is a b s o r b e d b y t h e p h o s p h o r p a r t i c l e s in t h e x - r a y s c r e e n s p r o d u c i n g a radiologic image. A fraction of this absorbed energy a

is c o n v e r t e d i n t o a n o p t i c a l i m a g e o f u l t r a v i o l e t - v i s i b l e l i g h t with an e n e r g y efficiency given b y r i c - A f t e r multiple scatteri n g a n d a b s o r p t i o n e v e n t s , a f r a c t i o n , nt# of this light reaches t h e f i r s t e m u l s i o n o f t h e d o u b l e e m u l s i o n film u s e d in medical radiography. A m a j o r f r a c t i o n , r\f\, o f t h i s l i g h t is a b s o r b e d b y the first silver halide emulsion which on development p r o d u c e s a p h o t o g r a p h i c image. A s i g n i f i c a n t f r a c t i o n , nf , ° f t h i s l i g h t c r o s s e s o v e r to t h e s e c o n d e m u l s i o n a n d is a b s o r b e d . 2



RABATIN

Figure 3.

X-Ray

Phosphors

for Medical

Radiology

207

Blue and green x-ray film sensitivities to ultraviolet and visible light


208

RARE E A R T H

This

c r o s s - o v e r p h e n o m e n o n d e g r a d e s t h e image d u e to s p r e a d -

ing

of the light.

the

radiologic image

Other

processes which

include:

Several attempts phosphor noise

efficiencies

(12,J6,17,18).,

quality

(20 2TJ.

phosphors

(I3_, I4_,I5_), x - r a y

absorption a n d quantum

screen performance

(I4,I6,I7,J8,I9_) a n d i m a g e

studies adequately

In t h i s

to final


image q u a l i t y

study,

primary

emphasis is g i v e n to

X-Ray Absorption.

(I). 7

noise *

1

T h e s e fluctuations a n d as "quantum

films s i n c e t h e films h a v e a g r a i n y fluctuations

= ^pr

energies must

a r e often

mottle"

about

400 x - r a y

20.

in F i g u r e

photons.

2

area.

T h u s t h e signal to

T o increase t h e signal to noise

filtered

the relative

ratio

x-ray ab-

s c r e e n s u s i n g a n 80 K V p e a k In t h e i n c i d e n t b e a m

photon energies a r e between

Since these photons a r e more readily more desireable c o n t r a s t s r e s u l t .

Also

regard,

about

with

K-edge

35 a n d 50 K e V ( Z f r o m

a

ab-

greater

be only achieved with cations h a v i n g

small ionic r a d i i

most o f t h e s e

region,

absorption The

6.0 which c a n

O n l y a small n u m b e r o f u s e f u l x - r a y

h a v e b e e n d i s c o v e r e d w h i c h meet

in this

55 t o 6 5 ) .

s h o u l d h a v e a h i g h d e n s i t y o f at least

and G d .

parts,

the K

since a

p h o s p h o r with high y

p h o s p h o r should contain elements

phosphor

profile,

p h o t o n s a r e a b s o r b e d a s c o m p a r e d to

for a good x - r a y

energies between

beam

40 a n d 50 K e V .

absorbed by body

In t h i s

sorption edges o f L a a n d B a a r e more useful fraction of these x - r a y

x-ray

b y o n e i n c h a l u m i n u m t o s i m u l a t e a 10

inch human body absorption.

La

According

2 are the calculated absorption curves for

100 m i c r o n t h i c k

most o f t h e x - r a y

the

These

photons of -70 K e V

screen thickness a n d speed,

which h a s been

Gd.

(I7_).

must be i n c r e a s e d .

Shown several

to

viewed o n

b e a b s o r b e d b y a C a W O i * s c r e e n to p r o d u c e a

is a b o u t

for a given sorption

referred

when

appearance

by

form the

(2)

d e n s i t y o f 0 . 6 f o r a 0.1 m m

noise ratio

amount o f

in part,

photons that

N is the number of absorbed x - r a y

t o C l e a r e e t a l (I8_) film

contribute speed a n d

c a n b e e x p r e s s e d a s a signal to noise ratios (12).

o where

is limited,

fluctuations of the x - r a y

image

"quantum

which

regarding

T h e maximum

c o d e d in a r a d i o l o g i c image

statistical

radiologic

screen performance

characteristics.

Intrinsic

as

quanti-

simple comparative analyses c a n be made o f t h e

used.

information

covers the

screens o r is sufficiently

evaluation o f measurable phosphor properties significantly

account

screen performances including;

complete performance o f x - r a y tative so that

degrade

a n d rjfj.

h a v e been made to q u a n t i t a t i v e l y

None o f these

#

significantly

Hfl*

r i o ^t'

for various aspects of x - r a y

the

ELEMENTS

such as

phosphors

criteria.

Listed in Table I a r e the absolute densities,

relative


x-ray

RABATIN

12.

X-Ray

Phosphors

a b s o r p t i o n s o f 80 K V p e a k nal

to noise ratios

phosphors. data

these

x-rays

thickness,

images

to noise ratios

e t al (18)

ratios

a n d equation

209

Radiology

at equal

for the radiologic

T h e signal

of Cleare

later,

for Medical

and sig-

for several

x-ray

were calculated 2.

using the

A s will b e d i s c u s s e d

will d e c r e a s e a s s c r e e n s p e e d s , e t c . a r e i n -

cluded. TABLE Absolute

Densities,

Relative

Radiologic

Image S i g n a l

Ratios o f Several


I X-Ray

X-Ray

Absorptions and

to Noise Phosphors

Signal

Relative Phosphor

Density

Noise

X-Ray Abs.

CaWOjj

6.1

1.0

20

BaFCI :Eu

4.7

I.I

21

7.3

1.7

26

L a O B r . 002Tb

6.3

1.9

28

LaOBr.003Tm

6.3

1.9

28

La 0 S:Tb

5.9

1.7

26

G

d

2 ° 2

2

S

:

T

b

2

The

results

in T a b l e

will b e s i g n i f i c a n t l y as

compared

must

to C a W O i * .

be used

absorption.

I indicate

improved

in t h i c k e r In g e n e r a l ,

B e c a u s e o f its lower thicker

Emission S p e c t r a . spectra of several examine x-ray

briefly

films.

F o r many

screens have

x-ray

years,

to be most

to by

absorption

improve

light

blue

such parameters

properties. phors,

as grain

In o r d e r

green

coupled system. of blue a n d green

quire that

blue

special safe

it is u s e f u l

sensitive x - r a y

2

Figure x-ray

a n d other

2

2

has high were

3 shows typical films.

Green

in the d a r k

rooms.

emulsion

emission spectra

shown

in Figures

phos-

using a d y e -

relative

sensiti-

films h a v e It

needed

primarily

2

films were d e v e l o p e d

film in s p e e d o r r e s o l u t i o n . lights

have

speeds were adjusted

Green

no a d films

should be

b l u e e m i t t i n g p h o s p h o r s will also e x p o s e g r e e n The

films

Since A g B r

size o f A g B r

to

with

ultraviolet-blue

no special efforts

Film

sensitive x - r a y

vities

over

the emission

to u s e G d 0 S : T b a n d L a 0 S : T b

AgBr

vantages

region,

absorption.

resolution.

of the p h o s p h o r emission

s e n s i t i v e to t h e

in this

BaFCI :Eu x-ray

poorer

phosphors,

emissions o f phosphors s u c h as C a W O i * . intrinsic

image phosphors

density,

to increase

Before comparing

important

the interaction

been developed

that the radiologic

b y the use of rare earth

screens in o r d e r

to Ratio

re-

noted

film.

4 through

7 were


210

RARE E A R T H

ELEMENTS

UJ


cr

WAVELENGTH (nm)

Figure 4.

Emission spectrum of LaOBr.003Tm

under CR excitation

600

WAVELENGTH (nm)

Figure 5.

Emission spectrum of LaOBr.002Tb

under CR excitation



RABATIN

X-Ray

500

400

Figure 6.

Phosphors

for Medical

Radiology

WAVELENGTH (nm)

600

Emission spectrum of GD O S.005Tb 2

350

2

400 WAVELENGTH (nm)

700

under CR excitation

450

Figure 7. Emission spectrum of BaFCI.05Eu under CR excitation


212

RARE E A R T H

obtained spectra used

using cathode practically

(I5_).

Figure

L a O B r . 003Tm where the

(22).

The

2

to

with green

screens and Shown


At

in

occur

at

380,

sitive

films.

This in

'

500

Picker

nm w h i c h

Spectra

comparable

Co.

Quanta

At

higher

20% c o m p a r a b l e

to

emitting phosphors are

for

III

L a O B r . 002Tb

principal

emissions

be used with blue

screens and

Co.

green

sig-

LaOBr.003Tm

sen-

u s e d in A g f a - G e v a e r t

e m i s s i o n s p e c t r a emit p r e d o m i n a n t l y of about

the

can

the

These

screens.

Since

phosphor

Lanex.

emission spectrum

concentrations,

440

nm

unsharp-

I).

this

with speeds DuPont

tric

efficiencies

Figure nm,

400

less

are

for

screens.

p h o s p h o r is b e i n g

B l u e Max

n

(nf2 and

u s e d in

Rapide

5 is the

terbium

415 a n d

screens,

being

below

greatest,

s e n s i t i v e films

Ltd.

Figure

these

460

emission excitations

emission spectrum

s c r e e n s s u c h as K o d a k

currently

11 f o r d

give

x-ray

emissions are

less c r o s s over

2

phosphors are

MR

principal

e m i s s i o n s a l s o o c c u r at

is a l s o u s e f u l

which

to t h o s e w h e n

film a b s o r p t i o n s a r e

to G d 0 S : T b g r e e n

(23).

excitations

4 shows a typical

intrinsic

ness occurs due nificant

ray

identical

ELEMENTS

in t h e

terbium

Co.

General

Elec-

concentrations,

green

at

542

nm

with

Z n C d S : A g phosphors

suitable

for

green

(24).

sensitive

films. Figure The

6 shows the

principal

trations,

the

with green

Lanex

green

in

divalent nm.

This

To

be

phosphors 1)

suitable

is u s e f u l

must

listed

in T a b l e

La 0 S:Tb 2

the

film

to a b o u t

6% f o r

application

efficiency

to u s e f u l

n

=

%

\

n

t

is

used

in

Co.

green

Co.

Kodak

screens.

for

BaFCI.05Eu.

peaking Quanta

The

screens,

rare

about blue

earth (n

in

c

absorptions of x - r a y s several

rare earth

conversion efficiencies 20% f o r

at II

only.

intrinsic

I have

LaOBr :Tb

from

and

phosabout

(5,26,24)

as com-

CaWOi*. Systems.

of x - r a y

energy

nm.

h i g h c o n v e r s i o n efficiencies

to h i g h

with which

light

concen-

440

phosphor

band

in D u P o n t

intensifying

to a b o u t

2

Screen-Film to t h e

with blue

also have

in addition

Trimax

2

This

emission spectrum

is u s e d

in x - r a y

2

Tb

415 a n d

is b e i n g

3 M Co.

Gd 0 S.005Tb.

lower

380,

e m i s s i o n is a b r o a d

phosphor

used

in

7 is the

for

At

decreases.

emission characteristics.

10% f o r pared

Figure

europium

screens and

phors

nm.

sensitive films a n d

screens and

Shown

Figure

542

phosphor efficiency

useful

380

at

some e m i s s i o n s will o c c u r at

However,

The

emission spectrum

emissions are

A c c o r d i n g to L u d w i g

phosphors for the

incident

as g i v e n

(

3

by

the

(5),

intensifying

x-ray

energy

is

crucial

screens

is

converted

expression:

)

w h e r e nt the e f f i c i e n c y with which the light e n e r g y is t r a n s mitted t h r o u g h t h e s c r e e n to t h e f i l m . T h e processes n a n d ric have been d i s c u s s e d . T h e transmission, scattering and absorpt i o n p r o c e s s e s , r]^, a r e e x t r e m e l y c o m p l e x i n v o l v i n g s u c h p a r a i

s

a


RABATIN

12.

meters

X-Ray

as particle

Phosphors

size,

particle

dispersion and structure

for

Presently

(I8_,

these processes individually.

transfer

function

measurements

refractive

25).

the quality

no measurements

213

Radiology

shape,

mottle

these processes degrade image.

for Medical

index,

particle

A s indicated

of the original c a n be made

radiographic

which

Comparisons using i n v o l v e all image

earlier, account

modulation

degrading

p r o c e s s e s (l_, 21). Table phors

II

lists several

fractive

index

should be fairly

(polymer

Rl "1.5)

creased.

In t h i s

the number respect,

t u r e mottle a n d i m p r o v e hedral


physical properties

a s c o m p a r e d to those f o r a n ideal

shaped particles

phosphor Finally,

packing,

average

quality.

In a d d i t i o n

low s o t h a t

phenomena

B a F C I : E u is b e s t .

particle

T o reduce Further,

distributions

sizes between

system

are d e struc-

spherical or poly-

a r e most d e s i r e a b l e .

narrow

phos-

The re-

in a polymeric

of scattering

phosphor packing,

particle

Clearly

of various

phosphor.

to

improve

are best.

5 a n d lOy g i v e

best

n o n e o f t h e p h o s p h o r s meet all o f t h e s e

to t h e a b o v e ,

other

including coating operations

factors also affect

quality

(25).

TABLE Physical

image

image

criteria.

II

Properties of Various X - R a y

Phosphors

Avg Refractive Phosphor

Index

Ideal

BaFCI :Eu

Particle

Particle

Diameter

Shape

1.6

Polyhedral,

1.7

Spheres

1.7

Irregular-

Particle Distribution

5-1 Oy

Narrow

I0-I5y

Broad

Plates Gd 0 S:Tb

1.8

Polyhedral

8-15y

Broad

LaOBr :Tb

2.0

Plates

3-1 Oy

Narrow

L a O B r :Tm

2.0

Plates

3-1 Oy

Narrow

CaWO

1.9

Polyhedral

5-1 Oy

Broad

efficiency

of given

2

2

T h e overall equation

3.

interaction

x-ray

Screen speed, however, steps

rit|

is b e s t

to determine

system

(26)

a

n

c

*

nt2 shown

the overall

screen is g i v e n

by

a l s o d e p e n d s o n t h e film

in Fiqure

speed,

I.

In p r a c t i c e

S . of the screen-film

using the expression:


it

214

RARE E A R T H

where S R

is

s

relative screen s p e e d , S f

is a c o r r e c t i o n In

Table

speeds was

set equal

experimental average nal

1.

particle

takes

in

-

s

t

where

ria >

screen

Par

Table

h

III

n

e

purposes,

Kodak

thick

screen pairs

(APD)

DuPont

Blue Brand

also lists the

when

and

screen film

radiologic

sig-

expression

absorption

thickness

various

indicated

following

x-ray

equal

the

The

u s i n g the

reduced

speed for

Par

BB-54

resolution of

were u s e d .

were calculated

screen

^

of

1 and

diameter

^400

=

to

to a c c o u n t t h e

because of the

0

comparative

100 m i c r o n

to n o i s e r a t i o s

which


for equal

to

is r e l a t i v e film s p e e d ,

factor.

III,

were set

ELEMENTS

required

screens.

T

£

"a

S,

(5)

relative x - r a y

set equal

absorption

to o n e ,

Sj

(Table

is s p e e d of

I),

S is

speed

experimental

screen. The

results

thickness, LaOBr particle all

in T a b l e

screens have size of the indicated,

noise ratios the

indicate

the

best

all

the

It

was

x-ray

Size APD CaWO^Par)

BaFCI :Eu

5

12

size.

Par

to

obtain

III Radiologic

Experimental

Screens

Speeds

Resolution,

Blue Film

Green Film

Line Pairs P e r mm

1.0

-

7.2

20

3.4

2.5

5.4

II

S i g n a l to Noise Ratio

7

4.2

5.6

13

La 0 S:Tb

9

3.0

5.8

15

2

the to

screens.

2

2

at

signal

Gd 0 S:Tb 2

smaller

prepare

Clearly, lower

equal

The

to t h e

required

t o CaWOi+

Resolution and

Noise of Various

due

p o s s i b l e to

photons are

TABLE

Particle Relative

partly not

speed.

phosphors have

same film e x p o s u r e s a s c o m p a r e d

Speed,

screens of

highest

same p a r t i c l e

rare earth

since fewer

that for

the

resolution

phosphors.

p h o s p h o r s with about

speeds

III

L a O B r : T b screens have

L a O B r :Tm

4

4.0

4.2

7.0

14

LaOBr :Tb

4

5.0

4.0

7.0

12


RABATIN

12.

X-Ray

Phosphors

for Medical

215

Radiology

No easy c o m p a r i s o n s c a n be made o f commercial s c r e e n s r e garding

quantum

mottle,

resolution and speed since

screen constructions are used and particle factors cannot be easily compared. discussions earth

was to c o m p a r e s e v e r a l

x-ray

phosphors which


image q u a l i t y .

The kilo

Other

physical properties of

have an important

benefits of rare

R e d u c e d patient

R e d u c e d motion u n s h a r p n e s s ( 2 7 ) .

3.

R e d u c e d focal spot size o f x - r a y

4.

L o n g e r life o f x - r a y

tubes

5.

Better

use of

lower

powered

6.

Fewer

retakes

(28).

for rare

Earth

earth

derlying

to d i f f e r e n c e s

center on development

associated development x-ray

such

black a n d white

for greater

image

the use of color

of suitable

fast

appropriate

study

b y the author

(31)

with

c h o i c e o f two o r more x - r a y

of

emission colors.

indicates that

studies. b y the

phosphors mixed

r a t i o s will g i v e a d e s i r e d c o l o r w h e n

excited

in

by x-rays.

If o n e o f t h e s e p h o s p h o r s h a d G d a n d t h e o t h e r s c o n t a i n or

lower

Z elements

different the

2),

then,

are differentially

energy

profile

emit a g r e a t e r

absorbed b y body parts.

is h a r d e n e d

proportion of light Z elements.

compared to the p h o s p h o r s c o n -

lower

further

T h i s unbalancing of color can be

enhanced b y use of body contrast

potassium iodide solutions.

appropriate

x-ray

color films,

When

Color

rare

earth

x-ray

and brightness.

p h o s p h o r s suitable

points a n d emission colors u n d e r

are also listed.

It

media

s u c h as BaSO**

suitably coupled with

t h e s e c o l o r u n b a l a n c i n g s will

changes in h u e , color saturation several

A s the

t h e G d c o n t a i n i n g p h o s p h o r will

taining and

La, Ba

d u e to t h e p r e s e n c e o f

a b s o r p t i o n e d g e s , u n b a l a n c i n g o f t h e c o l o r will o c c u r a s

x-rays

x-ray

(See Figure

con-

radio-

color films

have different

On

in h u e ,

techniques a n d on the availability

phosphors which

infor-

The un-

d e v e l o p m e n t s to take place r e q u i r e s additional

recent

suitable

allowing

problems regarding

films

( 3 £ ) have

in s h a d e s o f g r e y .

colored images c a n h a v e variations

T h e main

efficient

Radiography.

black a n d white r a d i o g r a p h s .

brightness a n d color saturation trasts.

A

Contrast

100,000

that colored r a d i o g r a p h s contain more

conventional

hand,

(29).

u s e black a n d white

reasons for these advantages a r e that

other

For

generators

Studies b y Prins and coworkers

image c o n t r a s t s a r e limited

graphy

generators.

Phosphors for Color

record the images. than

screens include:

p h o s p h o r is at least

to

mation

rare

o n final

(28).

p r e s e n t all radiological examinations

clearly demonstrated

effect

exposure.

At

the

earth

2.

Rare

Other

T h e intent of the previous

1.

potential market annually.

different

sizes differ.

should be noted

Table

for this

90 K V p x - r a y

that

IV

show lists

purpose. excitation

for blue colors

ultra-


RARE E A R T H

216 violet

emitting

phosphors are

emulsion of color

suitable

will e x p o s e t h e

blue

film. TABLE Colors and Rare

Under

Earth

90 K V

Phosphor


and

ELEMENTS

IV

Color Points X-Ray

Peak

of

Phosphors

X-Ray

Excitations

Colors

Color

Prints

n Y 0 :Eu

Red

.655

.355

Y 0 S:Tb

Green

.336

.536

L a O B r . 05Tb

Green

.350

.533

L a O B r . 005Dy

Yellow

.430

.466

L a O B r . 005Sm

Orange

.546

.384

Cd 0 :Eu

Red

.655

.355

Gd 0 S:Tb

Green

.336

.536

BaFCI :Eu

for

Blue

Near

UV

Near

UV

L a O B r . 003Tm

for

Blue

Near

UV

Near

UV

2

3

2

2

2

2

3

2

Summary

and Conclusions

The

desireable

nificantly (Using

to f i n a l

Figure

phosphor properties

speed and

1 as

The

desireable

atomic

which

quality

contribute

include the

sig-

following

reference).

Intrinsic X - R a y

sities and

image

Absorption,

rja«

characteristics

numbers

from

about

are

high

55-65 to

absolute

reduce

den-

quantum

noise.


12.

RABATIN

Emission

X-Ray

conversion

p r e f e r r e d emission

match the

spectral

to

cross

reduce

Scattering

and

I11

Radiology

of

are

ru,

should

in the

silver

near

color contrast

should

only

at

least

halide emulsions,

possess

Polyhedral shapes

structure

present

be

ultraviolet

15%.

to

rif|,

and

rjf2*

phosphors

these characteristics.


efficiencies, spectra

sensitivity

over,

scattering.

reduced At

Medical

and Transmission Characteristics,

The reduce

for

Characteristics. The

The

Phosphors

low

rit*

refractive

promote

better

indices

to

packing

mottle.

rare earth

x-ray

Rare earth

phosphors

phosphors

meet most

are also useful

of for

radiography.

Acknowledgments The a u t h o r is e s p e c i a l l y g r a t e f u l to R o b e r t E v a n s f o r making numerous preparations, measurements and related studies.

References 1.

Ter-Pogossian, M.M. "The Physical Aspects of Diagnostic Radiology". Harper and Row Publishers: New York, 1967.

2.

Alves, R.V.; Buchanan, R.A. IEEE Trans. Nuc. Sci., 1972, 19, 415.

3.

Buchanan, R.A., Finkelstein, S.I., Wickersheim, K.A., Radiology, 1972, 105, 185-190.

4. 5.

Chenot, D.F. Canadian Pat. 896453, 1972. Stevels, A.L.N., Pingault, F., Phillips Res. Repts., 1975, 30, 277.

6.

Rabatin, J.G., U.S. Pat. 3,617,743, 1971.

7.

Rabatin, J.G., U.S. Pat. 3,795,814, 1974.

8.

Rabatin, J.G., U.S. Pat. 3,591,516, 1971.

9.

Rabatin, J.G., Extended Abstract, #220, Electrochemical Meeting, May 1979.

10.

Storm, E., Irael, H.I., Report LA3753 Los Alamos Scientific Lab., Univ. of Calif., 1967.

11.

Clark, G. "The Encyclopedia of X-Rays and Gamma Rays", Reinhold Publishing Corp., New York, 1963, 9.


218

RARE E A R T H

ELEMENTS

12.

Swank, R.B., J. Appl. Phys. 1973, 44, 4199.

13.

Coltman., J.W., Ebbighausen, E.G., Altar, W.J., J. Appl. Phys., 1947,18,530.

14.

Grum, F., Costa, L.F., Donavan, J.L. J. Opt. Soc. Am.,


1969, 59, 848. 15.

Ludwig, G.W., J. Electrochem. So., 1971,118,1152.

16.

Venema, H.W., Radiology, 1979,130,765.

17. 18.

Lubberts, G., J. Opt. Soc. Am., 1968, 58, 1475. Cleare, H.M., Splettstosser, H.R., Seemann, H.E., Am. J. Roent. And Radium, 1962, 88, 168.

19.

Herz, R.H., Bri. J. Appl. Phys., 1956, 7, 182.

20.

Klasens, H.A., Philips Res. Rep., 1947, 2, 68.

21.

Morlotti, R., J. Photo. Sci., 1975, 23, 181.

22.

Rabatin, J.G., Extended Abstract #198, The Electrochemical Society Meeting, May 1975.

23.


24.

Ludwig, G.W., Kingsley, J.D., J. Electrochem. Soc., 1970,117,348. 25. Rabatin, J.G., Extended Abstract #220, The Electrochemical Society Meeting, May 1979.

26.

Moser, E.S., Holland, R.S., SPIE, 1975, 56, 26.

27.

Prasad, S., Marc Edwards, F., Hendel, W.R., Radiology, 1977, 123, 763.

28.

Thompson, T.T , Radford, E.L., Kirby, C.C., Applied Radiology, 1977 6, 71.

29.

Rucker, J.L., Applied Radiology, 1980, 9, 57.

30.

Prins, H.R., Katz, J.L., Billmeyer, F.W., Am. J. of Roent., 1966, 98, 966.

31.


RECEIVED

December 29, 1980.


Industrial Applications of Rare Earth Elements

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