Multichannel Image Detectors Volume 2 - ACS Publications

2 The Silicon-Intensified Target Vidicon Detector Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 27, 2018 | https://pubs.acs.org Publication Date: November 16, 1983 | doi: 10.1021/bk-1983-0236.ch002

Operation, Characterization, and Application in Atomic Spectroscopy Research J O H N W. O L E S I K 1 and J O H N P. WALTERS2 University of Wisconsin-Madison, Madison, WI 53706

In this paper we b r i e f l y discuss the operation of the s i l i c o n intensified target (SIT) vidicon, describing what we conclude to be the spectroscopically most important properties of the detector. We provide experimental evaluation of the SIT vidicon characteristics including two-dimensional image fidelity; channel-to-channel and pixel-to-pixel response as a function of position on the detector; and temporal resolution and gating. We illustrate how these unique properties enhance our ability to make spectroscopic measurements while at the same time impose limitations on the use of the SIT vidicon. We describe a variety of experiments which employed an SIT vidicon detection system for atomic spectroscopy with various degrees of spatial and temporal resolution.

The s i l i c o n i n t e n s i f i e d t a r g e t ( S I T ) v i d i c o n h a s a number o f u n i q u e p r o p e r t i e s w h i c h make i t a v a l u a b l e d e t e c t o r f o r a t o m i c spectroscopy. The S I T v i d i c o n p r o v i d e s t w o - d i m e n s i o n a l p h o t o e l e c t r i c d e t e c t i o n w i t h h i g h s e n s i t i v i t y and r a p i d s i g n a l readout. Time r e s o l u t i o n c a n be o b t a i n e d i n a t i m e - r e s o l v e d ( r e a l t i m e ) mode o n t h e m i l l i s e c o n d s c a l e and i n a t i m e - g a t e d ( e q u i v a l e n t t i m e ) mode o n t h e s u b m i c r o s e c o n d s c a l e . C o u p l i n g an S I T v i d i c o n d e t e c t o r w i t h a n o n - l i n e c o m p u t e r for d e t e c t o r c o n t r o l , experiment s y n c h r o n i z a t i o n , data a c q u i s i t i o n and d a t a p r o c e s s i n g r e s u l t s i n a p o w e r f u l and f l e x i b l e d e t e c t i o n s y s t e m . The f l e x i b i l i t y r e s u l t s f r o m t h e 1Current address: Department of Chemistry, Indiana University, Bloomington, IN 47405. 2Current address: Department of Chemistry, St. Olaf College, Northfield, MN 55057.

0097-6156/ 83 / 0236-0031 $06.00/ 0 © 1983 A m e r i c a n C h e m i c a l S o c i e t y

Talmi; Multichannel Image Detectors Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 27, 2018 | https://pubs.acs.org Publication Date: November 16, 1983 | doi: 10.1021/bk-1983-0236.ch002

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MULTICHANNEL IMAGE DETECTORS

s o f t w a r e c o n t r o l o f t h e speed o f s i g n a l a c q u i s t i o n , s p e c t r a l r e s o l u t i o n , photometric accuracy, exposure time, s p a t i a l r e s o l u t i o n , and t e m p o r a l r e s o l u t i o n . C o m p l e t e c o m m e r c i a l s y s t e m s i n c l u d i n g an S I T v i d i c o n d e t e c t o r , d e t e c t o r c o n t r o l l e r , h i g h v o l t a g e p u i s e r f o r t i m e g a t i n g , and a c o m p u t e r , i n c l u d i n g s o f t w a r e , a r e a v a i l a b l e . The P r i n c e t o n A p p l i e d R e s e a r c h C o r p o r a t i o n (PARC) OMA I I s y s t e m was u s e d i n t h e s e s t u d i e s , mounted i n t h e f o c a l p l a n e o f a one m e t e r M c P h e r s o n model 2051 monochromator. W h i l e t h e S I T v i d i c o n d e t e c t i o n s y s t e m p r o v i d e s u n i q u e and exciting spectroscopic detection c a p a b i l i t i e s , inherent l i m i t a t i o n s of v i d i c o n d e t e c t i o n prevent i t from r e p l a c i n g other d e t e c t o r s such as p h o t o g r a p h i c e m u l s i o n s , p h o t o m u l t i p l i e r tubes, or l i n e a r p h o t o d i o d e a r r a y s . R a t h e r , t h e SIT v i d i c o n i s c o m p l e m e n t a r y t o t h e more t r a d i t i o n a l s p e c t r o s c o p i c d e t e c t o r s . Operation

of t h e SIT

Vidicon

A l t h o u g h t h e o p e r a t i o n o f v i d i c o n d e t e c t o r s h a s been d e s c r i b e d e l s e w h e r e (_l-_4), we b r i e f l y s u m m a r i z e i t h e r e s i n c e t h e v i d i c o n components and o p e r a t i o n a f f e c t t h e c h a r a c t e r i s t i c s o f t h e S I T v i d i c o n as a s p e c t r o s c o p i c d e t e c t o r . S i m i l a r to other i m a g i n g d e t e c t o r s , t h e s i l i c o n v i d i c o n i s made up o f t h r e e m a i n components. A t r a n s d u c e r c o n v e r t s t h e p h o t o n s p e c t r a l image to i t s corresponding e l e c t r i c a l analog. I n the s i l i c o n v i d i c o n the s i l i c o n t a r g e t performs t h i s f u n c t i o n w h i l e i n t h e s i l i c o n i n t e n s i f i e d t a r g e t v i d i c o n , shown i n F i g u r e 1, t h e s c i n t i l l a t o r and p h o t o c a t h o d e a c t as t h e t r a n s d u c e r . The s i l i c o n t a r g e t a c t s as a s t o r a g e d e v i c e f o r t h e e l e c t r i c a l a n a l o g o f t h e s p e c t r a l image. The t a r g e t i s b u i l t on an η-type s i l i c o n w a f e r w h i c h s e r v e s as a common c a t h o d e . P - t y p e s e m i c o n d u c t o r anodes a r e grown on t h e w a f e r , a p p r o x i m a t e l y e i g h t m i c r o n s apart. The v i d i c o n s i g n a l i s a c q u i r e d by an e l e c t r o n beam, t w e n t y - f i v e m i c r o n s i n d i a m e t e r , w h i c h a c t s as t h e r e a d i n g device . The o p e r a t i o n o f a s i l i c o n v i d i c o n c o n s i s t s o f p r e p a r a t i o n , e x p o s u r e , d e v e l o p m e n t and r e a d i n g s t e p s . The t a r g e t i s r e s t o r e d t o an e q u i l i b r i u m p o t e n t i a l by s c a n n i n g t h e e l e c t r o n beam t o charge the p-type i s l a n d s t o a negative p o t e n t i a l , e r a s i n g p r e v i o u s l y s t o r e d i m a g e s . The η-type s u b s t r a t e i s h e l d a t ground p o t e n t i a l , r e s u l t i n g i n a r e v e r s e d - b i a s e d diode condition. T h i s p r e p a r e s t h e t a r g e t f o r e x p o s u r e by p r o d u c i n g a d e p l e t i o n l a y e r w h i c h a c t s as a s t o r a g e c a p a c i t o r . P h o t o e l e c t r o n s imaged on t h e t a r g e t p r o d u c e e l e c t r o n - h o l e p a i r s . The h o l e s d i f f u s e t h r o u g h t h e d e p l e t i o n l a y e r i n t o t h e p - t y p e i s l a n d s , d i s c h a r g i n g t h e c a p a c i t o r s . The d e v e l o p m e n t s t e p i s p e r f o r m e d by s c a n n i n g t h e t a r g e t w i t h t h e e l e c t r o n beam, t h e v i d i c o n analog t o a photographic developer. The change i n c u r r e n t due t o r e c h a r g i n g o f e a c h c a p a c i t o r i s a m p l i f i e d t o become t h e v i d e o s i g n a l . Since t h i s current i s proportional to


Talmi; Multichannel Image Detectors Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 1983. IMAGE SECTION

HIGH VELOCITY PHOTOELECTRON BEAM IMAGED ON TARGET

ANOOE

SCANNING SECTION

^SILICON TARGET LOW VELOCITY I SCANNING BEAM

FIELD MESH GUN FOCUSING GRID (#4) GRID (#3)

CONTROL GRIO (#1)

ELECTRON GUN

CATHODE

ACCELERATING GRID (#2)

Figure 1. Illustration of the silicon-intensified target (SIT) vidicon. (Reproduced with permission from Ref. 8. Copyright 1978, Princeton Applied Research Corporation.)

FIBER OPTIC FACEPLATE

LIGHT

IMAGE FOCUS GRIDS-, PHOTOCATHODE



34


t h e amount o f c a p a c i t o r r e c h a r g i n g n e c e s s a r y t o r e a t t a i n t h e e q u i l i b r i u m p o t e n t i a l , and t h e c a p a c i t o r s a r e d i s c h a r g e d i n p r o p o r t i o n t o t h e number o f p h o t o e l e c t r o n s , t h e r e c h a r g i n g c u r r e n t i s p r o p o r t i o n a l t o t h e i n c i d e n t l i g h t i n t e n s i t y . The v i d e o s i g n a l i s d i g i t i z e d and s t o r e d i n t h e c o m p u t e r random a c c e s s memory a n d / o r d i s p l a y e d on a CRT. The i n t e n s i f i e r s t a g e i n c r e a s e s t h e s e n s i t i v i t y o f t h e S I T v i d i c o n o v e r an u n i n t e n s i f i e d s i l i c o n v i d i c o n d e t e c t o r . The p h o t o e l e c t r o n s a r e a c c e l e r a t e d and e l e c t r i c a l l y f o c u s e d o n t o t h e t a r g e t by an e l e c t r o s t a t i c f i e l d o f s e v e n t o n i n e k i l o v o l t s . A p p r o x i m a t e l y 1500 e l e c t r o n - h o l e p a i r s a r e p r o d u c e d f o r e a c h e l e c t r o n which s t r i k e s the s i l i c o n t a r g e t . However, b e c a u s e t h e p h o t o c a t h o d e h a s a much l o w e r quantum e f f i c i e n c y t h a n t h e s i l i c o n t a r g e t , t h e p h o t o s i g n a l g a i n o f t h e SIT v i d i c o n i s o n l y a p p r o x i m a t e l y two h u n d r e d t i m e s t h a t o f t h e u n i n t e n s i f i e d s i l i c o n v i d i c o n (_3) a n d s u b s t a n t i a l l y l e s s i n t h e u l t r a v i o l e t region. The s e n s i t i v i t y o f t h e SIT v i d i c o n w i t h a s c i n t i l l a t o r has b e e n compared (_5) t o a 1P28 p h o t o m u l t i p l i e r t u b e i n t e g r a t e d s i g n a l . The SIT v i d i c o n s e n s i t i v i t y was f o u n d t o be s i m i l a r t o a 1P28 p h o t o m u l t i p l i e r t u b e f r o m 400 t o 600 nm, somewhat more s e n s i t i v e f r o m 600 t o 800 nm and a p p r o x i m a t e l y t w e n t y t i m e s l e s s s e n s i t i v e f r o m 200 t o 400 nm. W i t h . t h e new s c i n t i l l a t o r i n t h e OMA I I s y s t e m , t h e d i f f e r e n c e i n s e n s i t i v i t y i n the u l t r a v i o l e t r e g i o n i s f i v e t o t e n times l e s s than the 1P28 PMT. The s c i n t i l l a t o r i s n e c e s s a r y b e c a u s e o f t h e o t h e r w i s e p o o r r e s p o n s e b e l o w 400 nm, due m a i n l y t o l o s s e s i n t h e f i b e r optic faceplate. The f i b e r o p t i c f a c e p l a t e i s n e c e s s a r y b e c a u s e a curved surface i s required f o r e l e c t r o s t a t i c imaging i n the i n t e n s i f i e r s e c t i o n ( 6 0 o f t h e SIT v i d i c o n tube. The SIT v i d i c o n d e t e c t i o n c a n be g a t e d by c o n t r o l l i n g t h e i n t e n s i f i e r s t a g e i n a manner s i m i l a r t o a t r i o d e t u b e i n t h e g r o u n d e d g r i d c o n f i g u r a t i o n (7_) . A n e g a t i v e v o l t a g e p u l s e (-500 t o -1200 v o l t s ) i s a p p l i e d t o t h e p h o t o c a t h o d e t o t j i r n t h e t u b e o n . The o n / o f f g a t i n g r a t i o o f t h e d e t e c t o r i s 10 t o 10 . C o n t r o l of t h e SIT v i d i c o n . Regions of the v i d i c o n target t o be r e a d o u t , and t h e r e f o r e t h e s p e c t r a l a n d / o r s p a t i a l r e g i o n s t o be o b s e r v e d , a r e c o n t r o l l e d by s c a n n i n g o f t h e e l e c t r o n beam. I n t h e PARC OMA I I s y s t e m t h e e l e c t r o n beam s c a n n i n g i s u n d e r s o f t w a r e c o n t r o l . A 500 χ 500 a r r a y o f p o s i t i o n s on t h e v i d i c o n t a r g e t ( p i x e l s , o r p i c t u r e e l e m e n t s ) c a n be r a n d o m l y a d d r e s s e d and r e a d . I n d i v i d u a l r e g i o n s o f t h e t a r g e t t o be r e a d o u t , c a l l e d t r a c k s , a r e d e f i n e d by a n u m b e r o f c h a n n e l s ( c o l u m n s of p i x e l s ) o f a c h o s e n h e i g h t , y. A f r a m e c o n s i s t s o f one r e a d o u t s c a n o f t h e s e l e c t e d number o f t r a c k s . The e x p o s u r e t i m e i s d e t e r m i n e d b y t h e c h a n n e l t i m e , t h e t i m e t h e e l e c t r o n beam t a k e s t o r e a d e a c h c h a n n e l , t h e number o f c h a n n e l s r e a d o u t , t h e number o f c h a n n e l s g r o u p e d t o g e t h e r a s one r e s o l u t i o n e l e m e n t , a n d t h e n u m b e r o f s c a n s w h i c h a r e


2.

OLESIK A N D WALTERS

Silicon-Intensified

Target Vidicon

Detector

35

accumulated. The e x p o s u r e t i m e c a n a l s o be c o n t r o l l e d b y a s o f t w a r e - c o n t r o l l e d d e l a y b e f o r e t h e t a r g e t i s r e a d . However, c o o l i n g o f t h e v i d i c o n i s r e q u i r e d f o r d e l a y t i m e s o f more t h a n 100 ms a s d i s c u s s e d b e l o w .


V i d i c o n Components

Effect

on D e t e c t i o n

The most o b v i o u s l i m i t a t i o n o f v i d i c o n d e t e c t o r s i s t h e s m a l l d e t e c t o r a r e a . T y p i c a l l y , t h e SIT v i d i c o n d e t e c t o r a r e a i s 12.5 by 12.5 mm. However, t h e u s e f u l d e t e c t o r a r e a may be f u r t h e r l i m i t e d by d i s t o r t i o n w h i c h i s most s e v e r e n e a r t h e edges o f t h e d e t e c t o r , a s w i l l be d i s c u s s e d b e l o w . I fa p o l y c h r o m a t o r w i t h a d i s p e r s i o n o f 0.83 nm/mm i s u s e d , a w a v e l e n g t h r a n g e o f 10 nm c a n be m o n i t o r e d when t h e f u l l w i d t h o f t h e v i d i c o n i s used. Silicon target. The use o f a s i l i c o n t a r g e t f o r storage o f t h e e l e c t r i c a l s i g n a l image r e s u l t s i n a n i n t e g r a t i n g d e t e c t o r s e n s i t i v e t o e n e r g y r a t h e r t h a n power. The t a r g e t i s t h e m a i n s o u r c e o f d a r k c u r r e n t and l i m i t s t h e o n - t a r g e t s t o r a g e t i m e t o a p p r o x i m a t e l y one h u n d r e d m i l l i s e c o n d s a t room t e m p e r a t u r e ( V ) . The s t o r a g e t i m e c a n be i n c r e a s e d t o up t o two h o u r s by c o o l i n g t h e d e t e c t o r (_9 ,_10 ) . H o w e v e r , c o o l i n g i s i n c o n v e n i e n t due t o the s i z e o f t h e v i d i c o n d e t e c t o r ( i n comparison t o a photodiode a r r a y , f o r example). Some c o m m e r c i a l l y a v a i l a b l e c o o l e r s a r e c o n s t r u c t e d so t h a t t h e f o c a l p l a n e o f t h e p o l y c h r o m a t o r used must be t h r e e i n c h e s o u t s i d e o f where t h e c o o l e r and v i d i c o n a r e attached t othe polychromator housing o f t e n r e q u i r i n g a s p e c i a l l y designed polychromator o r a d d i t i o n a l o p t i c s . This i s due t o t h e need t o i n s u l a t e t h e v i d i c o n c o o l e r and p r e v e n t f r o s t i n g o f t h e window b e t w e e n t h e p o l y c h r o m a t o r and t h e vidicon. Cooling also results i n substantially increased l a g , w h i c h i s d i s c u s s e d below. Because t h e t a r g e t r a t h e r than t h e photocathode i s t h e main dark c u r r e n t source, t h e dark c u r r e n t s i g n a l i s independent o f the gate w i d t h used. The random n o i s e component o f t h e d a r k current can l i m i t the d e t e c t a b i l i t y o f l i g h t s i g n a l s . Multiple s c a n s o r a s y n c h r o n o u s a v e r a g i n g i n c r e a s e s d e t e c t a b i l i t y and s i g n a l - t o - n o i s e (_10 a s s h o w n i n F i g u r e 2. Blooming. M i g r a t i o n o f c h a r g e o n t h e s i l i c o n t a r g e t a n d , more i m p o r t a n t l y , u s e o f e l e c t r o n beam r e a d o u t r e s u l t s i n b l o o m i n g , o r c r o s s t a l k , b e t w e e n c h a n n e l s . The c r o s s t a l k i n t h e PARC s u p p l i e d S I T v i d i c o n i s s p e c i f i e d ( 8 ) t o be l e s s t h a n 60% b e t w e e n a c e n t r a l c h a n n e l and e a c h o f i t s two n e i g h b o r s . T a b l e I shows t h e measured b l o o m i n g f r o m a c e n t r a l c h a n n e l t o nearby channels. Blooming l i m i t s t h e r e s o l u t i o n o b t a i n a b l e w i t h v i d i c o n detectors. F o r e x a m p l e , i f a n a t o m i c l i n e w i t h a 0.02 nm l i n e w i d t h i s o b s e r v e d t h r o u g h a p o l y c h r o m a t o r w i t h 0.83 nm/mm


36

MULTICHANNEL IMAGE DETECTORS A

Β

C

D


©

Figure 2. Vidicon detected, copper hollow cathode spectra showing an increase in signalto-noise ratio as the detection time and number of readout scans is increased. Key: A, signalfrom a single readout scan (detection time, 16.6 ms); B, signalfrom 10 readout scans (detection time, 166 ms); C, signalfrom 100 readout scans (detection time, 1.66 s); and D, signalfrom 1000 readout scans (detection time, 16.6 s). The Y axes for all spectra are in arbitrary intensities units. The spectral lines observed are: 1, 329.30 nm; 2, 329.77 nm; 3, 330.80 nm; and 4, 331.13 nm. Detector conditions were: 80 μ$/channel readout; 500 channels read out; 10 prep scans before each spectrum was obtained; and delta Y - 100.

Table I .

Channel center

+ + + + + + + -

0 1 1 2 2 3 3 4 4 5 5 6 6 7 7

from

Blooming

Intensity

19935 10435 11133 2481 2360 624 622 251 399 92 346 62 185 43 99

i n SIT v i d i c o n . *

Percentage of center channel i n t e n s i t y 100.0% 52.3 55.8 12.4 11.8 3.1 3.1 1.3 2.0 0.4 1.7 0.0 0.1 0.0 0.0

* B l o o m i n g was m e a s u r e d by u s i n g an i r o n h o l l o w c a t h o d e lamp s o u r c e , a 20 m i c r o n e n t r a n c e s l i t on a M c P h e r s o n 1 m e t e r C z e r n y T u r n e r p o l y c h r o m a t o r u s e d i n f i r s t o r d e r . The 352.126 nm i r o n I l i n e was c e n t e r e d on a s i n g l e c h a n n e l (X = 2 3 0 ) . F i v e h u n d r e d c h a n n e l s , e a c h 80 p i x e l s h i g h (Y = 210 t o 290) were m o n i t o r e d i n PARC t i m i n g mode 1 and s c a n mode 1 ( n o r m a l ) . One h u n d r e d r e a d o u t s c a n s w e r e p e r f o r m e d a f t e r two p r e p a r a t i o n s c a n s w i t h a 20 m i c r o s e c o n d / c h a n n e l r e a d o u t t i m e . (Other channel times of 40, 6 0 , 8 0 , 100, 120 and 140 m i c r o s e c o n d s were a l s o u s e d w i t h s i m i l a r blooming r e s u l t i n g . )


2.

OLESIK AND WALTERS

Silicon-Intensified

Target Vidicon

Detector

37


d i s p e r s i o n , t h e measured h a l f - w i d t h o f t h e l i n e w i l l be a p p r o x i m a t e l y 0.06 nm, due t o t h e 6 0 % c r o s s t a l k t o n e i g h b o r i n g c h a n n e l s . T h i s e f f e c t w i l l be much l e s s s e v e r e f o r s i g n a l s w i t h larger linewidths. B l o o m i n g o c c u r s i n b o t h d i m e n s i o n s so t h a t i f t h e second dimension o f t h e v i d i c o n d e t e c t o r i s used f o r s p a t i a l r e s o l u t i o n , i t t o o w i l l be l i m i t e d by s i g n a l b l o o m i n g . The e f f e c t s o f b l o o m i n g a r e most s e v e r e when a weak s i g n a l o f i n t e r e s t i s o b s e r v e d n e a r an i n t e n s e s i g n a l o r when l a r g e changes i n i n t e n s i t y o c c u r o v e r s h o r t image d i s t a n c e s . Lag due t o i n c o m p l e t e s i g n a l r e a d o u t f r o m t h e t a r g e t . When t h e e l e c t r o n beam s c a n s o v e r t h e t a r g e t , t h e s i g n a l i s n o t c o m p l e t e l y r e a d o u t i n a s i n g l e p a s s . T h i s phenomenon, c a l l e d d i s c h a r g e l a g , i n a s i m p l i s t i c v i e w , i s due t o t h e r e s i s t a n c e o f t h e r e a d i n g e l e c t r o n beam, c a p a c i t a n c e o f t h e s i l i c o n t a r g e t , and t h e c a p a c i t i v e r e c h a r g e n a t u r e o f t h e s i g n a l r e a d o u t . The l a g depends o n t h e s i g n a l i n t e n s i t y (4_,_L2) b e c a u s e t h e t a r g e t c a p a c i t a n c e i s a f u n c t i o n o f t h e c h a r g e p r e s e n t on i t . The percentage of t h e s i g n a l read out i n each pass i s a f u n c t i o n o f t h e d w e l l t i m e , o r t h e t i m e t h e r e a d o u t beam s p e n d s a t e a c h p i x e l , t h e e l e c t r o n beam c u r r e n t ( L 2 ) , and t h e r e l a t i o n s h i p between t h e t a r g e t and beam v o l t a g e s . S i n c e t h e s c a n r a t e i s c o n t r o l l e d by t h e r e a d o u t t i m e p e r c h a n n e l , t h e d w e l l t i m e i s a f u n c t i o n o f t h e c h a n n e l t i m e and t h e number o f p i x e l s m a k i n g up each channel : d w e l l time = channel time / p i x e l s per channel. The l a g was measured f o r a number o f d i f f e r e n t d w e l l t i m e s . The r e s u l t s a r e shown i n F i g u r e 3. L a g l i m i t s t h e r a t e a t w h i c h t h e v i d i c o n t a r g e t c a n be r e a d o u t , and t h e r e f o r e , t h e t i m e r e s o l u t i o n a t t a i n a b l e i n t h e t i m e - r e s o l v e d , o r r e a l - t i m e , mode. A p p r o x i m a t e l y 30 s p e c t r a o f 500 c h a n n e l s e a c h c a n be a c q u i r e d i n a o n e - s e c o n d p e r i o d . S p e c t r a c a n be o b t a i n e d more r a p i d l y i f t h e s i z e o f t h e t a r g e t r e g i o n s c a n n e d , and t h e r e f o r e t h e w a v e l e n g t h r a n g e m o n i t o r e d , are reduced. With pulsed or t r a n s i e n t i l l u m i n a t i o n , l a g also r e s u l t s i n a n o n l i n e a r d e t e c t o r response t o i n c i d e n t l i g h t i n t e n s i t y . The e f f e c t s o f l a g on t h e l i n e a r i t y o f d e t e c t o r r e s p o n s e c a n be r e d u c e d by u s i n g m u l t i p l e r e a d o u t s c a n s , l o n g d w e l l t i m e s , b o t h r e s u l t i n g i n l o n g e r r e a d o u t t i m e s , o r o t h e r schemes (13,14, I D

· The v i d i c o n c a n be c o o l e d i n o r d e r t o r e d u c e t h e t h e r m a l l y generated dark c u r r e n t , a l l o w i n g long i n t e g r a t i o n times t o m o n i t o r weak s i g n a l s . However, l a g i s g r e a t l y i n c r e a s e d when t h e d e t e c t o r i s c o o l e d . As a r e s u l t , e r a s u r e o f p r e v i o u s images and t a r g e t p r e p a r a t i o n becomes e x t r e m e l y i m p o r t a n t . The e f f e c t s o f l a g c a n be m i n i m i z e d by u s i n g p r e i l l u m i n a t i o n . I n commercial c o o l e d h o u s i n g s , a l i g h t e m i t t i n g d i o d e (LED) i s p o s i t i o n e d n e a r




6000 5000 4000 3000 2000 1000 J

L 10

NUMBER OF READOUT SCANS Figure 3. Relative integrated signal showing the effect of lag as a function of dwell time controlled by channel time. Each channel consisted of500 pixels (delta Y- 500). Signal is due to a single pulse of a Xe strobe lamp (General Radio, 1539-A Stroboslave). Light was detected in zero order with a 20^m entrance slit. Dwell time: o, 40 ns; •, 80 ns; A, 160 ns; and o, 240 ns.


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39


t h e v i d i c o n f a c e . The LED i s f l a s h e d t o c o m p l e t e l y d i s c h a r g e t h e s i l i c o n diodes i n o r d e r t o t o t a l l y erase p r e v i o u s images. Readout scans a r e then used t o p r e p a r e the t a r g e t f o r exposure. A l t e r n a t i v e l y , t h e e l e c t r o n beam p o t e n t i a l c a n be s h i f t e d between t h e p r e p a r a t i o n and r e a d i n g p o t e n t i a l l e v e l s t o reduce t h e e f f e c t s of l a g ( 13, 14,15). E l e c t r o s t a t i c image intensifier» The i n t e n s i f i e r s t a g e o f t h e SIT v i d i c o n i n c r e a s e s s e n s i t i v i t y t h r o u g h e l e c t r o n g a i n and provides time g a t i n g c a p a b i l i t y . V a r i a b l e g a i n i s p o s s i b l e by a d j u s t m e n t o f t h e c a t h o d e v o l t a g e . However, e l e c t r o s t a t i c f o c u s i n g o f t h e i n t e n s i f i e d image i n t r o d u c e s l o s s e s i n r e s o l u t i o n and s e n s i t i v i t y n e a r t h e t a r g e t edges due t o p i n c u s h i o n d i s t o r t i o n (JJ>_,r7) o f t h e i m a g e . The d i s t o r t i o n a l s o r e s u l t s i n a n o n l i n e a r w a v e l e n g t h a x i s when an SIT v i d i c o n i s p l a c e d at the f o c a l plane of a polychromator. The n o n l i n e a r i t y o f 2 t o 3% c a n be c a l i b r a t e d and a f i t t e d e q u a t i o n g e n e r a t e d r e l a t i n g t h e c h a n n e l number t o w a v e l e n g t h . The s o f t w a r e f o r g e n e r a t i o n o f t h e c h a n n n e l t o w a v e l e n g t h f u n c t i o n i s i n c l u d e d i n t h e PARC OMA I I s y s t e m . Image f i d e l i t y when t h e SIT v i d i c o n i s u s e d i n t h e c o n t i n u o u s mode was t e s t e d by i m a g i n g a o n e - h u n d r e d t w e n t y - f i v e m i c r o n s c r e e n , b a c k l i t by a c o p p e r h o l l o w c a t h o d e lamp, on a 0.5 mm w i d e e n t r a n c e s l i t o f t h e o n e - m e t e r C z e r n y - T u r n e r monochromator. The t w e n t y - f i v e c e n t i m e t e r f o c a l l e n g t h q u a r t z i m a g i n g l e n s was p o s i t i o n e d f o r one t o one m a g n i f i c a t i o n a t 327.4 nm. The r e s u l t i n g image a t t h e f o c a l p l a n e was d e t e c t e d p h o t o g r a p h i c a l l y and w i t h t h e SIT v i d i c o n as shown i n F i g u r e 4. D i s t o r t i o n near the c e n t e r of the v i d i c o n t a r g e t i s l e s s than f i f t y m i c r o n s (2 c h a n n e l s ) . However as l a r g e r c h a n n e l h e i g h t s a r e u s e d d i s t o r t i o n i n c r e a s e s as shown i n F i g u r e 5 and T a b l e I I . The d i s t o r t i o n i s due t o a change i n m a g n i f i c a t i o n which causes a skewing of s p e c t r a l l i n e s toward the o u t s i d e of t h e t a r g e t a t l a r g e c h a n n e l h e i g h t s , as shown i n F i g u r e 6. I n t h e t i m e - g a t e d mode t h e d i s t o r t i o n due t o t h e e l e c t r o n i m a g i n g i s more s e v e r e . F o c u s i n g o f t h e p h o t o e l e c t r o n s i n t h e SIT v i d i c o n i s a f u n c t i o n o f t h e g a t e p u l s e w i d t h and v o l t a g e . T h e r e f o r e , f o r e a c h new t i m e g a t e - w i d t h t h e g a t e p u l s e v o l t a g e must be r e o p t i m i z e d . F o c u s i n g i s a l s o s e n s i t i v e t o t h e i n c i d e n t light i n t e n s i t y l e v e l f o r short gate widths (7,18). The r a t i o of photocathode v o l t a g e t o gate v o l t a g e c o n t r o l s the e l e c t r o n image f o c u s i n g and m a g n i f i c a t i o n . As a r e s u l t , s e v e r e p i n c u s h i o n d i s t o r t i o n o c c u r s when t h e g a t e t i m e i s o f t h e same m a g n i t u d e as t h e r i s e and f a l l t i m e s o f t h e g a t e p u l s e o r when t h e g a t e p u l s e has r i n g i n g o r a s l o p i n g v o l t a g e (7). The e f f e c t on t h e o b s e r v e d s p e c t r u m w i l l be most s e v e r e i f t h e i n c i d e n t l i g h t p u l s e i s l o n g e r t h a n t h e g a t e w i d t h . The e f f e c t at s h o r t g a t e t i m e s c a n be m i n i m i z e d by m o d i f i c a t i o n o f t h e g a t i n g c i r c u i t (7_) o r c o m b i n i n g a n o p t i c a l g a t e , s u c h as a P o c k e l s c e l l , w i t h t h e e l e c t r o n i c g a t e o f t h e SIT v i d i c o n ( 1 8 ) .



1

' 50

' 347

' 1

100

349 1

Γ

150

35Ι

'

200

' 353

1

χ

x

(fim)

CHANNEL

Figure 4. SIT vidicon detected signalfrom a 250 750-μιη portion of the image of a backlit 125-pm screen imaged with unity magnification at the focal plane. The SIT vidicon signal was obtained with the maximum attainable detector spatial resolution, with each picture element, or with pixel, 25 μm in diameter.

0

345

INTENSITY


(X)


2.

OLESIK AND

Silicon-Intensified

WALTERS

ι

1

0

100

Target Vidicon

1

1

200

300

r-

400

Detector

41

1

500

CHANNEL (X) Figure 5. Continuous (nongated) SIT vidicon video signal showing pincushion of the image of emission lines from an iron hollow cathode lamp.

Table I I .

distortion

D i s t o r t i o n o f t h e SIT v i d i c o n d e t e c t e d image i n t h e c o n t i n u o u s ( n o n g a t e d ) mode.* X

(Channel)

29

111

157

265

374

414

468

-5 -2 0 -4 -8 -13

-2 0 +1 -1 -5 -9

-2 0 0 -2 -4 -7

-1 0 0 -1 -1 -2

+1 0 0 0 0 +1

+3 0 0 -1 0 +1

+5 +1 +1 0 +1 +2

Y 75-125 125-175 175-225 325-375 375-425 425-475

A one m e t e r * An i r o n h o l l o w c a t h o d e lamp s o u r c e was u s e d . monochromator ( M c P h e r s o n 2051) was used w i t h a 20 m i c r o n E a c h s p e c t r u m was a c q u i r e d w i t h a s c a n h e i g h t entrance s l i t . ( d e l t a Y) of 50. B e l o w are l i s t e d t h e d i f f e r e n c e s b e t w e e n o b s e r v e d c h a n n e l of peak i n t e n s i t y and t h e c h a n n e l o f peak i n t e n s i t y f o r the c e n t e r e d (Y = 225-275) s c a n .




Ο Ο Ο

IlJlL

ILLLJ 100

200

300

CHANNEL

Ο Ο Ο

400

500

(X)

ο ~ ο

CO LU

100

200

300

CHANNEL

400

500

(Χ)

Ο Ο Ο

CO

U U

UJJI ιοο

200

300

CHANNEL

400

500

(Χ)

Figure 6. The effect of pincushion distortion on detected spectra. Key: A, spectrum obtained reading out the center 2.4 mm (delta Y- 100) of the vidicon target; B, spectrum obtained reading out the center 7.2 mm (delta Y- 300); and C, spectrum obtained reading out the complete 12.5 mm (delta Y - 500).



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W h i l e t h e s e v e r e p i n c u s h i o n d i s t o r t i o n was e x p e c t e d a t s h o r t g a t e t i m e s (100 ns o r l e s s ) , and has b e e n r e p o r t e d e a r l i e r ( 1 8 ) , we a l s o o b s e r v e d s e v e r e p i n c u s h i o n d i s t o r t i o n a t l o n g e r g a t e t i m e s . T h i s d i s t o r t i o n was o b s e r v e d w i t h a s p a r k d i s c h a r g e l i g h t s o u r c e w i t h a d u r a t i o n of a p p r o x i m a t e l y one-hundred m i c r o s e c o n d s and t o a l e s s e r e x t e n t w i t h a h e l i u m - n e o n l a s e r s o u r c e . The d i s t o r t i o n was most s e v e r e w i t h a o n e - m i c r o s e c o n d g a t e and l e s s s e v e r e a t l o n g e r and s h o r t e r g a t e w i d t h s , as shown i n F i g u r e 7. A f t e r c o n s i d e r a b l e e f f o r t and i n v e s t i g a t i o n we have b e e n u n a b l e t o i d e n t i f y t h e c a u s e o f t h i s d i s t o r t i o n o r t o reduce i t . U s i n g t h e PARC OMA I I s y s t e m t h e p u i s e r d u t y c y c l e i s l i m i t e d t o 0.1% i n t h e 10 n s e c t o 99.9 m i c r o s e c o n d r a n g e . The d u t y c y c l e i s l i m i t e d t o 1.0% i n t h e 1 m i c r o s e c o n d t o 999 microsecond range. T h e r e f o r e , when a o n e - h a l f m i c r o s e c o n d g a t e w i d t h i s u s e d , t h e maximum f r e q u e n c y o f g a t e s i s 200 Hz. When a o n e - h a l f m i l l i s e c o n d gate w i d t h i s used the d e t e c t i o n gate can be on o n l y t w i c e p e r s e c o n d . Channel-to-channel and p i x e l - t o - p i x e l r e s p o n s e . Variation i n c h a n n e l - t o - c h a n n e l and p i x e l - t o - p i x e l r e s p o n s e was m e a s u r e d u s i n g an Ε G & G model 590 c a l i b r a t e d , c o n t i n u o u s s o u r c e lamp system. W h i l e c h a n g e s i n r e s p o n s e a c r o s s t h e t a r g e t were w e l l w i t h i n t h e +/- 10% r a n g e s p e c i f i e d by PARC w i t h i n t h e c e n t e r 10 mm χ 10 mm a r e a (400 c h a n n e l s ) o f t h e t a r g e t , t h e r e was a d e f i n i t e dependence o f b o t h s p e c t r a l r e s p o n s e and c h a n n e l - t o c h a n n e l r e s p o n s e p r e c i s i o n on s p a t i a l p o s i t i o n , as s e e n i n F i g u r e 8. P i x e l - t o - p i x e l v a r i a t i o n was m e a s u r e d a t χ = 150 and 250 on t h e SIT t a r g e t among 10 p i x e l s a t e a c h χ p o s i t i o n . Relative s t a n d a r d d e v i a t i o n s i n t h e i n t e n s i t i e s w e r e f o u n d t o be 3.3% and 3.5% r e s p e c t i v e l y . A t r a d e o f f between t h e s e n s i t i v i t y i n t h e c e n t e r o f t h e d e t e c t o r and t h e u n i f o r m i t y a c r o s s t h e d e t e c t o r c a n be made (14) by a d j u s t i n g t h e i m a g e i n t e n s i f i e r f o c u s v o l t a g e and t h e e l e c t r o n gun f o c u s v o l t a g e (gun f o c u s i n g g r i d G3 and image f o c u s i n g g r i d s as shown i n F i g u r e 1 ) . F o r t h e measurements shown i n F i g u r e 8 t h e f o c u s i n g v o l t a g e s w e r e a d j u s t e d f o r an optimum c o m b i n a t i o n o f s e n s i t i v i t y and u n i f o r m i t y o f s e n s i t i v i t y . Any c h a n g e s i n réponse f r o m c h a n n e l - t o - c h a n n e l c a n be c o m p e n s a t e d by n o r m a l i z i n g e a c h s p e c t r u m a c q u i r e d u s i n g a s t a n d a r d s p e c t r u m s t o r e d i n t h e computer memory. As a r e s u l t , c h a n g e s i n r e s p o n s e o f t h e o p t i c a l s y s t e m , s u c h as g r a t i n g e f f i c i e n c y o r d e t e c t o r r e s p o n s e c a n be c o m p e n s a t e d i f known o r calibrated. Examples o f

E x p e r i m e n t s E m p l o y i n g SIT

Vidicon

Detection

We h a v e u s e d t h e SIT v i d i c o n d e t e c t i o n s y s t e m i n a number o f separate experiments. D e t e c t i o n requirements i n these




44

Figure 7. Distortion detected for the gated SIT-vidicon spark-excited spectra at gate times near 1 μ$. Key to detection gate width: A, 6.0 μ*; Β, 1.0 μ*; and C, 0.1 μ*.

100

200

300

400

CHANNEL (Χ)


500

2.

OLESIK A N D WALTERS

Silicon-Intensified

Target

Vidicon

45

Detector

4500 r-


4000

UJ

È

3500

UJ

3000

100

I ιI -L 2 0 0 300 400 500

STARTING CHANNEL

UJ CO

ζ ο

CL CO UJ

2.3

U

18

μ

oc

Multichannel Image Detectors Volume 2 - ACS Publications

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