B e t r i e b d e s G a s - P r o p e r t i o n a l z ä h l e r a l s o r t s e m p f i n d l i c
h e r D e t e k t o r ( " P S D " ). P r i n z i p : L a n g z e i t m e s s u n g d e r L ...
7 .3 R ö n tg e n d e te k to r e n E s g ib t e in e V ie lz a h l v e rs c h ie d e n e r S y s te m e , d ie je w e ils b e s tim m te S p e z ifik a tio n e n o p tim ie re n ; d ie s e S p e z ifik a tio n e n s in d :
1 ) B e trie b s z u v e rlä s s ig k e it / S ta b ilitä t / R e p ro d u z ie rb a rk e it 2 ) N a c h w e is w a h rs c h e in lic h k e it (d e te c tiv e q u a n tu m 3 ) G e n a u ig k e it d e r Z ä h lra te n m e s s u n g 4 ) M a x im a l v e ra rb e itb a re Z ä h lra te I 5 ) D y n a m is c h e r B e re ic h I 6 ) E n e rg ie a u flö s u n g , E 7 ) Z e ita u flö s u n g , t 8 ) O rts a u flö s u n g , x
m a x
/ I
m in
m a x
e ffic ie n c y D Q E )
V o rb e m e rk u n g e n G ru n d p rin z ip ie n - z u n ä c h s t A b s o rp tio n d e r S tra h lu n g - d a n n K o n v e rs io n d e r a b s o rb ie rte n E n e rg ie in S ig n a le ; z .B . e le k tr is c h e L a d u n g , L ic h tq u a n te n , r e d u z ie r te A g - I o n e n , ...
In te g rie re n d e v s . p h o to n e n z ä h le n d e D e te k to re n - in te g r ie r e n d e D e te k to r e n ( z .B . F ilm ) -> k e in e T o tz e itk o rre k tu r, a b e r n ic h t e n e rg ie a u flö s e n d - p h o to n e n z ä h le n d e D e te k to r e n ( z .B . S z in tilla to r ) -> T o tz e itk o rre k tu r b e i h o h e n Z ä h lra te n e rfo rd e rlic h , d .h . Z e it z u r V e r a r b e itu n g d e r P h o to n e n e n d lic h (in n e rh a lb d ie s e r Z e it k e in w e ite re s P h o to n d e te k tie rb a r) - > k ö n n e n i.d .R . s o b e tr ie b e n w e r d e n , d a ß P u l s h ö h e ~ E R ö n tg e n q u a n t, d . h . E n e r g i e a u f l ö s u n g m ö g l i c h
P h o to n e n s ta tis tik - fü r u n a b h ä n g ig e E re ig n is s e (P h o to n e n ) is t d e r F e h le r , N ~ N 1 /2 (P o is s o n -S ta tis tik )
7 .3 .1 F ilm e D ie s e rs te a u c h im E
is t h is to ris c h d e r D e te k to rty p ; h e u te n o c h in s a tz !
P rin z ip - R ö n tg e n q u a n te n re d u z ie re n A g (im A g B r) - N a c h E n tw ic k lu n g d e s F ilm s w ird R ö n tg e n in te n s itä t in F o rm v o n G ra u s tu fe n s ic h tb a r g e m a c h t
E ig e n s c h a fte n ; V o r- u n d N a c h te ile + m a x im a le I n te n s itä t ... n a c h o b e n u n b e s c h r ä n k t + o rts a u flö s e n d + s e h r e in fa c h e H a n d h a b u n g - k e in e E n e rg ie a u flö s u n g - S ig n a l s c h w e r z u q u a n tifiz ie re n - k le in e r d y n a m is c h e r B e re ic h (n u r ~ 2 0 :1 )
7 .3 .2 B ild p la tte n ( " I m a g e P la te s " ) D ie B ild p la tte w u rd e u rs p rü n g lic h a ls E rs a tz fü r d e n R ö n tg e n film fü r m e d iz in is c h e Z w e c k e v o n K o d a k u n d F u ji e n tw ic k e lt u n d e rs tm a ls 1 9 8 3 v o rg e s te llt.
P rin z ip D a s K ris R ö n D e r L ic h a b so
a k tiv e E le m e n ta lle ( z .B . B a F tg e n e n e rg ie in A u s le s e p ro z e s t, w o b e i e in e L rb ie rte n R ö n tg
t s in d k le in e , in e in e o rg a n is c h e M a trix e in g e b e tte te B r : E u 2+), in d e n e n e in T e il d e r a b s o rb ie rte n F o rm m e ta s ta b ile r F -Z e n tre n g e s p e ic h e rt w ird . s g e s c h ie h t d a n n d u rc h B e s tra h lu n g m it s ic h tb a re m u m in e s z e n z lic h tin te n s itä t p ro p o rtio n a l z u r e n in te n s itä t e m ittie rt w ird .
E ig e n s c h a fte n ; V o r- u n d N a c h te ile + g ro ß e r d y n a m is c h e r B e re ic h (~ 1 0 5 : 1 ) + o rts a u flö s e n d + u n e m p fin d lic h g e g e n " Ü b e rb e lic h tu n g " - k e in e E n e rg ie a u flö s u n g - e rfo rd e rlic h e r A u s le s e p ro z e s s
7 .3 .3 G a s d e te k to r e n - s e h r e in fa c h e r A u fb a u
-P rin z ip R ö n tg e n s tra h lu n g io n is ie rt G a s a to m e ; E le k tro n e n u n d Io n e n w e rd e n im ä u ß e re n F e ld g e tre n n t u n d ü b e r E le k tro d e n s tro m n a c h g e w ie s e n - z u m e is t w e r d e n E d e lg a s e e in g e s e tz t; z .B . A r
+
h n
A r +
+ e -
( + h n ' )
I + -
V
- J e n a c h F e ld s tä rk e E e rg e b e n s ic h u n te rs c h ie d lic h e B e trie b s b e re ic h e , d ie im fo lg e n d e n d is k u tie rt w e rd e n .
IV
I
III II
V
B e trie b im
B e re ic h I
D ie D riftg e s c h w in d ig k e it d e r L a d u n g s trä g e r is t s o k le in , d a ß e in ig e v o n ih n e n re k o m b in ie re n , b e v o r s ie d ie E le k tro d e n e re ic h e n .
B e trie b im
B e re ic h II: Io n is a tio n s k a m m e r
- T y p is c h e F e ld e r
E @ 1 0 0 V /c m
- A lle L a d u n g s trä g e r w e rd e n a n d e r E le k tro d e a u fg e s a m m e lt N ~ h n / e L m it z .B . h n = 8 k e V e
= 2 5 e V fü r A r N = 3 2 0 L
- N a c h w e is ü b e r A m p e re m e te r; b e i E m p f i n d l i c h k e i t v o n ~ 1 0 -1 5 A m in im a le r R ö n tg e n flu ß v o n I @ 1 5 0 P h o to n e n (1 k e V ) se c b z w . I @ 2 0 P h o to n e n (8 k e V ) se c
- k e in e B e g re n z u n g d e r Z ä h lra te n a c h o b e n ! - E in s a tz v o n Io n is a tio n s k a m m e rn b e i h o h e n R ö n tg e n in te n s itä te n a ls M o n ito rz ä h le r (u m b e i I0 = I0( t) d a s g e s tre u te S ig n a l a u f I0(t) n o rm ie re n z u k ö n n e n )
B e trie b im
B e re ic h III: G a s -P ro p o rtio n a l-Z ä h le r
- In d ie s e m B e re ic h is t d ie a n g e le g te S p a n n u n g s o g ro ß , d a s " G a s v e rs tä rk u n g " e in tritt; d ie s is t a b E l ~> U s w o b e i l = m ittle re fre ie W e g lä n g e d e r E le k tro n e n U s= S c h w e lle n s p a n n u n g z u r I o n is ie r u n g d e s F ü llg a s e s is t T o w n s e n d -L a w in e - L ö su n g :
- " P P Z G
ro p u ls h ä h lr e o m
B e im is c h u n g F lu o re s z e n z p P h o to n e n e m a u s lö s e n k ö n
o rtio n ö h e p a te h ä e trie
v o n " L ö h o to n e n itie rt, w e n e n ) ; z .B
sc h g a s" , w e " v e rs c h lu c k lc h e k e in e P .: A r + 1 0 %
lc h e s t" (u n h o to e C H 4
d ie d n u r e n e rg ie a rm e le k tro n e n (k ä u flic h )
a lb e trie b " ro p o rtio n a l z u r E n e rg ie d e s R ö n tg e n q u a n ts (N ~ E ), d a h e r N a m e ; n g t v o n R ö n tg e n e n e rg ie a b (u n d v o n G a s d ru c k , S p a n n u n g , ...) E n e rg ie d is k rim in ie ru n g m ö g lic h D E = D E = N E
ty p is c h e rw e is e
N = N 1
N
=
1
3 0 0
=
6 %
z u s a m m e n m it w e ite re m B e itra g v o n G a s v e rv ie lfa c h u n g (V e rs c h m ie ru n g ) e re ic h t m a n D E @ 1 5 % E to ta l
(
- B e trie b d e s G a s -P ro p e rtio n a lz ä h le r P rin z ip : L a n g z e itm e s s u n g d e r L a d u (lin k s v w e n n D
)
a ls n g s. t=
o rts e m p f a u f D ra h re c h ts ) D R ö 0 D ra
in d lic h e t a u s L a O rt n tg e n q u h tm itte
r D e te k to r u fz e itd iffe x a n t w u rd e d e te k tie rt.
(" P S D " ) re n z n a h e :
V o rs ic h t: B e s c h ä d ig u n g d e s D e te k to rs b e i h o h e r In te n s itä t
B e re ic h IV - D ie s e r B e re ic h , d e r d e n Z u s a m m e n b ru c h d e s " P ro p o rtio n a lb e re ic h s " k e n n z e ic h n e t, is t fü r R ö n tg e n z ä h le r v o n g e rin g e m In te re s s e .
B e trie b im
B e re ic h V : G e ig e r-Z ä h le r
- In d ie s e m B e re ic h is t d e r G a s v e rs tä rk u n g s fa k to r u n a b h ä n g ig v o n d e r A n f a n g s io n is a tio n , d .h . E n e r g ie d is k r im in ie r u n g is t n ic h t m e h r m ö g lic h .
- T o t z e i t n i c h t u n t e r 1 0 -6 s e c r e d u z i e r b a r ( a u c h n i c h t m i t L ö s c h g a s e n ) w e il d ie E n tla d u n g e in e g ro ß e rä u m lic h e A u s d e h n u n g a u fw e is t. - W e s e n tlic h e r V o rte il d e s G e ig e r-Z ä h le rs : H o h e N a c h w e is e m p fin d lic h k e it
7 .3 .4 H a lb le ite r d e te k to r e n P rin z ip
- ä h n lic h G a s d e te k to r; R ö n tg e n s tra h l e rz e u g t E le k tro n -L o c h -P a a re ; d ie s e w e rd e n im e le k tris c h e n F e ld g e tre n n t
E ig e n s c h a fte n , V o r- u n d N a c h te ile g u te E n e rg ie a u flö s u n g (b e s s e r a ls G a s d e te k to r), w e il N = h n /e l @ 1 0 k e V @ 3 3 0 0 3 e V +
A b s c h ä tz u n g D E / E @ 1 / N F ü r G e rm a n iu m
@ 2 %
fin d e t m a n a ls in trin s is c h e W e rte
D E = 1 4 0 e V b e i 1 0 k e V D E = 4 4 e V b e i 1 k e V ( a b e r : e l e k t r o n i s c h e s R a u s c h e n ( =^ ~ 1 0 0 e V ) k o m m t n o c h d a z u ) - K ü h lu n g ( L N 2 ) e r f o r d e r lic h , w e il E gap @ 0 .6 6 e V - M a x im a le Z ä h lra te lim itie rt a u f < 5 x 1 0 4 c p s w e g e n e n d lic h e r G e s c h w in d ig k e it d e r S ig n a lv e ra rb e itu n g +
a ls o rts a u flö s e n d e r D e te k to r k o n s tru ie rb a r
7 .3 .5 S z in tilla tio n s d e te k to r e n P rin z ip : - A b s o rp tio n v o n R ö n tg e n q u a n te n in S z in tilla to rm a te ria l
E rz e u g u n g h o c h e n e rg e tis c h e r E le k tro n e n A n re g u n g e n in S z in tilla to rm a te ria l A b re g u n g d u rc h E m is s io n s ic h tb a re n L ic h ts (~ 4 0 L ic h tq u a n te n / k e V in N a J ) N a c h w e is d e r L ic h tq u a n te n a u f P h o to k a th o d e V e rs tä rk u n g d e r d o rt a u s g e lö s te n E le k tro n e n in S e k u n d ä re le k tro n e n v e rv ie lfa c h e r (S E V )
S z in tilla to rm a te ria lie n : -N a J (T l) -G d 2O 2 S (T b ) - ....... E ig e n s c h a fte n , V o r-u n d N a c h te (+ ) E n e rg ie a u flö s u n g m ö g lic h + D ire k te In fo rm a tio n (o h n e + R e la tiv h o h e E m p fin d lic h k ( + ) I m a x (th e o re tis c h ) ( 1 0 6 c p s w e g e in R e a litä t a b e r o ft ~ 1 0 0
ile : (a b e r sc h w a c h e rfo rd e rlic h e n e it v e rg lic h e n n D a u e r d e s S 0 0 ....1 0 0 0 0 0 c
, n u r 3 0 % ) A u s le s e p ro z e ß ) m it G a s d e te k to re n z in tilla tio n s p ro z e s s e s p s
(-) (ü b lic h e rw e is e ) n ic h t o rts a u flö s e n d (K o n s tru k tio n je d o c h m ö g lic h )
