A large area imaging gas scintillation proportional counter (IGSPC) was constructed for use in X-ray astronomy. The IGSPC consists of a gas scintillation proportional counted (GSPC) with a micron polyprotylene window coupled to a multiwire proportional counter (MWPC) via a calcium fluoride window. Over a sensitive area of 21 cu cm the instrument has a measured energy resolution of 17.5% (FWHM) and 1.9 mm (FWHM) spatial resolution at 1.5 keV

Hailey, C. J.

Ku, W. H. M.

Vartanian, M. H.

English

NASA Technical Reports Server

AN IMAGING GAS SCINTILLATION PROPORTIONAL COUNTER 
FOR THE DETECTION OF SUBKILOELECTRON-VOLT X-RAYS 
Charles J. Hailey, William H.-M. Ku, and Michael H. Vartanian 
Columbia Astrophysics Laboratory, Columbia University 
ABSTRACT 
A large area imaging gas scintillation proportional counter (IGSPC) has 
been constructed for use in X-ray astronomy. The IGSPC consists of a gas 
scintillation proportional counter (GSPC) with a 1 wm 
coupled to a multiwire proportional counter (MWPC) via a calcium fluoride 
window. The MWPC, filled with a mixture of argon, methane, and tetrakis 
(dimethylamino) ethylene, detects the W photons emitted by the xenon gas in 
the GSPC. 
energy resolution of 17.5% (FWHM) and 1.9 mm (FWHM) spatial resolution at 1.5 
keV. 
polypropylene window 
Over a sensitive area of 21 c d  the instrument has a measured 
https://ntrs.nasa.gov/search.jsp?R=19820018192 2020-03-21T07:37:01+00:00Z
, 
302 
I. INTRODUCTION 
Recent research into gas scintillation proportional counters by ourselves 
(Anderson et al. 1978; Ku et al. 1979; Hamilton et al. 1980) and others 
(Charpak, Policarpo, and Sauli 1980; Andresen et a1 1978; Hoan et al. 1980; 
Peacock et al. 1980; Maneo et al. 19801, has demonstrated the enormous versa- 
tility of this device for X-ray detection. These counters offer a factor of 2 
improvement in energy resolution over conventional proportional counters in 
the energy range 0.1 to 60 keV (Ku et al. 1979; Hamilton et al. 19801. The 
use of a photomultiplier tube array to collect the W light from the GSPC has 
demonstrated the feasibility of achieving mlllimeter resolution imaging 
capabilities (Hamilton et al. 1980; Charpak, Policarpo, and Sauli 1980; 
Andresen et al. 1978; Hoan et al. 19801. Finally, the well defined pulse 
shape corresponding to X-ray capture in the GSPC allows rise-time discrimina- 
tion, enhancing the usefulness of thi.3 instrument in high background environ- 
ments such as outer space (Ku et al. 1979, Mane0 et al. 1980). 
Three years ago, Policarpo (1978) suggested using a photoionization 
chamber in place of a photomultiplier tube to detect the W radiation. TEA 
was found to be a suitable gas for krypton scintillation (Policarpo 1978; 
Charpak, Policarpo, arld Sauli 1980) but a more versatile gas ("MAE) was sug- 
gested by Anderson (1981). We extended the usefulness of the GSPC by combin- 
ing the good endrgy resolution capabilities of the GSPC with the good spatial 
resolution capabilities of the MWPC I:Ku and Hailey 1981). 
with its 100 pm aluminum foil window, was aseful above several kilovolts. We 
have extended the spectral range of our instrument to the subkiloelectron-volt 
That instrument, 
X-ray region. We discuss  the  design and operation of the IGSPC a3d present  
da ta  acquired with our counter. 
11. INSTRUMENT 
The instrument we have designed is shown i n  Figure 1. The GSPC is a 
pillbox-shaped chamber of 15O-mm diameter. 
mic material (Macor). 
dow. 
ensure t h a t  the  window is an e l e c t r i c a l  ground. 
O-ring seal so t ha t  the  X-ray window may be replaced. 
absorbed by xenon atoms i n  the  d r i f t  region. 
xenon atoms d r i f t  t o  a l i g h t  producing region defined by two s t a i n l e s s  steel 
meshes held a t  high voltage.  
photons by excimer deexci ta t ion.  
f l uo r ide  window i n t o  the 30 c m  X 30 cm x 5 e m  MWPC. The MWPC is f i l l e d  with 
TMAE and P-20. 
Photoelectrons e j ec t ed  from the  TMAE molecdes  d r i f t  down through a low f i e l d  
region defined by a s t a i n l e s s  s t e e l  mesh located on the  lower sur face  of the  
calcium f l u o r i d e  window and a cathode w i r e  plane located below the  window. 
The photoelectrons are acce lera ted  through the cathode plane toward a high 
voltage anode plane. 
wires on the  upper cathode plane def ines  the t o t a l  ac t ive  region. 
ching e l ec t rons  a r e  co l lec ted  on t h e  anode while ions c o l l e c t  on the  cathode 
wires. The charge signals induced on :he two cathode planes set perpendicu- 
l a r ly  t o  one another are processed d i g i t a l l y  t o  yiefd the  cent ro id  of t h e  
It is made of a machineable cera- 
X-rays e n t e r  the  counter through a polypropylene win- 
The polypropylene is coated with approximately 100 A of aluminum t o  
The GSPC is designed with an 
The X-ray photons are 
Photoelectrons e j ec t ed  from the  
The e l ec t rons  are acce lera ted ,  producing W 
The W photons escape through the  calcium 
"MAE moleculres absorb a la rge  f r ac t ion  of the  W photons. 
A lower cathode plane with wires set orthogonal t o  t h e  
The avalan- 
! 
304 
charge d i s t r i b u t i o n  i n  orthogonal d i r ec t ions  (Reid et al. 1979). Energy 
information is obtained by co l l ec t ing  the  e l ec t rons  i n  a charge-sensit ive 
preamplif ier  a t tached t o  the  anode plane. Important parameters of the  IGSPC 
are summarized i n  Table 1. 
111. DESIGN 
The goal was t o  bui ld  a s o f t  X-ray IGSPC with 18% (FWHM) energy resolu- 
t i o n  and 1.0 mm (FWHM) s p a t i a l  r e so lu t ion  a t  1 keV. Good energy r e so lu t ion  
requi res  maximizing W l i g h t  output from the GSPC and de tec t ion  by the  MWPC. 
Good W l i g h t  production is obtained by using as high a d i f f e r e n t i a l  vol tage 
as possible  on the GSPC without allowing charge mul t ip l ica t ion .  The W 
photons produced are proport ional  t o  the  thickness  of the  l i g h t  producing 
regicn so t h i s  must be l a rge  (Conde, Fe r re i r a ,  and Ferreira 1977). 
co l l ec t ion  requi res  t ransparent  W window and g r i d s  and a la rge  s o l i d  angle of 
in te rcept ion .  The calcium f luo r ide  d isk  is made as t h i n  as mechanical con- 
s t r a i n t s  allow and provides approximately 80% transmission. 
reso lu t ion  s e t s  add i t iona l  cons t r a in t s  on design (Ku and Hailey 1981). A 
f l a s h l e s s  g e t t e r  i n  a separa te  chamber connected by two tubes t o  the  main 
counter chamber maintains the  pu r i ty  of the xenon gas. The upper O-ring s e a l  
i n  the  GSPC allows easy replacement of t h i n  wlndows which a re  mounted on 
aluminum carrier p l a t e s .  
an O-ring and may be removed t o  provide access t o  the  MWPC chamber. 
Good W 
Optimal pos i t i on  
The e n t i r e  GSPC assembly is at tached t o  the  MWPC v i a  
305 
I V .  MEASURED PERFORMANCE OF THE IGSPC 
a. Gain and Energy Resvlution 
The GSPC was f i l l e d  with 760 t o r r  of high pu r i ty  (99.995%) xenon gas. 
The MWPC was f i l l e d  with 6?0 t o r r  of P-20 and 0.3 torr of TMAE. Resul t s  
similar t o  those described below were a l s o  obtained with an MWPC f i l l  of 608 
t o r r  of argon, 152 t o r r  of isobutane,  and 0.3 t o r r  of TMAE. 
source (5.9 keV Mn-K) was epoxied t o  a window support r i b  t o  allow cont inual  
A small '5Pe 
monitoring of gain while measurements were taken at  lower energies  with a 
var i e ty  of fluorescence X-ray sources. GSPC s e t t i n g s  of 0.5 kV and 3.8 kV on 
the  upper and lower g r ids ,  respec t ive ly ,  and 0.5 kV on the  MWPC cathode were 
chosen as nominal. Figure 2 shows a typ ica l  energy reso lu t ion  versus anode 
high voltage curve a t  0.94 keV (Cu-L). 
2.2 kV, which was chosen as the nominal anode opera t ing  voltage.  
The curve e x h i b i t s  a minimum at  about 
Figure 2 
a l s o  shows the  t o t a l  cathode plane output as a funct ion of MWPC anode HV. The 
system gain tends t o  run away above about 2.3 kV. Figure 3 summarizes the  
best  energy reso lu t ion  performance of the  IGSPC a t  the energies  measured: 0.28 
keV (C-K), 0.68 keV (F-K), 0.94 keV (Cu-L), 1.5 keV (Al-K), 2.3 keV (S-K), 5.9 
keV (Mn-K). The 8.5% 
(FWHM) reso lu t ion  obtained a t  5.9 keV is cons is ten t  with energy r e so lu t ion  
The r e s u l t s  agree w e l l  with the  expected E'1'2 sca l ing .  
measurements taken on a photomult ipl ier  tube using the  same polypropylene 
window. The gain of the counter was measured as a funct ion of posi t ion.  The 
gain is uniform t o  b e t t e r  than 10% across  the  e n t i r e  51-mm diameter of t he  X- 
r a y  entrance window. The good gain uniformity is a result of the  small X-ray 
window diameter with respect t o  the  t o t a l  GSPC diameter (ensuring minimal 
f i e l d  f r ing ing  a t  the window edge) as well as the  l a rge  area of t he  MWPC which 
306 
ensures e f f i c i e n t  W co l l ec t ion  even f o r  events  near the  edge of the  window. 
Figure 4 shows the energy reso lu t ion  as a funct ion of pos i t i on  i n  the  d i rec-  
t i o n  p a r a l l e l  t o  the  anode wire d i rec t ion .  The energy r e so lu t ion  is uniform 
across  the e n t i r e  window as expected from the  good gain uniformity. S imi la r  
r e s u l t s  are obtained i n  the  orthogonal d i rec t ion .  
b. Pos i t ion  Resolution and S e n s i t i v i t y  
We have mapped the  counter response over the e n t i r e  51- polypropylene 
window. Figure 5 shows a p lo t  of the  weighted average pos i t ion  versus the  
source pos i t ion  p a r a l l e l  t o  the  anode wire d i rec t ion .  
were obtained f o r  t he  d i r ec t ion  peroendicular t o  the  anode wire d i rec t ion .  
The measurements were taken a t  0.94 keV (Cu-L) with a copper anode Henke tube 
mounted approximately 300 cm from the  window plane of the  counter and con- 
nected v i a  a vacuum vesse l  t o  the  test chamber. Movable slits defined a 150 
pm x 150 pm beam s ize .  The counter response is l i n e a r  over almost the  e n t i r e  
range, slowly f l a t t e n i n g  towards the  window edges. This f l a t t e n i n g  is due t o  
the  lo s s  of l i g h t  in te rcepted  by the  130 mm x 130 am MWPC as the  X-ray source 
is displaced from the center .  The e f f e c t  is q u i t e  small, i n  marked con t r a s t  
t o  r e s u l t s  we  had previously obtained with a 75 rn x 75 am MWPC (Ku and Hailey 
1981). 
i n  going t o  a l a rge r  s ized  MWPC. 
obtained at  d i f f e r e n t  X-ray energies.  Various col l imat ion schemes allowed us  
t o  obta in  measurements a t  5.9 keV (Mn-K), 1.5 keV (Al-K),  and 0.29 keV (C-K), 
i n  add i t ion  t o  our measurement at 0.94 keV (Cu-L). The da ta  have been cor- 
rec ted  f o r  the  e f f e c t s  of f i n i t e  source size. 
E11'2. 
Again, similar r e s u l t s  
The des i r e  t o  improve our pos i t i on  s e n s i t i v i t y  was a prime motivation 
Figure 6 shows posi t ion-resolut ion da ta  
The da ta  scales roughiy as 
Figure 4 shows the  pos i t ion  r e so lu t ion  versus source pos i t i on  p a r a l l e l  
307 
t o  the anode wlres. 
obtained ( i n  both direct ions) .  
As v i t h  the energy resolut ion,  good uniformity is 
C. Bi8e-Tf- and Backgroud-Rejection Efficiency: 
Pulso-shape d i sc r i a ina t ion  has been appl ied with good success i n  GSPCS to  
reject non-X-ray background events (Andresen et al. 1977). T k  t i m e  between 
i a i t i a t i o n  of secondary l i g h t  m l t i p l i c a t i o n  and the  complete co l l ec t ion  of 
a l l  the l i g h t  is a function of the e l ec t ron  cloud size, t h e  d r i f t  ve loc i ty  i n  
the  absorption region, the s ize  of the s c i n t i l l a t i o n  region, and the d r i f t  
ve loc i ty  i n  the s c i n t i l l a t i o n  region. For aur  counter geometry and xenon, 
t h i s  rise time is 2.1 mfcroseconds f o r  X-rays captured near the  vindow. The 
spread i n  the rise time is a funct ion of the variance on the s i z e  of t h e  
e lec t ron  cloud as it  en te r s  the s c i n t i l l a t i o n  region, va r i a t ions  i n  d r i f t  
ve loc i t i e s  a r i s i n g  from d i s t o r t i o n s  i n  the  electric f i e l d s ,  and va r i a t ions  i n  
the Ce:h a t  which the X-rays are absorbed. 
by maximizing the d r i f t  ve loc i ty  and minimizing the f i e l d  d i s to r t ions .  
p a r a l l e l  g r id  geometry of our present counter is i d e a l  f o r  t he  formation of X- 
ray  pulses. 
The rise tjme is e s s e n t i a l l y  the  same f o r  a l l  X-rays i n  the  energy range 0.1 
to 6 keV within the cen t r a l  51- a c t i v e  area of the counter, although the  
spread i n  rise times increases  f o r  lower energy X-rays due t o  the  s t ronger  
e f f e c t  of poor f i e l d s  near the  polypropylene window. 
times f o r  minimum ionizing particles and y-raps is m c h  less well-def ined and 
ravges from very shor t  times €or those events hJsorbed i n  the  s c i n t i l l a t i o n  
region t o  very long times f o r  those events which t rack  through the whole 
counter. The narrowness of the r ise-t ie d is t r i t ru t ic  f o r  X-rays then, allows 
us t o  set a very narrow tlse-time window f o r  ' , c e p t t ~ c e  and reject a la rge  
These va r i a t ions  can be minimized 
The 
The spread i n  rise times is less than 0.06 microsecoids (1  u). 
The pulse  formation 
308 
percentage of t h e  non-X-ray background events .  Background rejection r a t e s  of 
95% have been achieved while  maintaining 90% X-ray acceptance rates. More- 
over ,  more than ha l f  of t h e  cosmic ray induced background events  occur  near 
t h e  edge of t h e  GSPC an4 can be r e j e c t e d  by b i r t u e  of t h e i r  l o c a t i o n  o u t s i d e  
t h e  c e n t r a l  5!-mm f i e l d  of view. 
V. CONCLUSION 
The prel iminary r e s u l t s  from our  s o f t  X-ray IGSPC demonstrate t h e  poten- 
t i a l  usefu lness  of t h i s  instrument.  However, t h e r e  are s e v e r a l  s t e p s  we w i l l  
t a k e  t o  improve t h e  energy and p o s i t i o n  reso lu t ion .  Improvement of our 
g e t t e r i n g  system should lead t o  purer  xenon gas and thus  l a r g e r  W l i g h t  
output .  Reducing t h e  calcium f l u o r i d e  th ickness  through t h e  use  of double 
pumping and reducing t h e  d i s t a n c e  from t h e  lower GSPC g r i d  t o  t h e  calcium 
f l u o r i d e  window should allow better W c o l l e c t i o n  e f f i c i e n c y  and also lessen 
t h e  i n t r i n s i c  l i g h t  cloud spread. F i n a l l y ,  a sys temat ic  program t o  examine 
var ious  f i l l  gases  f o r  our IGSPC w i l l  be undertaken. These s t e p s  should h e l p  
us  a t t a in  our goal  of 18% (FWHM) energy r e s o l u t i o n  and 1.0 mm (FWHM) s p a t i a l  
r e s o l u t i o n  a t  1 keV. 
The au thors  wish t o  acknowledge h e l p f u l  d i s c u s s i o n s  with R. Novick. This  
work was supported by t h e  Nst iona l  Aeronautics and Space Adminis t ra t ion under 
g r a n t  NCR 33-008-102. This  is Columbie Astrophysics Laboratory Cont r ibu t ion  
No. 214. 
1ABLE 1 
Parameters  of ICSPC's Design 
Parameter  IGSPC 
GSPC 
- Gas mixture"  
- Gas p r e s s u r e  ( to r r )*  
- Absorpt ion  d e p t h  (m) 
- Ligh t  m u l t i p l i c a t i o n  d e p t h  (mm) 
- Bottom g r i d  t o  W window (mm) 
- W window material 
- W window dinenst ions (mm) 
- X-ray window material (vm)  
- X-ray window dimension (mm) 
- Grid  t r a n s p a r e n c y  
- Nominal HV (kV)*V3, V1, V2 
-
Gas mixture"  
c.is p r e s s u r e  ( torr  )* 
Absorpt ion  d e p t h  (mm)* 
Anode-cathode d i s t a n c e  (mm) 
Anode wire dimension (mm) 
Cathode wire dimension (mm) 
Cathode group s i z e  
Anode wire s p a c i n g  (mm) 
Cathode w i r e  s p a c i n g  
Grid t r a n s p a r e n c y  (ground)  
Nominal *dV (kV)* 
va, vc, vG 
x t  ion 
760 
10 
1 1  
4 
CaF2 
7.0 x 150 
i polypropylene  
51 die. 
9LZ 
0, 0.5,  4.0 
"MAE + P-20 
0 3 + 620 
12 
4.5 
0.02 x 130 
0.063 x 130 
6.0 
2.0 
0 .55  
90% 
2.2, 0.5 ,  0 
*Ad j u s t  able  
31 0 
REFERENCES 
Anderson, D. F. 1981, Nucl. Ins t r .  Methods, submitted. 
Anderson, D. F., Ku, W. H.-M., Novick, R., and Scheckman, M. 1978, - IEEE 
Trans. Nucl. Sci., -15, 813. 
Andresen, R. D., Leimann, E. A., Peacock, A., and Taylor, B. G. 1977, Nucl. 
Ins t r .  Methods, 146. 
Andresen, 8. D., Leimann, E. A., Peacock, A., and Taylor, B. G. 1978, - IEEE 
Trans. Nucl. Sci., RS-25, 800. 
Charpak, G., Policarpo, A,, and Sau l i ,  F. 1980, IEEE Trans. Nucl. Sci.,  
19s-28. 
Conde, C. A. N., Fer re i r a ,  L. R., and Fe r re i r a ,  F. A. 1977, IEEE Trans. 
IJucl. Sci., As-24, 221. 
Hamilton, T. T., Hailey, C. J., Ku, W. H.-M., and Novick, R. 1980, - IEEE 
Trans. Nucl. S c i . ,  19s-27, 190. 
Hoan, N. N., Jeanjean, J., I toh,  H., and Charpak, G. 1980, Nucl.Instr. 
Methods, 172, 603. 
Ku, W. H.-M., and Hailey, C. J., 
Ku, W. €?.-H., Anderson, D. F., Hamilton, T. T., and Novick, R. 
1981, IEEE Trans. Nucl. Sci., US-28. 
1979, - IEEE 
Trans. Nucl. Sc i . ,  -26, 490. 
Manzo, G . ,  Peacock, A., Andresen, R. D., and Taylor, 8. G. 1980, Nucl. 
Ins t r .  Methods, 173.. 301. 
Peacock, A., Andresen, R. D., kimenn,  E. A. , tong, A,, Mdneo, G . ,  and 
Taylor,  B. G. 1980, Nucl. Instr. Methods, 169, 613. 
Policarpo, A. J. P. L. 1978, Nucl. Instr .  Methods, 153, 389. 
Reid, P. B., Ku, W. R.-M., Long, K. S., Novick, R., and P l sa r sk i ,  R. L. 1979, 
IEEE Trans. Nucl. Sci., IS-26, 46. 
FIGURE CAPTIONS 
311 
FIG. 1 - Schematic diagram of the imaging gas scintillation proportional 
counter (IGSPC). 
FIG. 2 - Total pulse height from the cathode planes plotted as a function of 
anode high voltage (XI; energy resolution plotted as a function 
anode high voltage (*I. 
the GSPC at (3.8 - 0 .5 )  kV. 
The data are for Cu-L with Vc = 0.5 kV and 
FIG. 3 - The energy resolution vs.X-ray photon energy of the IGSPC. The 
solid line is proportinal to E’“*. 
The energy ( 0 )  and position ( * )  resolution for Cu-L as a function of 
source position parallel to the anode w i r e  direction. 
FIG. 4 - 
FIG. 5- Position sensitivity and linearity in the direction parallel to the 
anode wires in the MUPC. The divider output is plotted VS. the 
source positon. 
FIG. 6 - The IGSPC position resolution VS. X-ray photon energy. The solid 
line in proportional to ~ - 1 1 2 .  

-.e.-.”.- 
ORlGlNAP PAPUT ?$ 
OF POOR QUALITY 
MWPC ANODE HV ( K V )  
3 13 
3 14 
I 1 * b l . . l  
I 0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 IO 
X-RAY ENERGY ( k e V )  
FIGURE 3 
315 
CI 
v) 
!= z 
a6 - 140- 
a 
s120,i 
3 80- 
cn 
E 100 
c 
0 
W 
PT 
* 
c3 
W z 
W 
1E 
I 
3 
LL 
a 
CI 
v) r- z 
3 
cri 
0 a 
5 0 0 o o o o O  0 z 
0 
c- - - 
=? s - I40 
-120 z 
-eo Z 
a 
b 0 .  
‘IooE -1 -  . . .  . . .  0 .  0 .  
0. 
H 
I * 
LL 
- 1  1 I 1 I 1 1 I 1 
POSITION (cm) 
FIGURE 4 
3 16 
2950 6- 
----I t - 2750 !z f 2550 m a S A  _ _  v) 
2350 I- J 
POSITiGY (cm) 
FIGURE 5 
317 
FIGURE 6 


An imaging gas scintillation proportional counter for the detection of subkiloelectron-volt X-rays

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server