Results of intial performance tests on X-ray sensing properties of charge-coupled devices (CCDs) are presented. CCDs have demonstrated excellent spatial resolution and good spectral resolution, superior to that of non-imaging proportional counters

Garmire, G. P.

Liewer, K.

Nousek, J. A.

Riegler, G. R.

Stern, R. A.

Vescelus, F.

English

NASA Technical Reports Server

PERFORMANCE OF SINGLE PHOTON-COUNTING X-RAY CHARGE COUPLED DEVICES 
G .  R .  Riegler 
R .  A. S t e m  
K .  L5ewer 
F .  Vesct?lus 
J e t  Propulsion Laboratory 
California Ins t i tu te  of Technology 
Pasadena, CA 91109 
J .  A. Nousek and G .  P .  Garrnire 
Department of Astronom;? 
Pennsylvania State University 
State College, Pennsylvania 
https://ntrs.nasa.gov/search.jsp?R=19820018191 2020-03-21T07:36:59+00:00Z
290 
PERFORMANCE OF SINGLE PHOTON-COUNTIN3 X-RAY CHARGE COUPLED DEVICES 
G. R. Riegler ,  R. A. Stern,  K. Liewer, and F. Vescelus 
Pasadena, CA 91109 
Jet Propulsion Laboratory, Ca l i fo rn ia  I n s t i t u t e  of Technology 
and 
J. A. Nousek and G. P. Garmire 
S t a t e  College, Pennsylvania 
Department of Astronomy, Pennsylvania S t a t e  Universi ty  
ABSTRACT 
Resul ts  of i n i t i a l  performance tests on x-ray sensing p rope r t i e s  of 
charge-coupled devices  (CCDs) are presented. CCDs have demonstrated ex- 
c e l l e n t  spa t ia l  r e so lu t ion  and good s p e c t r a l  reso lu t ion ,  super ior  t o  t h a t  
of non-imaging proport ional  counters .  
I. INTRODUCTION 
A charged coupled device (CCD) can be expected t o  combine t h e  bes t  
p rope r t i e s  of high-resolut ion imaging de tec to r s  and so l id - s t a t e  spectro-  
meters, namely high s p a t i a l  reso lu t ion  and moderate s p e c t r a l  reso lu t ion ,  
a t  high e f f ic iency .  Because of t h e  importance of such a device f o r  x-ray 
astronomy, a number of researchers  have begun laboratory s t u d i e s  of x-ray 
CCDs ( G r i f f i t h s  et .  al.  1981 and re ference  the re in ) .  This  paper de- 
s c r i b e s  t h e  s t a t u s  of x-ray CCD s t u d i e s  a t  t h e  Jet Propulsion Laboratory. 
Elsewhere wi th in  t h i s  conference, reviews of t h e  known prope r t i e s  and 
open quest ions f o r  a wide range of g a l a c t i c  and ex t ra -ga lac t ic  sources  
have been presented. 
EIAE - 40 a t  E " 6 keV, which CCDs are expected t o  provide. The a b i l i t y  
t o  produce a spectrum f o r  each image element (p ixe l )  is important f o r  t h e  
s t u d i e s  of supernova remnants, c l u s t e r s  of ga lax ies ,  and wherever source 
confusion would be a problem f o r  non-imaging s o l i d  state spectrometers.  
These reviews have c a l l e d  for moderate r e so lu t ion ,  e . @ . ,  
11. DESCRIPTION OF THE TEST CAMERA 
Several  CCD cameras have been b u i l t  a t  JPL  f o r  t h e  study of CCDs and 
f o r  t he  t e s t i n g  of CCDs t o  be used by the  Gal i leo  J u p i t e r  o r b i t e r  and t h e  
Wide Fie ld  Planetary Camera of t h e  Space Telescope. 
i n  these  tests, one f o r  t h e  v i r t u a l  phase devices (Janesick e t  a l . ,  1981) 
and the  o ther  f o r  t he  th ree  phase devices (Blauke e t  a l . ,  1981). 
and amplitude of t he  clock pulses  required t o  s h i f t  t he  da ta  out of the  two 
types of CCDs d i f f e r ,  but  a l l  the  ampl i f f e r s  and s i g n a l  processing c i r c u i t s  
are ideiit ical  . 
Two cameras were used 
The timing 
The s igna l  processing c i r c u i t r y  uses  a double co r re l a t ed  sampler where 
output is d i g i t i z e d  t o  16 b i t s .  
5 x lo4 p i x e l s  per  second which minimizes t h e  noise  assoc ia ted  with sampler. 
Thcse itells p lus  very c a r e f u l  camera layout  allow us  t o  achieve a no i se  
fip(ure of 4.5 e lec t rons  RMS f o r  the  camera sys tem and an ampl i i i e r  no i se  
of 7 e lec t rons  f o r  t h e  v i r t u a l  phase CCD o r  13  e l ec t rons  f o r  t he  th ree  
phase CCD. The d i f fe rence  i n  read noise  is due t o  d i f f e rences  i n  t h e  on 
chip ampl i f ie rs .  
The p ixe l s  are clocked out a t  a r a t e  of 
The CCDs are contained i n  a vacuum chamber which is evacuated t o  a 
Cooling is accomplished by a regulated flow of pressure of a few Torr. 
l i q u i d  ni t rogen.  
with a s t a b i l i t y  of l? . l °C.  
exposure f o r  all. the  sources used. 
The temperature can be var ied  from room temp t o  -190°C 
A s h u t t e r  i n  t h e  vacuum chamber con t ro l s  t h e  
The removable f ron t  f ace  of t h e  vacuum chamber can hold an o p t i c a l  
window, a beryll ium f o i l  window, o r  t h e  window can be replaced by a f lour -  
escent X-ray source with interchangeable t a r g e t s  which is ac t iva t ed  by a 
lmCu source of a lpha particles. 
The CCDs used i n  these  tests were a l l  manufactured by T e x a s  Ins t ru-  
The t h r e e  phase devices were thinned 
ments Co. 
square p ixe l s  i n  800 x 800 ar rays .  
t o  approximately l o p  and operated with t h e  r ad ia t ion  inc ident  from t h e  non- 
c i r c u i t  s i d e  (backside). The v i r t u a l  phase CCD is a t h i c k  device and t h e  
r ad ia t ion  is  inc ident  on t he  c i r c u i t  s ide .  Because t h e  v i r t u a l  phase CCDs 
have only a s i n g l e  ga t e  s t r u c t u r e ,  t h e  c i r c u i t  l aye r  i s  t h i n  (0.6 micron). 
There is an add i t iona l  0.5 micron p ro tec t ive  overcoat.  While t h e  absorb- 
t i o n  i n  these  layers would requi re  t h a t  t h e  CCD be thinned and run i n  t h e  
back i l luminated mode f o r  energies  below one k i l o v o l t ,  th inning was not  re- 
quired f o r  these  tests (80% transmission a t  1.5 keV). 
Both t h e  th ree  phase and v i r t u a l  phase devices  have 15.2 micron 
Although t h e  v ix tua l  phase device i s  250 microns t h i c k  it is a layered 
s t r u c t u r e  with only a 10 micron th ick  layer adjacent  t o  the  ga t e  s t r u c t u r e  
chat a c t s  as an a c t i v e  de t ec to r  region €or x-rays. 
t h i s  temperature t h e  dark current  is near 10’2e-/pixel/sec f o r  t he  3-phase 
device and 10-3e’/pixel/sec f o r  t h e  v i r t u a l  phase. 
vary from 1 t o  30 seconds depending on which source is used and so dark 
cur ren t  is completely negl ig ib le .  The v i r t u a l  phase devices  have an ad- 
d i t i o n a l  no ise  source which comes froru cu r ren t s  generated within t h e  de- 
v i ce  when it is clocked. This noise  source amounted t o  40 e lec t rons  RMS 
f o r  t he  device used i n  these  tests, 
For the  tests described here  the  CCDs were cooled t o  about - lOO°C.  A t  
Our exposure times 
A more complete descr ip t ion  of t he  3-phase device was given by Blauke 
e t  a l .  (1981). 
(1981). 
The v i r t u a l  phase device w a s  described by Janesick e t  a l .  
292 
111. X-RAY PERFORMANCE 
Both three-phase and virtual-yhAse CCDs were il luminated with 
5.9 keV x-rays from a Fe 55 aource. 
pseudo-image which d isp lays  r e l a t i v e  amount of charge f o r  each p ixe l .  
With the  a r r ay  o r i en ta t ion  shown i n  Figure 1, events are t ransfer red  
out (charge-coupled) i n  a downward d i r ec t ion  i n  each column. Some 
"smearing" along the  v e r t i c a l  d i r ec t ion  is apparently due t o  incomplete 
charge t r a n s f e r  and is frequent ly  re fer red  t o  as "deferred charge". 
The gray scale of the image is highly "stretched" t o  reveal  t h i s  
low-level incomplete charge t r ans fe r .  
Figure 1 shows a port ion of a 
The d i s t r i b u t i o n  of individual  p ixe l  event s i z e s  is shown i n  
The "background" curve is off scale below r e l a t i v e  event 
The response t o  Fe 55 x-rays is shown by the  peak near 
Figure 2. 
s i z e  7200. 
event s i z e  7720. 
To account f o r  charge smeo:ing along the  read-out d i r ec t ion  and 
Figure 3 shows the  
the f i n i t e  probabi l i ty  of charge deposi t ion i n  t w o  adjacent  p ixe ls ,  
var ious da ta  summation techniques have been used. 
histogram of the number of events  as a funct ion of r e l a t i v e  event s i z e  
f o r  p i x e l s  when the  deferred charge is included with the  main event 
charge. S t a r t i n g  with the  main event, the  computer algorithm f o r  t h i s  
approach adds the  charge f o r  successive p i x e l s  i n  a v e r t i c a l  coluum 
below the  i n i t i a l  event u n t i l  the p ixe l  charge is less than two standard 
deviat ions above an average background l eve l .  Figure 4 shows the  same 
set of da t a  as i n  Figure 3, except that those events have been excluded 
f o r  which the main charge deposit ion was observed i n  two adjacent p ixe l s .  
It is apparent t h a t ,  compar2d t o  the  histogram i n  Figure 3, the  background 
events below the  energy peak have been s i g n i f i c a n t l y  reduced. 
Figure 4 represents  the  bes t  performance obtained so f a r  i n  an 
x-ray CCD. The f u l l  width a t  ha l f  maximum response (FWHM) is - 12%,  
corresponding t o  an energy reso lu t ion  of 700 e V  FWHM. In order t o  obta in  
the  equivalent RMS noise  f igure ,  which is frequent ly  used t o  charac te r ize  
CCDs, w e  use a conversicn f ac to r  of 8.5 (3.6 evlelectron-hole p a i r  x 2.35, 
the number of standard deviat ions per FI.rHM). The noise  performance in  
Figure 4 corresponds t o  74 e lec t rons  equivalent RMS noise.  Figure 5 was 
derived from the same set  of da t a  as i n  Figure 2 through 4 except t h a t  a 
3 x 3 mask was used -33 the  a r ray  of p ixe l s  t o  be included f o r  each event. 
A somewhat b e t t e r  performance is obtained i n  Figure 6 where a rectairgular 
mask of 2 x 6 p ixe l s  w a s  used; the  mask was aligned along the  charge readout 
d i rec t ion .  In  the  l a t t e r  two cases, no add i t iona l  cor rec t ion  f o r  incomplete 
charge t r ans fe r  w a s  made. 
The performance f o r  virtual-phase CCDs Is comparable t o  t h a t  of 
3-phase CCDs except that a poorel- noise  performance was obtained. From 
tests a t  o p t i c a l  wave lengths  it  was observed that addi t iona l  charge is 
created in  the serial r e g i s t e r  under the  clocked w e l l .  
charge, which appears t o  be a funct ion of t he  clock p u l m  shape, contr ibuted 
t o  poor noise  performance e t  o p t i c a l  and x-ray wavelengths. Figure 7 shows 
the response of a vir tual-phase CCD t o  Fe 55 x-rays. Comparable graphs f o r  
s u l f u r  x-rays (2.3 keV) and alumj.num x-rays (1.49 keV) are shown i n  Figures 
8 and 9. 
This amount of 
IV. PRESENT AND EXPECTED PERFoRr4ANCE 
The noise performance achieved t o  date with v i s i b l e  l i g h t  
was 8.8 electrons IPS f o r  a three-phase device and 22 electrons r m s  
fo r  a virtual-phase CCD. 
tests are 22 electrons and 44 electrons,  respectively.  Compared t o  
these values, the r e su l t s  f o r  mono-energetic x-rays of 5.9 Lev are 
st ill s igni f icant ly  poorer. 
The average widths observed in  vis ible- l ight  
me most recent analysis  of x-ray ca l ibra t ions  shows that the  depth 
of the ac t ive  regions deduced from op t i ca l  tests is i n  f a c t  consis tent  
with the detection efficiency as determined from x-ray absorption measure- 
ments and the manufacturer's specif icat ions,  i.e., 10 microns. 
Spat ia l  resolution obtained is comparable t o  the  s i z e  of a p ixe l ,  
i-e.,  I5 microns x 15 microns. 
resolution of ! /3  arc second f o r  a telescope of 10 meter focal  length,  
for example. fo r  an AXAF. 
This pixel  s i z e  corresponds t o  an angrlar  
The preceding data show conclusively that CCDs have excel lent  s p a t i a l  
resolution and good spec t ra l  resolution, superior t o  non-imaging proportional 
caunter resolution. However, fur ther  research is required t o  understand the 
difference between the noise performance a t  op t ica l  and x-ray wavelengths. 
Various technfques t o  optimize CCD performance f o r  x-ray appl icat ions need 
t o  be explored, 5 u t  these preliminary r e s u l t s  a r e  c l ea r  evidence tha t  CCDs 
w i l l  becora an iaportant too l  fo r  x-ray astronomy. 
i 
1 
294 
REFERENCES 
Blauke, M . ,  Janesick, J . ,  Hall, J . ,  and Cowens, M. 1981, Prc:. S . P . I . E . ,  
290. -
G r i f f i t h s ,  R.  E . ,  Polucci,  C . ,  Mak, A., Pturrdy, S .  S., and Schwartz, D. A. 
1981, Proc. S.P.I.E., 290 ( i n  press) .  
Janesick, J . ,  Hynecek, J . ,  Blouke, M. 1981, Proc. S.P.I .E. ,  290. 

296 
ORIGINAL PAGE If 
OF POOR  QUAL^^ 
3-PHASE CCD: Fe55, ADJACENT EVENTS INCLUDED 
3-PKQSE CCD: Fe55, ADJACENT EVENTS EXCLUDED 
I t  1
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20 
10 
0 
0 200 400 6al 800 loo0 1200 
MNl SIZE (Dn) 
O9!CINAL GAGE 1E 
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3-PHASE CCD: Fe55, 3 x 3 MASK 
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MNl SIZE (On) 
FIGURE 5 
3-PHASE CCD: Fe55, 2 x 6 MASK 
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FIGURE 6 
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VIRTUAL PHASE CCD: FeSS 
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FIGURE 7 
VIRTUAL PHASE CCD: S K a  
FIGURE 8 
VIRTUAL PHASE CCD: AI K a  
la- 
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FIGURE 9 


Performance of single photon-counting X-ray charge coupled devices

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