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State-of-the-art and application of wire chambers
Nuclear Instruments and Methods 217 (1983) 65-76 North-Holland Publishing Company
65
STATE-OF-THE-ART AND APPLICATION OF WIRE CHAMBERS A.H. WALENTA
Universi O, of Siegen, 59 Siegen, Germat~v
Recent improvements in chamber design and readout electronics have allowed the construction of position sensitive detectors with very high rate capabilities as used in beam spectrometers and at high intensity X-ray sources. The measurement of the spatial ionisation structure (time expansion chamber) with a waveform digitizer allows high precision position measurement for charged particles and -{-rays. Applications in particle physics and nuclear medicine are discussed.
I. Introduction More than a decade ago a new generation of large area detectors, the proportional chambers (]-ef. 1) and muhiwire drift chambers (ref. 2), were introduced providing information mainly in the time and space domain about ionizing particles considerably surpassing the prevailing spark chambers. But new projects initiated by the capabilities of the wire chambers stimulated a continuous improvement which could only be supported by a better understanding of the basic processes in the detector and an application of advanced electronics. It is of interest to follow in some detail the contribution of both aspects to the "state-of-the-art" of wire chambers. In the following one example will be given in the time domain focusing on the high rate behaviour and another in the space domain focusing on position and double track resolution for charged particles.
2. High rate chamber There are two different aspects of MPWCs and multiwire drift chambers operating at high rates. The first refers to the fundamental limits of saturation effects of gas gain, limited lifetime, and limitations due to pulse formation and shaping, problems which are principally the same as in ordinary cylindrical proportional counters and are described in the literature. The second is typical of multiwire chambers providing additional information such as position in one or two coordinates, track angle, m o m e n t u m and mass of particles. This very complex information has to be processed and stored. If only rare events out of a high intensity background are of interest (as is mostly the case in particle physics) the trigger principle provides a powerful solution and then only the signals occurring during a memory time set by the whole system has to be recorded. For most uses in applied fields (medical and biological 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland
research, crystallography etc.) the high rate events themselves are the subject of interest and, as we will see, new aways of dealing with this information have to be found.
2. l. High rate limitations As the count-rate in ordinary proportional counters is increased occasionally a drop in pulse height will be observed. This effect has been explained (ref. 3) by the accumulation of space charge in the drift region produced by the positive ions generated in the gas amplification which lowers the electric field strength near the anode. The calculiation of the effect can be simplified by determining the equivalent drop of anode voltage causing the same lower field strength and inserting this drop of anode voltage into the formula for gas amplification as a function of anode voltage. Then one obtains for the relative gas gain G / G o with Go the value at very low rates:
G / G o = exp( - F~a2QoN ), where
F
kp In( r , / r i ) 8~rzc0~ + V
and p is the pressure, r, and r~ the outer and inner radii of the counter, respectively, #+ the ion mobility, V the applied voltage, Q0 the charge in a single avalanche and N the count-rate. A simplified formula for the gas amplification defines k and V0: G = exp[k(V-
V0) ].
F changes only little with the geometry and choice of gas (at given pressure) and therefore the most important design parameter is ra which corresponds in a multiwire drift chamber with roughly rectangular cell structure to half the gap width or distance to the potential wires. I. PLENARY TALKS
66
.1. I1. II a l O l l a
S t , l h > o~ - I/iv
In addition to) a homogeneous reduction c>f pulse height an efl'icienc3 lo,,,s has been obser,,ed in proportional chambers operated at high rates (ref. 4). A possible explanation is the shielding of the anode v>ire bx the positive space charge as long a~, it is ch)~,e to the ~ire such that subsequent arriving electrons tire not sufficiently nmhiplied. It is expected that this effect d e p e n d s on the spatial extension of the a,+alanchc near the ~irc. T h e gas c o m p o s i t i o n , gas gain and the aiiodc radiu,,, ~, occaskmallv ~\erlap and therefore the 5 cannot be counted individually, an effect which is usually referred to as deadtime loss. Tile essential factors influencing the dead time tire the spatial extensi,m t>l ionisatic,n, diffusiou, tl~c gas a m p l i f i c a t i o n proce.~s wiii~ signal f o r l t i a l i o l l aild ieadot.lt electronics. Let us first examine the gas amplification alld sigllal f o r u l a t i o n . A pointlike charge produces an a,,alailclle at the ;.tllode of total charge (_)~. T//e scparatioll o i tile clccti<,lls and the positixe ion.,, illducc chdrgcs oil the anode_', t o l s i m p l i c i t y the charge densitx, d i s t r i b u t i o n e l the a~ahinctre as a t u n c t i o n of r is lepLlccd b~ :l p o i r i t l i k e cl~,.llge Qo at a distance 1],, #. a_ 2 . ~ Il-Ol/i tiae a n o d e surface where ~ is the first l o~'.ll~,el'id coefficient *. F l a the charge induced lit the allode .,>no o b ta il i s Q" ( tin r i -ln
O,.d(t)~
ra
~lil(t,'.'<>
# 1)] •
,ailere s'tJ=#<-~,~,/2lt li~ is a characieristic time o f ti~e signal developlllcnt, c = In G / t i , p ' is the ion nlobilit,, and li~ the applied anode vohage. One notices for t = 0 a signal of Qc.l(O) = Q ( l n
r i - In ;'a, )
v, hich is called the electron c o m p o n e n t generated b \ the fast nlo',ing electrons. This signal has a ri.setinle c o n > parable to the formation of the awtlanche during tile last few mean free paths of ionisation which can be con,sMered here to be prompt. ]'he signal for s' > 0 is culled the ion c o m p o n e n t which has a rather steep rise but reaches the maxilnum only after a relative hmg time. For fast counting this signal has to be differentiated. This can be simply done by terminating the anode by a low impedance (usually a coirimon base ainplifier is used, but the same results are in principle obtained bv sharp differentiation after a voltage or charge sensitive amplifier) and one obtains the "current'" signal
,(~)__'4_0= dt
G, 2c
i,,,< 1
t/t o
* A nlorc detailed treatnlent can be found in textbooks, fo~ example see ref, 5.
art
, u l d a p f U u~trv , h w l l l ~ c t ~
1 his ,signal alrcad\ has a narroxg x~idth at half maxinmnl of l~ btlt il still tlas a long tall. F u r t h e r sll~.tpillg b\ d i f f e i e n t i a l i o n or by i/lille im~,l~ed cilcuit~ (ief ~,) icmo~es thi~ tail. D e p e n d i n g on tile l i m i t e d b a n d ~ i d t l l of the a x a i k i b l e p rca l l i p l i fi crs OllC obtairis a MgllaI ~ i t h a base w i d t t / o r a f0~ tO a b o u t lO{i~. Since /{,, ~- r, t "~.r the range t~f ~ecol3dar\ electrons, l:c,r charged particles cros,~ing the chail/bei ~.~. i~, of the ordt.,r <,f half the gap w i d t h ,~r ~ i l e ~p;.l~_lng ~ l / i c h e x e r is largcl, t tir i(lit ellcrgs, X-ra\ diffli~ion is d~,ininanL .A ~ -~ 2 0 0 #.inl t~pl-
C~.til\ [701" hlghL'r ciIcrg\ lhc f a i l l e t)l thc pht)lt)L'lccirt,l'i dcfili¢~, "i <. ];ii3all\. increased rates raise tt/c quesliol~ <,1 the l i m i t e d iiletiine of tile COUl'ltt~rs. fqnal mol-iclllMoii~ are itee.n ftlund (tc'f 7) tiiat i.ii~dcr ct_'l!ain Ct,lldition.~ e l clean c<~tllltel ~, and p u r i f i e d ga-,e~ ,t hfctilllC c<,uld be o b t a i n e d sufficic'i~t to tipt:rate ~uch a c h a i l l b e r it)r ,,ears at peak rates. It :';t.'Cill>, lhat ~,c, tar the ~,pacc chalgc ,.tlid dead time problcn>, aie the i m > t .,>e,,C'lc !lll/itathln~ tl. leal liigtl rah: cot.intcls, i'hcrel'c>le tilt f o l l o w i n g design c l l t c r i a sliould bc o h s e r \ e d : small x~lr~, spacing to reduce the rate per \t. liC. small gap w i d t h t~, reduce space churge effects and redtlcc signal tails caused h~ late cirri~ing ioni,,atlOil. l(',",', liOiSC p r e a m p l i f i e r to o p e r a t e at lo\t glia aniplification< slri,lll alil)dc r~.lditi~+ it, pro~ldc tam anode ,,i~lial,, and li, redtlce l<,ccil ~pacc charge, large b a n d \ t i d t i l of a l n p l i f i c r to allm~ laM c l i p p u i g of signals. dead time of t_tiscliniinator m a t c h e d h> ~ i d t i l at bw, c ~f clipped signal, double (multiplej-hit TI)C ,xith double hit resoluti~m at least equal to double pulse resolulion ~)1" tile amplifier disciimmator chain. l ' h c requirements of speed and low noise at the sanic time can only be accommodated v+ith aver,, lee, input capacity which means that the preamplifier has to be mounted right at the end of the anode wire. Therefore lhe design of chamber and amplifier has to tie c~m.,idered as one unit. The small wire spacing leads to a design of an emitter f o l l o w e r consiMing of one lllillial u r e tranaiMor ( S O 1 - 2 3 case) and one resistor iiieOlp o r a t c d iilh/ the challlbcr frame thus lllillillllZing spate a n d i n p u t ca p a ci ty Fhe o u t p u t ha.~ a rchitivclv Io,a inlpedalice and the signal Call be carried over honle di,,huice t<, the main amplifiei and shaping circuit~,
67
,4.11. Walenta ./ Ntale- of the-art and application ,{[ wire chamherv 2.2. ( ' h a m h e r construction
~'254mm I
The c h a m b e r was designed for the focal plane of a magnetic spectrometer covering a beam profile of about ' " × 4". A c h a m b e r with a sensitive surface of 1" x 5" is planned * and a test c h a m b e r of 1" × 2" has been built and tested with wires 1" long (ref. 9). The wires are stretched over a frame made of standard PC-material ( i ' , " thick) which carried at the same time the printed circuit for the amplifier. The construction parameters are given in the following list (see also fig. 1): anode wire: gold-plated tungsten, ~, rail diam., anode wire spacing: ~',,", potential wire: ( ' u - B e , 2 rail diam., cathode: carbon coated aluminized mylar, gap width (cathode +cathode): ' " . The opened c h a m b e r is shown in fig. 2. The emitter follower circuits are visible in the spaced out part of the c h a m b e r frame. The signals are transmitted through a f l a t b a n d cable with a shielding grounded on one side to reduce crosstalk between channels.
I
32mm .
.
.
.
:
.
f
c
t
A
PW
Fig. 1. Flectrode arrangement of high rate drift chamber.
2.3. R e a d out electronics
In order to minimize the n u m b e r of parts and consequently space, there are only a few design parameters left to be optimized, with the emitter follower given as the input stage. Since as a second stage a low noise c o m m o n base amplifier is used the d o m i n a n t noise source is the series noise from the emitter follower +. It could be shown (ref. 11) that a good approximation for the series equivalent noise charge is given by , .2
1 e2 +2
E?v(" =~t~ " C i " [ 1
+ ]
Im
3 R,~,~]
2
'
with e n equivalent series noise voltage per { H . , ('i,, tile input capacity, R,~ the input resistance and t~ a typical shaping time which is, for a triangular shape, equal to the full width at half maximum. Since t m corresponds approximately to the full width at half m a x i m u m of the clipped signal (tm = 10... 15 ns) it is possible to choose Rin('in << t m and then E N Q (x enCin, e , is determined by the operating conditions of the transistor and can be found in data sheets. For the high b a n d w i d t h required here it was found that the transistor BFT25 would be profitable because of the low noise figure at low currents and the high g a i n - b a n d w i d t h product. Fig. 3 shows the equivalent noise of the emitter follower as a function of input capacity compared with the performance of a c o m m o n base low noise amplifier * It turned out that the rate requirements were more modest and a somewhat less ambitious chamber with twice the wire spacing has been built (ref. 8). + A more detailed analysis of the noise of this front end circuit has been described elsewhere (ref, 10).
Fig. 2. View on the opened chamber. Only a few preamplifiers are assembled.
14
E
-c:,
12] /" /
/
/
•
•
//[/
o
/ oJ
a} / J
/ • •b}
t~ LZU
6"
//
•
//, 4•
2
/
,/ 0
10
20
30
~0
SO
60
Co (pFl Fig. 3. Equivalent noise charge (rms) measured for full bandwidth (100 MHz), (a) common base, (b) emitter follower. (ref. 6). This m e a s u r e m e n t shows an additional bonus for the emitter follower of somewhat lower noise over the whole range of input capacity. In the present chamber a total input capacity of 2.7 p F and a noise of E N C = 103% was obtained. I. PLENARY TALKS
68
.4. H. Walenta / State- of- the- art and application of wire chamhers
pr eamp[ifier on chamber
amplifier & shGper
discriminator (MVL 100)
shiftregister (E]L 8 )
100
4++
a) ___
-
i
-
_
÷
o~
!
+ ÷
v
102L, hit i
uJ
50-
6V
#
twisted pair cable + Fig. 4. Read out electronics (schematically). 0
•'7
6
'8
scintillator q c0[[imated beam
'9 1'0 UA (kV)
111
1'2
1'3
prop. counter
I
~ead ~J~source2 (10mCi
bl
10
bloc"er c
+ i ".'." ." .r.]1~........ ; testchamber L~J~source
+
1 (.1 mCi
Fig. 5. Set-up for measurement with minimum ionizing tracks.
The read out electronics is shown in fig. 4. The shaping is adjusted to provide a signal with 30 ns at the base necessary to avoid double or afterpulses with minimum ionising particles. A slight undershoot has been added to make the system easily adjustable and reliable for larger numbers of channels. The discriminator (LeCroy MVL-100) was set at a threshold corresponding to a signal charge of 10% o at the input, about a factor of two above the peak noise in order to guarantee stable operation. With an ECL-output signal of the MVL-100 with a width of 40 ns, which equals the double pulse resolution reached over a twisted pair cable, the multihit TDCs * consisting of shift registers operated with a clock frequency up to 140 MHz. 2.4. M e a s u r e m e n t s
The system has been tested with minimum ionizing tracks in a set-up shown in fig. 5. A fine collimated beam of B-rays from a weak 9°Sr-source crosses the c h a m b e r and provides the trigger signal. The efficiency and drift time spectra as a function of beam position can be measured. Two additional hot 9°Sr-sources (10 mCi) are arranged to irradiate the chamber from the back producing a high background rate without disturbing the trigger. For a background rate up to 3 × 105
* These units are basically the same as the time digitizing electronics of the JADE experiment (ref. 12).
~;
.5
+, 950
9;0
1'00
x(inch)
~o x
2-
c)
+
c c
~n
== o
+
-k
+ +
6'0
70
8'0
9'0
1;0
110
t Ins) Fig. 6. Results obtained with minimum ionizing tracks. II, low rate (about 10 events/s), +, high rate (3x 105 cts/s on 4 mm wire length ×2.5 mm drift space, corresponding to 3× 106 cts/s cm2); (a) plateau, (b) scan over drift space, (c) drift time distribution for x = 0 . 4 mm, i x = 0 . 8 1 mm (fwhm), gas A r / C 2 H~, (50/50).
over an area of 2.54 mm X 4 mm (corresponding to a rate of 3 x 106/s cm 2) no effects on the plateau, the position d e p e n d e n t efficiency or drift time spectra, could be observed (fig. 6). The limits of the chamber have been investigated
A.H. Walenta / State- of- the- art and application of wire chambers
69
1.0
0 ~ " ) ~~./*.2.10 5 5.6-106
~") ~ ~ 6 . 1 0 ~ ,
J0.5-
"~, \ 210~
t..O
~.10~
i
2 '10 ~'
10"
i
10 s N (ram'1 s-1 )
1
t
10 6
10"
Fig. 7. Pulse height (normalised to pulse height at low rates) versus rate/wire length. Parameter: avalanche size (electrons). (a) standard drift chamber, (b) high rate chamber.
with an X-ray generator (50 kV, Cu-target) providing 8 keV X-rays within a spot of 2.6 m m diameter and intensities up to 2 X 107 s-1. F o r simplicity it is assumed that this intensity is spread evenly over an anode wire length of Al = 2.6 m m which is used to calculate the rate per wire length. Fig. 7 shows the pulse height as a function of this rate for a s t a n d a r d c h a m b e r ( r a = 5 mm, r i = 10 # m ) and for the high rate chamber. For the same avalanche size (here 4 x 105 electrons) the high rate c h a m b e r is able to h a n d l e with the same 2% amplitude drop 1½ orders of m a g n i t u d e higher rates. If the same performance is required the curves for an avalanche size of 5 x 106 in the s t a n d a r d c h a m b e r - a m p l i f i e r unit have to be c o m p a r e d with 105 in the high rate c h a m b e r with low noise electronics allowing it to handle almost 3 orders of m a g n i t u d e higher rates. With a m o n i t o r c h a m b e r at m u c h larger distance the X-ray flux has been measured without saturation effects allowing the determination of the efficiency of the high rate chamber. Fig. 8 shows the measured rate in the test c h a m b e r against the true rate (determined by the monitor-chamber). The absolute value of the true rate can be o b t a i n e d at low intensity. Up to a rate of 106 s - ] no loss can be observed. At the highest rate the loss of efficiency agrees within ___1% with the dead time losses calculated from the k n o w n discriminator dead time. Therefore one can conclude that the real efficiency loss due to limitations in the c h a m b e r is less than 1%. As a conclusion it is reasonable to assume that multiwire (drift) c h a m b e r s can be built to handle up to 10 6 c o u n t s / m m s which translates in the geometry of the test c h a m b e r to an area rate of 4 x 10 7 c o u n t s / s cm 2. Higher rates of 108 c o u n t s / s cm 2 are possible if
10! ea E c~
=
/
/
10~
,3.)
L
106-
/
/
f
1c 10 ~
10 ~
10 ~
rQte(s-1) monitor chamber Fig. 8. Rate in test chamber versus intensity (rate in monitor chamber). (a) true rate, (b) measured rate, +, true rate with dead time correction; Qo = 103 electrons.
the requirements on energy resolution are somewhat relaxed.
2.5. Application of high rate chambers These high rate c h a m b e r s with their relatively short dead time of 40 ns a n d even shorter m e m o r y time of 28 1. PLENARY TALKS
70
A,tt. H~Uenta / State-q/-the-arl and apphcation ,1 wire chamher~
ns are certainly useful in particle physics as beam chambers, target chambers or in experiments at p p-colliders with very high rates in the innermost ring around the beam pipe. The readout can be accomplished with the multihit TDCs mentioned above or with even higher resolution shift registers (ref. 13). The interesting question is, however, can the chamber possibly compete with other detectors in applied fields where the high intensity radiation has to be recorded without the help of a "'trigger'" system. For comparison a picture containing 512 × 512 pixels (:area resohition elements) of a good television picture quality (5% intensity resolution) would contain in total n,,,t - 10 ~ counts. A television picture is generated in At = 40 ms and therefore a countrate of 4 × 10'~/s would be equivalent to a television s\'stem. A high rate chamber with a sensitive area of 10 ~< 10 cm ~ m principle could collect 10 m c o u n t s / s and therefore be competitive with other image producing systems. The important advantage lies in the fact that it is a quantum counting system while all other systems are integrating devices. Counting individual quanta allows the measurement of additional properties such as energy, angle or timing, providing subsamples or rejecting background. An even greater advantage of quantum counting systems is the low noise which is by definition given by quantum statistics and the high linearity and dynamic range which are only limited by dead time losses. In the foilowing a possible application of the high rate chamber is discussed where the last points are crucial In the diagnosing of heart diseases important progress could be made by improvement of the coronary angiography which is the visualisation of coronary arteries. Usually a catheter is inserted into a femoral artery and positioned such that the tip reaches the ostium o[ the coronary artery. Then the maximum strength of iodine contrast agent is injected and an X-ray image is acquired. This method carries a high risk of mortality and morbidity: consequently it is only used in special cases. Therefore a method has been developed (ref. 14) where the contrast of arteries against the background could be enhanced by subtracting the background quantitatively such that injection of the iodine contrast agent through a catheter directly into the coronary arteries occasionally may be replaced by peripheral venous injection. The method consists of taking two pictures one with a monochromatic X-ray beam slightly above the K-edge of iodine ( E K = 33,2 keV), the other slightly' below (fig. 9). The mass attenuation coefficient of iodine changes drastically while for soft tissue and bone the changes are negligible if the two X-ray energies differ by small amounts. This can be accomplished at sufficient intensity with synchrotron radiation sent through a switchable monochromator. A successfully operated test set-up (ref. 15) is shown in fig. 10. After the monochromator
i00-
iodine 10~E to,
~on
musc(e
7o
7o
E(keV}
<
~c
Fig. 9. Mass attenualioil coefficient versus X-rax, energY. the beam is narrowed by' a slit collimator to aboul I ran3 in vertical direction and fanned out in the horizontal direction covering the object and a one-dimensional position sensitive integrating silicon strip detector. Vertical mechanical movement of the object provides the scan m the second dimension. First results are shown in fig. l l. A straightforward calculation gives for the signal-tonoise ratio of the iodine struclure S'
1
a s = '~ ( v/°
)"c' j2
N
.~,'< "
where A ll_t/p ).l is the change of mass attenuation c'oefficient for iodine, Cj the iodine thickness and 5; the countrale with , I N its error. For S / A S _>_3 and ( j <_ 1 m g / c m 2 one obtains A N / N < 6.7 x 10 ~. The linearit~ errors and the noise introduced by the detector have to be ~<2× l0 ~ in order to have the quantum noise as the dominant contribution which is necessary I o keep the required beam intensity and the radiation exposure of the patient at a minimum. This requirement is relatively easy to fulfill with a wire chamber. The set-up in fig. t2 shows the principal arrangement for a two-dimensional detector, assuming the synchrotron beam can be fanned out by a curved mirror also in vertical direction to cover the whole object. A detector of dimensions 10 cm × 10 cm is sufficient. With the accumulation of 2CLLIH ATOP
i'
SYNCHROTRO N
R ADLATION
I
/ N INOEHRONATOP
T,
i ?
Fig. tO. Set-up for dichromatic angiograph'~ (rcf. {5L
A.H. Walenta / State- of- the- art and application o[ wire charnberf
a
b
71
c
Fig. I1. X-ray images obtained for the coronary arteries (a) above K-edge, (b) below K-edge, (c) image after logarithmic subtraction (ref. 15).
N = 2.2 x 10 a counts in one resolution element (pixel) of 0.5 m m × 0.5 m m in the quiet period of the heart (400 ms) one calculates a rate of N = 2.2 × 10 v c o u n t s / s cm 2. Although this rate still gives good pulse height m e a s u r e m e n t (fig. 7} the rate per wire will be 5.5 × 107/s resulting in too high deadtime losses. If the total rate is to be distributed over a stack of, for example, 10 chambers, the dead time losses of 5%. can be corrected to give an overall precision of 2 × 10 3 In order to increase the total thickness of the counters a drift space is added to the thin amplification region which is separated by a fine grid. In this way only a very small fraction ( = 1%) of the positive ions accumulates in the drift space and does not disturb the gas amplification. Since the wire spacing of 2.5 m m is too coarse for the expected resolution of 0.5 m m (fwhm) the position will
be interpolated using the difference signal from the potential wires in the same way as explained below in the time expansion chamber. The second coordinate can be read out by cathode strips arranged at an angle + 7.5 ° with respect to the anode or by fast delay lines or by charge division on the anode wire. There is another reason for distributing the total rate o n t o several planes (fig. 13). The memories receiving the information after the derandomizer handle a fraction of the total rate. The subdivision of the detector makes it
apW,~¢
QRID
ANODES
AMPLIFIER
!!!: ii!
a:
,
L_
......
:l-
!
Fig. 12. Chamber set-up for noninvasive angiography. Total number of wires: 400; gas filling: Xe, 4 atm.
Fig. 13. Read out of a single chamber plane. Circuits for two wires are shown. 1. PLENARY TALKS
72
A. tt. Walenta / State-of-the-art and apphlcatton of wire chambers
possible to use m e d i u m speed m e m o r y associated to detector sections in parallel. The electronics, particularly the memory, may appear to be rather " b u l k y " but it is i m p o r t a n t to realize that such a system is currently feasible. A rapid advance in electronics can be expected in the next few years, allowing a more concentrated memory to be used. It is i m p o r t a n t to note, that the typical limitations of p r o p o r t i o n a l wire c h a m b e r s in imaging applications can be overcome in this example: the relative low X-ray energy gives already an efficiency of e = 95% for xenon at 4 atm and a total detector d e p t h of 20 cm. The range of the photoelectron is less than the desired resolution of 0.5 m m a n d the specific imaging principle avoids parallax problems. Keeping in mind the problems of c o m p e t i n g integrating systems for this application it seems that the multiwire c h a m b e r s have a good chance of application in the field of noninvasive angiography.
3. Time expansion chamber In the spatial d o m a i n for muhiwire c h a m b e r s and drift c h a m b e r s the quantities of interest are position resolution and double track resolution. A somewhat more careful definition of these quantities reveals a more complex situation. As k n o w n from cloud c h a m b e r a n d b u b b l e c h a m b e r pictures a charged particle leaves b e h i n d in the detector m e d i u m a n u m b e r of ionisation clusters. A fit through all these points gives the best guess of a segment of the particles' real trajectory. The double track resolution is ultimately given by the possible separation of the individual clusters. In a s t a n d a r d drift c h a m b e r only incomplete information is obtained: the leading edge discrimination yields the distance of the cluster from the a n o d e which has the shortest drift time. The best guess for the trajectory uses assumptions on the cluster density a n d the angle of the trajectory. The most severe d r a w b a c k is that the diffusion of the drifting electrons enters fully into the error of measurement. This effect limits the overall resolution of d r i f t c h a m b e r s to a b o u t o = 150 ffm at atmospheric pressure (ref. 16) and to a b o u t o = 50 ,am at p = 4 atm (ref. 17). A better resolution could he o b t a i n e d by measuring a more a p p r o p r i a t e fit to the track segment which means ultimately the spatial reconstruction of all ionisation electrons. This spatial reconstruction of the ionisation structure is the aim of the time expansion c h a m b e r (see ref. 18). The principle of operation is shown in fig. 14. The amplification region is separated from the drift region by a fine grid. The electric field in the drift region is adjusted to give a low drift velocity while in the amplification region a fast drift velocity is obtained. Gases with large inelastic cross-sections are well suited. This a r r a n g e m e n t has the following advantages:
;
/~_~
-
D P-I~T
' - - .
-
"
- H V
pw
DETECTION REGION
DRIFT REGION
Fig. 14. Time expansion chamber (principle). A: anode ,,,,'ire. PW: pick-up wire.
- the drift path distortions near the anode wire are negligible when the effect is translated into the drift region; the anode signal is fast enough to follow the time development of the spatial structure of arriving ionisation; recording of the anode signal shape with transient recorders is in the time scale which can be handled by m o d e r n electronics. The introduction of two additional wires (pick-up wires) very close to the a n o d e wire provides the measurement of the.v coordinate needed for tracks under angles 0 :~ 0 with respect to the grid plane (fig. 14). 3.1. Test chamber
Fig. 15 shows the electrode a r r a n g e m e n t of the c h a m b e r which is housed in a larger gas vessel. The dimensions of the sensitive surface (20 cm x 20 cm) are of the order which will be used in vertex c h a m b e r s close
/FRAME i
/ GRIS 2mr~
±
,~
o
F
o!2 ~ •
..............
671<
Fig. 15. Electrode arrangement of the test chamber.
A.H. Walenta / State-~-the-art and ~plication ~ wire chambers
RUN ANODE SIGNAL
47
EUEHT
AHODE SIGNAL
12
64.0
$1.~.
St . 2
O.B8 mm
Z
eS.6 • Le.BQ.0
717
11
64.0"
u
73
~N
• 100
, tSO
f 200
2Se
~Z
38.4'
O u
25.6
0.41 m m
--I-
A
12"81 e.Q
MDC CELLS
......
1S0 ADC CELLS
r ....
2Q0
a-O 2S0
Fig. 16. Display of digitized signals from two adjacent anodes for a double track event, 10.5 ns per channel.
to the b e a m pipe in experiments at colliding b e a m facilities. The anode wire a n d the two pick-up wires are connected every 5 cm by a small epoxy droplet to avoid electrostatic forces causing instabilities. The potential of the two field wires can be adjusted for the desired range of ionisation collected on the anode wire. Measurements were made with a collection of 2.5 m m track length for a particle parallel to the grid. The anode signal is read out by a flash A D C after passing a shaper a n d stored in a fast memory. The difference signal of the pick-up wires is h a n d l e d the same way. Four flash A D C - m e m ory units * digitizing the signals in 10 ns intervals have been connected to two a n o d e wires and the corresponding pick-up wires. 3.2. Measurements Fig. 16 shows the signals on the two anode wires for a double track event recorded in a test beam. It has been selected to show the limit where the reconstruction program still finds two tracks with more than 95%. The m i n i m u m double track resolution of A x = 400 ~ m agrees well with calculations based on b r o a d e n i n g of the signals by diffusion. For the m e a s u r e m e n t of position resolution the a r r a n g e m e n t of fig. 17 has been used. The two outer wires with s t a n d a r d leading edge discrimination determine the track angle 0 over a long lever-arm requiring a coincidence between these signals. The two inner wires equipped with flash A D C s measure the same angle over a shorter lever-arm enabling higher resolution to be measured. F u r t h e r m o r e the angular distribution of the tracks defined by the outer wires has been
recorded and can be used to unfold the m e a s u r e m e n t of the inner wires. The data of the inner wires have been treated by simulating the leading edge m e t h o d taking the timing of the first bin above noise a n d by calculating the center of gravity of the whole signal (centroid method). Fig. 18 shows the unfolded results expressed in s t a n d a r d deviations of the c o n t r i b u t i o n of a single wire. The dashed line represents the calculation following the formula for leading edge discrimination (ref. 19). '77
o
__%, 23 ~ n e
(1)
where o 0 is the diffusion of a single electron and ne is the n u m b e r of electrons contained in the signal. The full line is a calculation of the formula for the centroid method o = Oo/q~ee.
(2)
Clearly o 0 is a function of drift distance x. A literature value of o o = 2.2 x 1 0 - 2 ~ x (cm) a n d n c = 14
[o
o,
"
- -
~
I~
{i--[~--
tuner wires
L
ter wires * °
FADC's TRLGGE#
* The units were obtained from the University of Lund, Sweden.
Fig. 17. Read-out for measurement of position resolution. 10 = 7 cm, l i = 1 cm. I. PLENARY TALKS
74
4.11. 14"alenta ' ,S'lat{, :{/-the art and aiv?/icatlon q/wire ct/amher~
~ooi
apprc, x i n m t i o n of the m a p p i n g of t folh+wing tile field lines o n t o ~t gives tga' - }//.v. with s the d i s t a n c e of tire a n o d e to tile grid. T h i s yields
u,i
II
i
7
3PIt
i
loo~
b)
i
b
I
-.,.t i s
(3
.
24V2
I
F o r r < I~ it c a n be replaced b~ A P t ! A l, ~ a n d t h e r e f o r e the c e n t e r of g r a v i t y for an e x t e n d e d track b. given b \
i
0
5
10 x {ram)
15
20
2S
+: ,f.x/',
Fig IN. L;nflqded resolution of single tuner wire ',ersus drift length, (a) calcuhued resolution [eq. (1)1.
electrons/2.5 m m has been used. The comparison w i t h tile measured points shows that the centroid method works as expected and that the systematic errors are certainly below 40 fire. Fig. 19 shows the expected resolution for other gases. It is of interest to note that increased pressure would improve the position resolution by 1/p while the leading edge method follows at best 1 / ' T p . The above described measurements were done with tracks almnst parallel to the grid and therefore the ionisation fluctuation along tile track is almost invisible. At somewhat larger angles these fluctuation introduce errors since the centroid measurement i,] the drift direction (.v-coordinate, fig. ]4) refers to a point oil the track whose +' coordinate shows fluctuations. With the pick-up wires this coordinate is measured. The difference signal on the pick-up wires for a point charge is given by (ref. 20) A p I + ' c x A sin ~ with A the anode signal. A simple
(4)
T h e track a n g l e is n o w c a l c u l a t e d hv tg 0 --
(.,, > (,-~> /,+ ( n ) (yes
(5)
w h e r e { ) i n d i c a t e s the c e n t r o i d g i v e n by fig. 17.
t l S,
Clll
d ,/.[:~ d,
m e a s u r e m e n t and /,
Fig. 20 s h o w s the r e s u h s . (~nforttulatelv tile contrib u t i o n of tile a n g l e d e f i n i n g the o u t e r n l e a s u r e n l e n t s w a s n o t k n o w n well e n o u g h to p e r m i t an u n f o h t i n g as for the parallel track e v e n t s . T h e w i d t h of this c o t l | r i b u tion n e v e r t h e l e s s is of r o u g h l y the s a m e size as before a n d it c a n be seen t h a t t h e ( ) ' ) - c o r r e c t i o n gives a c o n s i d e r a b l e i m p r o v e m e n t a n d is c o m p a t i b l e v, ith the r e s o l u t i o n of fig. 18. F o r larger a n g l e s the t i m e d i f f e r e n c e of arrival of i n d i v i d u a l c l u s t e r s b e c o m e s large e n o u g h that the,,' app e a r as i n d i v i d u a l s p i k e s in the a n o d e signal as well as in t h e p i c k - u p wire signal (fig. 21). T h e n tile q u a n t i t y A P H / ' A c a n be c a l c u l a t e d for each p e a k (fig. 221. F h i s d i s p l a y s h o w s the s i g n a l s of a d o u b l e track event. U s i n g relation {3) r, ix c a l c u l a t e d a n d f r o m the a n o d e signals
~001 ,
-30
2'5
i
15 q 5 eIclegeee)
15
8O b) 2c
O": 08;+
oJ > oa
0
.
0
.
.
.
-
5
•
~
10
15
T
20
25
x (ram)
Fig. 19. ('alculated resolution [eq. {2)[ for other ga~,es, t n , l cm ">s. (I) 0.475 Ar+0.475 C~H~, 40.05 Methvlal. (2) 0.5 A r + 0 . 5 CO,. <3) 0 9 (702 .F0.1 i-Call >. (4) 0.8 ( ' ~ H s + 0 2 <('H~)20. (5) same as 14) but 1, 2 aim.
30
-25
15
S
5
e {degree) Vig. 20. Measured angular spread for tracks at angles () : 5 ' (a) without corrections, (b) v, ith corrections fi,r ionisalion fluctuation [eqs. {4) and (5)1.
,4. H. Walenta / State-oJ-thc-art am~ applicatton of wtre ~hamher,s 75
,,, 60 g z,5 2 ~ 3o <
along the centroid .~,,. Fig. 22 shows that this point wise (cluster wise) reconstruction reproduces two straight tracks. This demonstrates that the time expansion chamber with the pick-up wires is capable of measuring m two dimensions the spatial distribution of extended ionisation down to the scale of individual ionisation clusters.
ai
!5. 50
100
150
200
250
f (channeLs) 3O
b)
E 18 D 6 --
-6
-18
i
-3oi . . . . . . . . . . . . . . . . . . . 0 SO 100 1S0
200
4 250
f (channels)
Fig. 21. Display of anode signal and corresponding A Plf" signal for large angle double track event.
1.0-
/"
/' T1
~
T2 g,'
O-
ck
j-
-1.0 -
75
--50
1;0 t;0 t (channets)
b)
2;0
250
3.3. Application of the ttme expansmn
Apart from high precision position measurement the time expansion chamber will find application where the spatial distribution and its precise localisation gives new information. As an example the position measurement of y-rays as in the positron camera with parallel converter plates (ref. 21) is considered. It has been shown, that the position resolution is mainly influenced by the fact that the photoelectrons or Compton-electrons produced in the converter have a considerable angular divergence and multiple scattering. The point where they emerge from the converter, however, is well defined and can be selected with a time expansion chamber. The principle is shown in fig. 23. The signal recorded on the anode wires (here for simplicity connected together) is a pulse train produced by slowly arriving ionisation. Selecting with a linear gate the last portion of the signal also selects the ionisation close to the converter. Fig. 24a shows the position measurement as obtained with the full anode signal for a collimated y-source (ZYNa) anti displays the large variations in the position measurement. Fig. 24b shows the resolution with the gate switched on reproducing well the width of the source. This method in principle is expected to permit a position measurement only limited by diffusion of the order of 100 #m. This cannot be exploited at the moment in positron tomography but may be useful in autoradiography where until now only film can provide appropriate resolution.
. j T2 :I ~
? mrr
~i
/ J
/
"~" ~
'
14 brr,rn
[
/,,
"i-~ converter
i
/-'" < e - . ~ : :,~": d r i f t
~naL-~-ca 1jus - - ~ P-'A
P+
Fig. 22. Event from fig. 21. (a) A p W / A for each identified cluster as a function of drift time, (b) in space reconstructed event, cluster by cluster.
--4Fgate Fig, 23. Principle of time expansion chamber for localisation of y-rays.
I. PLENARY TALKS
A.H. Walenta / State-of-the-art and application of wire chambers
76
References
i*'k
/t
×
!
\
Z ~cJ
°~ I '
-~'
,
' 5 - ~ 1 0 x(mm)
i'i ×
I "l ; 'L
Z
11ram
.
, , --'c~: 5
×
//
\
,
°\
, ~', 10
(ram}
Fig. 24. Measured position resolution for a collimated (1 m m slit) 22Na source. (a) without gate, (b) with gate at the end of the signal.
4. Conclusion It has been shown that wire chambers have a long w a y to go to reach their limits. Better understanding of signal formation and application of modern electronics permit a considerable increase in countrate capability which eventually may challenge integrating imaging systems. On the other hand the electronics reaches a speed and scale of integration where information hidden in the signal shape can be extracted, h could be shown that a large fraction of the spatial extension of the ionisation is conserved in the signal and can be recovered leading to a new dimension in the application of wire chambers which was possible until now only for bubble or cloud chambers or emulsions.
[1] G. Charpak, J. Bouclier, T. Bressani, J. Favier and C. Zupancic, Nucl. Instr. and Meth. 62 (1968) 262. [2] A.H. Walenta, J. Heintze and B. Sch0rlein, Nucl. Instr. and Meth. 92 (1971) 373. [3] R.W. Hendricks, Rev. Sci. Instr. 40 (1969) 1216. [4] G. Schuhz, Thesis, Strasbourg (1976). [5] H. Neuert, Kernphysikalische Messverfahren (G. Braun, G m b H , Karlsruhe, 1966). [6] R.A. Bole, A.T. Hrisoho and P. Rehak, IEEE Trans. Nucl. Sci. NS-28 (1981) 603. [7] H.B. Jensen, H, Okuno and A.H. Walenta, Proc. 1978 lsabelle Summer Workshop, BNL 50885 (1979) 139. [8] P.H. Pile et al., to be published. [9] A.H. Walenta, R. Hackenburg, V. Manzella, D. Maurizio and H. Pierkarz, to be published. [10] W.D. Farr and G.C. Smith, Nut1. Instr. and Meth 206 (1983) 159. [11] V. Radeka, IEEE Trans. Nucl. Sci. NS-21 (1974) 51. [12] W. Farr and J. Heintze, Nucl. Instr. and Meth. 156 (1978) 30l. [13] A. Etkin, IEEE Trans. Nucl. Sci. NS-26 (1979) 54: E. Platner, Proc. 1981 Isabelle Summer Workshop. BNL 51443 (1981) 1243. [14] E. Rubenstein, E.B. Hughes, L.E. Campbell, R. Hofstadter, R.L. Kirk, T.J. Krolicki, J.P. Stone, S. Wilson, H.D. Zeman, W.R. Body, A. Macovski and A.C. Thompson, Synchrotron Radiation and its Application to Digital Subtraction Angiography, SPIE Vol. 314, Conf. on Digital Radiography, 42 ( 1981 ). [15] H.D. Zeman, E.B. Hughes, L.E. Campbell, R. Hofstadter, R.L. Kirk, T,J. Krolicki, J. Rolfe, J.P. Stone, S. Wilson, E. Rubenstein, A.C. Thompson and J.T. Walton, 1EEE Trans. Nucl. Sci. NS-29 (1982) 442. [16] A.H. Walenta, IEEE Trans. Nucl. Sci. NS-22 (1975) 251. [17] W. Farr, J. Heintze, K.H. Hellenbrandt and A.H. Walenta, Nucl. lnstr, and Meth. 154 (1978) 175. [18] A.H. Walenta, IEEE Trans. Nucl. Sci. NS-26 (1979) 73: A.H. Walenta, J. Paradiso, H. Hofer, G. Viertel and J. Fehlmann, Proc. Int. Conf. on Instrumentation for Colliding Beam Physics, Stanford 1982, SLAC-250, UC-34d. p. 34; D. Mattern, A.H. Walenta, J. Fehlmann, H. Hofer, J. Paradiso and G. Viertel, to be published. [19] V. Palladino and B. Sadoulet, Application of the Classical Theorie of Electrons in Gases to Multiwire Proportional and Drift Chambers, LBL-3013 (1974). [20] A.H. Walenta, Nucl. Instr. and Meth. 151 (1978) 461. [21] J.E. Bateman and J.F. Connolly, Nucl. Instr. and Meth. 156 (1978) 27.
65
STATE-OF-THE-ART AND APPLICATION OF WIRE CHAMBERS A.H. WALENTA
Universi O, of Siegen, 59 Siegen, Germat~v
Recent improvements in chamber design and readout electronics have allowed the construction of position sensitive detectors with very high rate capabilities as used in beam spectrometers and at high intensity X-ray sources. The measurement of the spatial ionisation structure (time expansion chamber) with a waveform digitizer allows high precision position measurement for charged particles and -{-rays. Applications in particle physics and nuclear medicine are discussed.
I. Introduction More than a decade ago a new generation of large area detectors, the proportional chambers (]-ef. 1) and muhiwire drift chambers (ref. 2), were introduced providing information mainly in the time and space domain about ionizing particles considerably surpassing the prevailing spark chambers. But new projects initiated by the capabilities of the wire chambers stimulated a continuous improvement which could only be supported by a better understanding of the basic processes in the detector and an application of advanced electronics. It is of interest to follow in some detail the contribution of both aspects to the "state-of-the-art" of wire chambers. In the following one example will be given in the time domain focusing on the high rate behaviour and another in the space domain focusing on position and double track resolution for charged particles.
2. High rate chamber There are two different aspects of MPWCs and multiwire drift chambers operating at high rates. The first refers to the fundamental limits of saturation effects of gas gain, limited lifetime, and limitations due to pulse formation and shaping, problems which are principally the same as in ordinary cylindrical proportional counters and are described in the literature. The second is typical of multiwire chambers providing additional information such as position in one or two coordinates, track angle, m o m e n t u m and mass of particles. This very complex information has to be processed and stored. If only rare events out of a high intensity background are of interest (as is mostly the case in particle physics) the trigger principle provides a powerful solution and then only the signals occurring during a memory time set by the whole system has to be recorded. For most uses in applied fields (medical and biological 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland
research, crystallography etc.) the high rate events themselves are the subject of interest and, as we will see, new aways of dealing with this information have to be found.
2. l. High rate limitations As the count-rate in ordinary proportional counters is increased occasionally a drop in pulse height will be observed. This effect has been explained (ref. 3) by the accumulation of space charge in the drift region produced by the positive ions generated in the gas amplification which lowers the electric field strength near the anode. The calculiation of the effect can be simplified by determining the equivalent drop of anode voltage causing the same lower field strength and inserting this drop of anode voltage into the formula for gas amplification as a function of anode voltage. Then one obtains for the relative gas gain G / G o with Go the value at very low rates:
G / G o = exp( - F~a2QoN ), where
F
kp In( r , / r i ) 8~rzc0~ + V
and p is the pressure, r, and r~ the outer and inner radii of the counter, respectively, #+ the ion mobility, V the applied voltage, Q0 the charge in a single avalanche and N the count-rate. A simplified formula for the gas amplification defines k and V0: G = exp[k(V-
V0) ].
F changes only little with the geometry and choice of gas (at given pressure) and therefore the most important design parameter is ra which corresponds in a multiwire drift chamber with roughly rectangular cell structure to half the gap width or distance to the potential wires. I. PLENARY TALKS
66
.1. I1. II a l O l l a
S t , l h > o~ - I/iv
In addition to) a homogeneous reduction c>f pulse height an efl'icienc3 lo,,,s has been obser,,ed in proportional chambers operated at high rates (ref. 4). A possible explanation is the shielding of the anode v>ire bx the positive space charge as long a~, it is ch)~,e to the ~ire such that subsequent arriving electrons tire not sufficiently nmhiplied. It is expected that this effect d e p e n d s on the spatial extension of the a,+alanchc near the ~irc. T h e gas c o m p o s i t i o n , gas gain and the aiiodc radiu,,, ~,
O,.d(t)~
ra
~lil(t,'.'<>
# 1)] •
,ailere s'tJ=#<-~,~,/2lt li~ is a characieristic time o f ti~e signal developlllcnt, c = In G / t i , p ' is the ion nlobilit,, and li~ the applied anode vohage. One notices for t = 0 a signal of Qc.l(O) = Q ( l n
r i - In ;'a, )
v, hich is called the electron c o m p o n e n t generated b \ the fast nlo',ing electrons. This signal has a ri.setinle c o n > parable to the formation of the awtlanche during tile last few mean free paths of ionisation which can be con,sMered here to be prompt. ]'he signal for s' > 0 is culled the ion c o m p o n e n t which has a rather steep rise but reaches the maxilnum only after a relative hmg time. For fast counting this signal has to be differentiated. This can be simply done by terminating the anode by a low impedance (usually a coirimon base ainplifier is used, but the same results are in principle obtained bv sharp differentiation after a voltage or charge sensitive amplifier) and one obtains the "current'" signal
,(~)__'4_0= dt
G, 2c
i,,,< 1
t/t o
* A nlorc detailed treatnlent can be found in textbooks, fo~ example see ref, 5.
art
, u l d a p f U u
1 his ,signal alrcad\ has a narroxg x~idth at half maxinmnl of l~ btlt il still tlas a long tall. F u r t h e r sll~.tpillg b\ d i f f e i e n t i a l i o n or by i/lille im~,l~ed cilcuit~ (ief ~,) icmo~es thi~ tail. D e p e n d i n g on tile l i m i t e d b a n d ~ i d t l l of the a x a i k i b l e p rca l l i p l i fi crs OllC obtairis a MgllaI ~ i t h a base w i d t t / o r a f0~ tO a b o u t lO{i~. Since /{,, ~- r, t "~.
C~.til\ [701" hlghL'r ciIcrg\ lhc f a i l l e t)l thc pht)lt)L'lccirt,l'i dcfili¢~, "i <. ];ii3all\. increased rates raise tt/c quesliol~ <,1 the l i m i t e d iiletiine of tile COUl'ltt~rs. fqnal mol-iclllMoii~ are it
67
,4.11. Walenta ./ Ntale- of the-art and application ,{[ wire chamherv 2.2. ( ' h a m h e r construction
~'254mm I
The c h a m b e r was designed for the focal plane of a magnetic spectrometer covering a beam profile of about ' " × 4". A c h a m b e r with a sensitive surface of 1" x 5" is planned * and a test c h a m b e r of 1" × 2" has been built and tested with wires 1" long (ref. 9). The wires are stretched over a frame made of standard PC-material ( i ' , " thick) which carried at the same time the printed circuit for the amplifier. The construction parameters are given in the following list (see also fig. 1): anode wire: gold-plated tungsten, ~, rail diam., anode wire spacing: ~',,", potential wire: ( ' u - B e , 2 rail diam., cathode: carbon coated aluminized mylar, gap width (cathode +cathode): ' " . The opened c h a m b e r is shown in fig. 2. The emitter follower circuits are visible in the spaced out part of the c h a m b e r frame. The signals are transmitted through a f l a t b a n d cable with a shielding grounded on one side to reduce crosstalk between channels.
I
32mm .
.
.
.
:
.
f
c
t
A
PW
Fig. 1. Flectrode arrangement of high rate drift chamber.
2.3. R e a d out electronics
In order to minimize the n u m b e r of parts and consequently space, there are only a few design parameters left to be optimized, with the emitter follower given as the input stage. Since as a second stage a low noise c o m m o n base amplifier is used the d o m i n a n t noise source is the series noise from the emitter follower +. It could be shown (ref. 11) that a good approximation for the series equivalent noise charge is given by , .2
1 e2 +2
E?v(" =~t~ " C i " [ 1
+ ]
Im
3 R,~,~]
2
'
with e n equivalent series noise voltage per { H . , ('i,, tile input capacity, R,~ the input resistance and t~ a typical shaping time which is, for a triangular shape, equal to the full width at half maximum. Since t m corresponds approximately to the full width at half m a x i m u m of the clipped signal (tm = 10... 15 ns) it is possible to choose Rin('in << t m and then E N Q (x enCin, e , is determined by the operating conditions of the transistor and can be found in data sheets. For the high b a n d w i d t h required here it was found that the transistor BFT25 would be profitable because of the low noise figure at low currents and the high g a i n - b a n d w i d t h product. Fig. 3 shows the equivalent noise of the emitter follower as a function of input capacity compared with the performance of a c o m m o n base low noise amplifier * It turned out that the rate requirements were more modest and a somewhat less ambitious chamber with twice the wire spacing has been built (ref. 8). + A more detailed analysis of the noise of this front end circuit has been described elsewhere (ref, 10).
Fig. 2. View on the opened chamber. Only a few preamplifiers are assembled.
14
E
-c:,
12] /" /
/
/
•
•
//[/
o
/ oJ
a} / J
/ • •b}
t~ LZU
6"
//
•
//, 4•
2
/
,/ 0
10
20
30
~0
SO
60
Co (pFl Fig. 3. Equivalent noise charge (rms) measured for full bandwidth (100 MHz), (a) common base, (b) emitter follower. (ref. 6). This m e a s u r e m e n t shows an additional bonus for the emitter follower of somewhat lower noise over the whole range of input capacity. In the present chamber a total input capacity of 2.7 p F and a noise of E N C = 103% was obtained. I. PLENARY TALKS
68
.4. H. Walenta / State- of- the- art and application of wire chamhers
pr eamp[ifier on chamber
amplifier & shGper
discriminator (MVL 100)
shiftregister (E]L 8 )
100
4++
a) ___
-
i
-
_
÷
o~
!
+ ÷
v
102L, hit i
uJ
50-
6V
#
twisted pair cable + Fig. 4. Read out electronics (schematically). 0
•'7
6
'8
scintillator q c0[[imated beam
'9 1'0 UA (kV)
111
1'2
1'3
prop. counter
I
~ead ~J~source2 (10mCi
bl
10
bloc"er c
+ i ".'." ." .r.]1~........ ; testchamber L~J~source
+
1 (.1 mCi
Fig. 5. Set-up for measurement with minimum ionizing tracks.
The read out electronics is shown in fig. 4. The shaping is adjusted to provide a signal with 30 ns at the base necessary to avoid double or afterpulses with minimum ionising particles. A slight undershoot has been added to make the system easily adjustable and reliable for larger numbers of channels. The discriminator (LeCroy MVL-100) was set at a threshold corresponding to a signal charge of 10% o at the input, about a factor of two above the peak noise in order to guarantee stable operation. With an ECL-output signal of the MVL-100 with a width of 40 ns, which equals the double pulse resolution reached over a twisted pair cable, the multihit TDCs * consisting of shift registers operated with a clock frequency up to 140 MHz. 2.4. M e a s u r e m e n t s
The system has been tested with minimum ionizing tracks in a set-up shown in fig. 5. A fine collimated beam of B-rays from a weak 9°Sr-source crosses the c h a m b e r and provides the trigger signal. The efficiency and drift time spectra as a function of beam position can be measured. Two additional hot 9°Sr-sources (10 mCi) are arranged to irradiate the chamber from the back producing a high background rate without disturbing the trigger. For a background rate up to 3 × 105
* These units are basically the same as the time digitizing electronics of the JADE experiment (ref. 12).
~;
.5
+, 950
9;0
1'00
x(inch)
~o x
2-
c)
+
c c
~n
== o
+
-k
+ +
6'0
70
8'0
9'0
1;0
110
t Ins) Fig. 6. Results obtained with minimum ionizing tracks. II, low rate (about 10 events/s), +, high rate (3x 105 cts/s on 4 mm wire length ×2.5 mm drift space, corresponding to 3× 106 cts/s cm2); (a) plateau, (b) scan over drift space, (c) drift time distribution for x = 0 . 4 mm, i x = 0 . 8 1 mm (fwhm), gas A r / C 2 H~, (50/50).
over an area of 2.54 mm X 4 mm (corresponding to a rate of 3 x 106/s cm 2) no effects on the plateau, the position d e p e n d e n t efficiency or drift time spectra, could be observed (fig. 6). The limits of the chamber have been investigated
A.H. Walenta / State- of- the- art and application of wire chambers
69
1.0
0 ~ " ) ~~./*.2.10 5 5.6-106
~") ~ ~ 6 . 1 0 ~ ,
J0.5-
"~, \ 210~
t..O
~.10~
i
2 '10 ~'
10"
i
10 s N (ram'1 s-1 )
1
t
10 6
10"
Fig. 7. Pulse height (normalised to pulse height at low rates) versus rate/wire length. Parameter: avalanche size (electrons). (a) standard drift chamber, (b) high rate chamber.
with an X-ray generator (50 kV, Cu-target) providing 8 keV X-rays within a spot of 2.6 m m diameter and intensities up to 2 X 107 s-1. F o r simplicity it is assumed that this intensity is spread evenly over an anode wire length of Al = 2.6 m m which is used to calculate the rate per wire length. Fig. 7 shows the pulse height as a function of this rate for a s t a n d a r d c h a m b e r ( r a = 5 mm, r i = 10 # m ) and for the high rate chamber. For the same avalanche size (here 4 x 105 electrons) the high rate c h a m b e r is able to h a n d l e with the same 2% amplitude drop 1½ orders of m a g n i t u d e higher rates. If the same performance is required the curves for an avalanche size of 5 x 106 in the s t a n d a r d c h a m b e r - a m p l i f i e r unit have to be c o m p a r e d with 105 in the high rate c h a m b e r with low noise electronics allowing it to handle almost 3 orders of m a g n i t u d e higher rates. With a m o n i t o r c h a m b e r at m u c h larger distance the X-ray flux has been measured without saturation effects allowing the determination of the efficiency of the high rate chamber. Fig. 8 shows the measured rate in the test c h a m b e r against the true rate (determined by the monitor-chamber). The absolute value of the true rate can be o b t a i n e d at low intensity. Up to a rate of 106 s - ] no loss can be observed. At the highest rate the loss of efficiency agrees within ___1% with the dead time losses calculated from the k n o w n discriminator dead time. Therefore one can conclude that the real efficiency loss due to limitations in the c h a m b e r is less than 1%. As a conclusion it is reasonable to assume that multiwire (drift) c h a m b e r s can be built to handle up to 10 6 c o u n t s / m m s which translates in the geometry of the test c h a m b e r to an area rate of 4 x 10 7 c o u n t s / s cm 2. Higher rates of 108 c o u n t s / s cm 2 are possible if
10! ea E c~
=
/
/
10~
,3.)
L
106-
/
/
f
1c 10 ~
10 ~
10 ~
rQte(s-1) monitor chamber Fig. 8. Rate in test chamber versus intensity (rate in monitor chamber). (a) true rate, (b) measured rate, +, true rate with dead time correction; Qo = 103 electrons.
the requirements on energy resolution are somewhat relaxed.
2.5. Application of high rate chambers These high rate c h a m b e r s with their relatively short dead time of 40 ns a n d even shorter m e m o r y time of 28 1. PLENARY TALKS
70
A,tt. H~Uenta / State-q/-the-arl and apphcation ,1 wire chamher~
ns are certainly useful in particle physics as beam chambers, target chambers or in experiments at p p-colliders with very high rates in the innermost ring around the beam pipe. The readout can be accomplished with the multihit TDCs mentioned above or with even higher resolution shift registers (ref. 13). The interesting question is, however, can the chamber possibly compete with other detectors in applied fields where the high intensity radiation has to be recorded without the help of a "'trigger'" system. For comparison a picture containing 512 × 512 pixels (:area resohition elements) of a good television picture quality (5% intensity resolution) would contain in total n,,,t - 10 ~ counts. A television picture is generated in At = 40 ms and therefore a countrate of 4 × 10'~/s would be equivalent to a television s\'stem. A high rate chamber with a sensitive area of 10 ~< 10 cm ~ m principle could collect 10 m c o u n t s / s and therefore be competitive with other image producing systems. The important advantage lies in the fact that it is a quantum counting system while all other systems are integrating devices. Counting individual quanta allows the measurement of additional properties such as energy, angle or timing, providing subsamples or rejecting background. An even greater advantage of quantum counting systems is the low noise which is by definition given by quantum statistics and the high linearity and dynamic range which are only limited by dead time losses. In the foilowing a possible application of the high rate chamber is discussed where the last points are crucial In the diagnosing of heart diseases important progress could be made by improvement of the coronary angiography which is the visualisation of coronary arteries. Usually a catheter is inserted into a femoral artery and positioned such that the tip reaches the ostium o[ the coronary artery. Then the maximum strength of iodine contrast agent is injected and an X-ray image is acquired. This method carries a high risk of mortality and morbidity: consequently it is only used in special cases. Therefore a method has been developed (ref. 14) where the contrast of arteries against the background could be enhanced by subtracting the background quantitatively such that injection of the iodine contrast agent through a catheter directly into the coronary arteries occasionally may be replaced by peripheral venous injection. The method consists of taking two pictures one with a monochromatic X-ray beam slightly above the K-edge of iodine ( E K = 33,2 keV), the other slightly' below (fig. 9). The mass attenuation coefficient of iodine changes drastically while for soft tissue and bone the changes are negligible if the two X-ray energies differ by small amounts. This can be accomplished at sufficient intensity with synchrotron radiation sent through a switchable monochromator. A successfully operated test set-up (ref. 15) is shown in fig. 10. After the monochromator
i00-
iodine 10~E to,
~on
musc(e
7o
7o
E(keV}
<
~c
Fig. 9. Mass attenualioil coefficient versus X-rax, energY. the beam is narrowed by' a slit collimator to aboul I ran3 in vertical direction and fanned out in the horizontal direction covering the object and a one-dimensional position sensitive integrating silicon strip detector. Vertical mechanical movement of the object provides the scan m the second dimension. First results are shown in fig. l l. A straightforward calculation gives for the signal-tonoise ratio of the iodine struclure S'
1
a s = '~ ( v/°
)"c' j2
N
.~,'< "
where A ll_t/p ).l is the change of mass attenuation c'oefficient for iodine, Cj the iodine thickness and 5; the countrale with , I N its error. For S / A S _>_3 and ( j <_ 1 m g / c m 2 one obtains A N / N < 6.7 x 10 ~. The linearit~ errors and the noise introduced by the detector have to be ~<2× l0 ~ in order to have the quantum noise as the dominant contribution which is necessary I o keep the required beam intensity and the radiation exposure of the patient at a minimum. This requirement is relatively easy to fulfill with a wire chamber. The set-up in fig. t2 shows the principal arrangement for a two-dimensional detector, assuming the synchrotron beam can be fanned out by a curved mirror also in vertical direction to cover the whole object. A detector of dimensions 10 cm × 10 cm is sufficient. With the accumulation of 2CLLIH ATOP
i'
SYNCHROTRO N
R ADLATION
I
/ N INOEHRONATOP
T,
i ?
Fig. tO. Set-up for dichromatic angiograph'~ (rcf. {5L
A.H. Walenta / State- of- the- art and application o[ wire charnberf
a
b
71
c
Fig. I1. X-ray images obtained for the coronary arteries (a) above K-edge, (b) below K-edge, (c) image after logarithmic subtraction (ref. 15).
N = 2.2 x 10 a counts in one resolution element (pixel) of 0.5 m m × 0.5 m m in the quiet period of the heart (400 ms) one calculates a rate of N = 2.2 × 10 v c o u n t s / s cm 2. Although this rate still gives good pulse height m e a s u r e m e n t (fig. 7} the rate per wire will be 5.5 × 107/s resulting in too high deadtime losses. If the total rate is to be distributed over a stack of, for example, 10 chambers, the dead time losses of 5%. can be corrected to give an overall precision of 2 × 10 3 In order to increase the total thickness of the counters a drift space is added to the thin amplification region which is separated by a fine grid. In this way only a very small fraction ( = 1%) of the positive ions accumulates in the drift space and does not disturb the gas amplification. Since the wire spacing of 2.5 m m is too coarse for the expected resolution of 0.5 m m (fwhm) the position will
be interpolated using the difference signal from the potential wires in the same way as explained below in the time expansion chamber. The second coordinate can be read out by cathode strips arranged at an angle + 7.5 ° with respect to the anode or by fast delay lines or by charge division on the anode wire. There is another reason for distributing the total rate o n t o several planes (fig. 13). The memories receiving the information after the derandomizer handle a fraction of the total rate. The subdivision of the detector makes it
apW,~¢
QRID
ANODES
AMPLIFIER
!!!: ii!
a:
,
L_
......
:l-
!
Fig. 12. Chamber set-up for noninvasive angiography. Total number of wires: 400; gas filling: Xe, 4 atm.
Fig. 13. Read out of a single chamber plane. Circuits for two wires are shown. 1. PLENARY TALKS
72
A. tt. Walenta / State-of-the-art and apphlcatton of wire chambers
possible to use m e d i u m speed m e m o r y associated to detector sections in parallel. The electronics, particularly the memory, may appear to be rather " b u l k y " but it is i m p o r t a n t to realize that such a system is currently feasible. A rapid advance in electronics can be expected in the next few years, allowing a more concentrated memory to be used. It is i m p o r t a n t to note, that the typical limitations of p r o p o r t i o n a l wire c h a m b e r s in imaging applications can be overcome in this example: the relative low X-ray energy gives already an efficiency of e = 95% for xenon at 4 atm and a total detector d e p t h of 20 cm. The range of the photoelectron is less than the desired resolution of 0.5 m m a n d the specific imaging principle avoids parallax problems. Keeping in mind the problems of c o m p e t i n g integrating systems for this application it seems that the multiwire c h a m b e r s have a good chance of application in the field of noninvasive angiography.
3. Time expansion chamber In the spatial d o m a i n for muhiwire c h a m b e r s and drift c h a m b e r s the quantities of interest are position resolution and double track resolution. A somewhat more careful definition of these quantities reveals a more complex situation. As k n o w n from cloud c h a m b e r a n d b u b b l e c h a m b e r pictures a charged particle leaves b e h i n d in the detector m e d i u m a n u m b e r of ionisation clusters. A fit through all these points gives the best guess of a segment of the particles' real trajectory. The double track resolution is ultimately given by the possible separation of the individual clusters. In a s t a n d a r d drift c h a m b e r only incomplete information is obtained: the leading edge discrimination yields the distance of the cluster from the a n o d e which has the shortest drift time. The best guess for the trajectory uses assumptions on the cluster density a n d the angle of the trajectory. The most severe d r a w b a c k is that the diffusion of the drifting electrons enters fully into the error of measurement. This effect limits the overall resolution of d r i f t c h a m b e r s to a b o u t o = 150 ffm at atmospheric pressure (ref. 16) and to a b o u t o = 50 ,am at p = 4 atm (ref. 17). A better resolution could he o b t a i n e d by measuring a more a p p r o p r i a t e fit to the track segment which means ultimately the spatial reconstruction of all ionisation electrons. This spatial reconstruction of the ionisation structure is the aim of the time expansion c h a m b e r (see ref. 18). The principle of operation is shown in fig. 14. The amplification region is separated from the drift region by a fine grid. The electric field in the drift region is adjusted to give a low drift velocity while in the amplification region a fast drift velocity is obtained. Gases with large inelastic cross-sections are well suited. This a r r a n g e m e n t has the following advantages:
;
/~_~
-
D P-I~T
' - - .
-
"
- H V
pw
DETECTION REGION
DRIFT REGION
Fig. 14. Time expansion chamber (principle). A: anode ,,,,'ire. PW: pick-up wire.
- the drift path distortions near the anode wire are negligible when the effect is translated into the drift region; the anode signal is fast enough to follow the time development of the spatial structure of arriving ionisation; recording of the anode signal shape with transient recorders is in the time scale which can be handled by m o d e r n electronics. The introduction of two additional wires (pick-up wires) very close to the a n o d e wire provides the measurement of the.v coordinate needed for tracks under angles 0 :~ 0 with respect to the grid plane (fig. 14). 3.1. Test chamber
Fig. 15 shows the electrode a r r a n g e m e n t of the c h a m b e r which is housed in a larger gas vessel. The dimensions of the sensitive surface (20 cm x 20 cm) are of the order which will be used in vertex c h a m b e r s close
/FRAME i
/ GRIS 2mr~
±
,~
o
F
o!2 ~ •
..............
671<
Fig. 15. Electrode arrangement of the test chamber.
A.H. Walenta / State-~-the-art and ~plication ~ wire chambers
RUN ANODE SIGNAL
47
EUEHT
AHODE SIGNAL
12
64.0
$1.~.
St . 2
O.B8 mm
Z
eS.6 • Le.BQ.0
717
11
64.0"
u
73
~N
• 100
, tSO
f 200
2Se
~Z
38.4'
O u
25.6
0.41 m m
--I-
A
12"81 e.Q
MDC CELLS
......
1S0 ADC CELLS
r ....
2Q0
a-O 2S0
Fig. 16. Display of digitized signals from two adjacent anodes for a double track event, 10.5 ns per channel.
to the b e a m pipe in experiments at colliding b e a m facilities. The anode wire a n d the two pick-up wires are connected every 5 cm by a small epoxy droplet to avoid electrostatic forces causing instabilities. The potential of the two field wires can be adjusted for the desired range of ionisation collected on the anode wire. Measurements were made with a collection of 2.5 m m track length for a particle parallel to the grid. The anode signal is read out by a flash A D C after passing a shaper a n d stored in a fast memory. The difference signal of the pick-up wires is h a n d l e d the same way. Four flash A D C - m e m ory units * digitizing the signals in 10 ns intervals have been connected to two a n o d e wires and the corresponding pick-up wires. 3.2. Measurements Fig. 16 shows the signals on the two anode wires for a double track event recorded in a test beam. It has been selected to show the limit where the reconstruction program still finds two tracks with more than 95%. The m i n i m u m double track resolution of A x = 400 ~ m agrees well with calculations based on b r o a d e n i n g of the signals by diffusion. For the m e a s u r e m e n t of position resolution the a r r a n g e m e n t of fig. 17 has been used. The two outer wires with s t a n d a r d leading edge discrimination determine the track angle 0 over a long lever-arm requiring a coincidence between these signals. The two inner wires equipped with flash A D C s measure the same angle over a shorter lever-arm enabling higher resolution to be measured. F u r t h e r m o r e the angular distribution of the tracks defined by the outer wires has been
recorded and can be used to unfold the m e a s u r e m e n t of the inner wires. The data of the inner wires have been treated by simulating the leading edge m e t h o d taking the timing of the first bin above noise a n d by calculating the center of gravity of the whole signal (centroid method). Fig. 18 shows the unfolded results expressed in s t a n d a r d deviations of the c o n t r i b u t i o n of a single wire. The dashed line represents the calculation following the formula for leading edge discrimination (ref. 19). '77
o
__%, 23 ~ n e
(1)
where o 0 is the diffusion of a single electron and ne is the n u m b e r of electrons contained in the signal. The full line is a calculation of the formula for the centroid method o = Oo/q~ee.
(2)
Clearly o 0 is a function of drift distance x. A literature value of o o = 2.2 x 1 0 - 2 ~ x (cm) a n d n c = 14
[o
o,
"
- -
~
I~
{i--[~--
tuner wires
L
ter wires * °
FADC's TRLGGE#
* The units were obtained from the University of Lund, Sweden.
Fig. 17. Read-out for measurement of position resolution. 10 = 7 cm, l i = 1 cm. I. PLENARY TALKS
74
4.11. 14"alenta ' ,S'lat{, :{/-the art and aiv?/icatlon q/wire ct/amher~
~ooi
apprc, x i n m t i o n of the m a p p i n g of t folh+wing tile field lines o n t o ~t gives tga' - }//.v. with s the d i s t a n c e of tire a n o d e to tile grid. T h i s yields
u,i
II
i
7
3PIt
i
loo~
b)
i
b
I
-.,.t i s
(3
.
24V2
I
F o r r < I~ it c a n be replaced b~ A P t ! A l, ~ a n d t h e r e f o r e the c e n t e r of g r a v i t y for an e x t e n d e d track b. given b \
i
0
5
10 x {ram)
15
20
2S
+: ,f.x/',
Fig IN. L;nflqded resolution of single tuner wire ',ersus drift length, (a) calcuhued resolution [eq. (1)1.
electrons/2.5 m m has been used. The comparison w i t h tile measured points shows that the centroid method works as expected and that the systematic errors are certainly below 40 fire. Fig. 19 shows the expected resolution for other gases. It is of interest to note that increased pressure would improve the position resolution by 1/p while the leading edge method follows at best 1 / ' T p . The above described measurements were done with tracks almnst parallel to the grid and therefore the ionisation fluctuation along tile track is almost invisible. At somewhat larger angles these fluctuation introduce errors since the centroid measurement i,] the drift direction (.v-coordinate, fig. ]4) refers to a point oil the track whose +' coordinate shows fluctuations. With the pick-up wires this coordinate is measured. The difference signal on the pick-up wires for a point charge is given by (ref. 20) A p I + ' c x A sin ~ with A the anode signal. A simple
(4)
T h e track a n g l e is n o w c a l c u l a t e d hv tg 0 --
(.,, > (,-~> /,+ ( n ) (yes
(5)
w h e r e { ) i n d i c a t e s the c e n t r o i d g i v e n by fig. 17.
t l S,
Clll
d ,/.[:~ d,
m e a s u r e m e n t and /,
Fig. 20 s h o w s the r e s u h s . (~nforttulatelv tile contrib u t i o n of tile a n g l e d e f i n i n g the o u t e r n l e a s u r e n l e n t s w a s n o t k n o w n well e n o u g h to p e r m i t an u n f o h t i n g as for the parallel track e v e n t s . T h e w i d t h of this c o t l | r i b u tion n e v e r t h e l e s s is of r o u g h l y the s a m e size as before a n d it c a n be seen t h a t t h e ( ) ' ) - c o r r e c t i o n gives a c o n s i d e r a b l e i m p r o v e m e n t a n d is c o m p a t i b l e v, ith the r e s o l u t i o n of fig. 18. F o r larger a n g l e s the t i m e d i f f e r e n c e of arrival of i n d i v i d u a l c l u s t e r s b e c o m e s large e n o u g h that the,,' app e a r as i n d i v i d u a l s p i k e s in the a n o d e signal as well as in t h e p i c k - u p wire signal (fig. 21). T h e n tile q u a n t i t y A P H / ' A c a n be c a l c u l a t e d for each p e a k (fig. 221. F h i s d i s p l a y s h o w s the s i g n a l s of a d o u b l e track event. U s i n g relation {3) r, ix c a l c u l a t e d a n d f r o m the a n o d e signals
~001 ,
-30
2'5
i
15 q 5 eIclegeee)
15
8O b) 2c
O": 08;+
oJ > oa
0
.
0
.
.
.
-
5
•
~
10
15
T
20
25
x (ram)
Fig. 19. ('alculated resolution [eq. {2)[ for other ga~,es, t n , l cm ">s. (I) 0.475 Ar+0.475 C~H~, 40.05 Methvlal. (2) 0.5 A r + 0 . 5 CO,. <3) 0 9 (702 .F0.1 i-Call >. (4) 0.8 ( ' ~ H s + 0 2 <('H~)20. (5) same as 14) but 1, 2 aim.
30
-25
15
S
5
e {degree) Vig. 20. Measured angular spread for tracks at angles () : 5 ' (a) without corrections, (b) v, ith corrections fi,r ionisalion fluctuation [eqs. {4) and (5)1.
,4. H. Walenta / State-oJ-thc-art am~ applicatton of wtre ~hamher,s 75
,,, 60 g z,5 2 ~ 3o <
along the centroid .~,,. Fig. 22 shows that this point wise (cluster wise) reconstruction reproduces two straight tracks. This demonstrates that the time expansion chamber with the pick-up wires is capable of measuring m two dimensions the spatial distribution of extended ionisation down to the scale of individual ionisation clusters.
ai
!5. 50
100
150
200
250
f (channeLs) 3O
b)
E 18 D 6 --
-6
-18
i
-3oi . . . . . . . . . . . . . . . . . . . 0 SO 100 1S0
200
4 250
f (channels)
Fig. 21. Display of anode signal and corresponding A Plf" signal for large angle double track event.
1.0-
/"
/' T1
~
T2 g,'
O-
ck
j-
-1.0 -
75
--50
1;0 t;0 t (channets)
b)
2;0
250
3.3. Application of the ttme expansmn
Apart from high precision position measurement the time expansion chamber will find application where the spatial distribution and its precise localisation gives new information. As an example the position measurement of y-rays as in the positron camera with parallel converter plates (ref. 21) is considered. It has been shown, that the position resolution is mainly influenced by the fact that the photoelectrons or Compton-electrons produced in the converter have a considerable angular divergence and multiple scattering. The point where they emerge from the converter, however, is well defined and can be selected with a time expansion chamber. The principle is shown in fig. 23. The signal recorded on the anode wires (here for simplicity connected together) is a pulse train produced by slowly arriving ionisation. Selecting with a linear gate the last portion of the signal also selects the ionisation close to the converter. Fig. 24a shows the position measurement as obtained with the full anode signal for a collimated y-source (ZYNa) anti displays the large variations in the position measurement. Fig. 24b shows the resolution with the gate switched on reproducing well the width of the source. This method in principle is expected to permit a position measurement only limited by diffusion of the order of 100 #m. This cannot be exploited at the moment in positron tomography but may be useful in autoradiography where until now only film can provide appropriate resolution.
. j T2 :I ~
? mrr
~i
/ J
/
"~" ~
'
14 brr,rn
[
/,,
"i-~ converter
i
/-'" < e - . ~ : :,~": d r i f t
~naL-~-ca 1jus - - ~ P-'A
P+
Fig. 22. Event from fig. 21. (a) A p W / A for each identified cluster as a function of drift time, (b) in space reconstructed event, cluster by cluster.
--4Fgate Fig, 23. Principle of time expansion chamber for localisation of y-rays.
I. PLENARY TALKS
A.H. Walenta / State-of-the-art and application of wire chambers
76
References
i*'k
/t
×
!
\
Z ~cJ
°~ I '
-~'
,
' 5 - ~ 1 0 x(mm)
i'i ×
I "l ; 'L
Z
11ram
.
, , --'c~: 5
×
//
\
,
°\
, ~', 10
(ram}
Fig. 24. Measured position resolution for a collimated (1 m m slit) 22Na source. (a) without gate, (b) with gate at the end of the signal.
4. Conclusion It has been shown that wire chambers have a long w a y to go to reach their limits. Better understanding of signal formation and application of modern electronics permit a considerable increase in countrate capability which eventually may challenge integrating imaging systems. On the other hand the electronics reaches a speed and scale of integration where information hidden in the signal shape can be extracted, h could be shown that a large fraction of the spatial extension of the ionisation is conserved in the signal and can be recovered leading to a new dimension in the application of wire chambers which was possible until now only for bubble or cloud chambers or emulsions.
[1] G. Charpak, J. Bouclier, T. Bressani, J. Favier and C. Zupancic, Nucl. Instr. and Meth. 62 (1968) 262. [2] A.H. Walenta, J. Heintze and B. Sch0rlein, Nucl. Instr. and Meth. 92 (1971) 373. [3] R.W. Hendricks, Rev. Sci. Instr. 40 (1969) 1216. [4] G. Schuhz, Thesis, Strasbourg (1976). [5] H. Neuert, Kernphysikalische Messverfahren (G. Braun, G m b H , Karlsruhe, 1966). [6] R.A. Bole, A.T. Hrisoho and P. Rehak, IEEE Trans. Nucl. Sci. NS-28 (1981) 603. [7] H.B. Jensen, H, Okuno and A.H. Walenta, Proc. 1978 lsabelle Summer Workshop, BNL 50885 (1979) 139. [8] P.H. Pile et al., to be published. [9] A.H. Walenta, R. Hackenburg, V. Manzella, D. Maurizio and H. Pierkarz, to be published. [10] W.D. Farr and G.C. Smith, Nut1. Instr. and Meth 206 (1983) 159. [11] V. Radeka, IEEE Trans. Nucl. Sci. NS-21 (1974) 51. [12] W. Farr and J. Heintze, Nucl. Instr. and Meth. 156 (1978) 30l. [13] A. Etkin, IEEE Trans. Nucl. Sci. NS-26 (1979) 54: E. Platner, Proc. 1981 Isabelle Summer Workshop. BNL 51443 (1981) 1243. [14] E. Rubenstein, E.B. Hughes, L.E. Campbell, R. Hofstadter, R.L. Kirk, T.J. Krolicki, J.P. Stone, S. Wilson, H.D. Zeman, W.R. Body, A. Macovski and A.C. Thompson, Synchrotron Radiation and its Application to Digital Subtraction Angiography, SPIE Vol. 314, Conf. on Digital Radiography, 42 ( 1981 ). [15] H.D. Zeman, E.B. Hughes, L.E. Campbell, R. Hofstadter, R.L. Kirk, T,J. Krolicki, J. Rolfe, J.P. Stone, S. Wilson, E. Rubenstein, A.C. Thompson and J.T. Walton, 1EEE Trans. Nucl. Sci. NS-29 (1982) 442. [16] A.H. Walenta, IEEE Trans. Nucl. Sci. NS-22 (1975) 251. [17] W. Farr, J. Heintze, K.H. Hellenbrandt and A.H. Walenta, Nucl. lnstr, and Meth. 154 (1978) 175. [18] A.H. Walenta, IEEE Trans. Nucl. Sci. NS-26 (1979) 73: A.H. Walenta, J. Paradiso, H. Hofer, G. Viertel and J. Fehlmann, Proc. Int. Conf. on Instrumentation for Colliding Beam Physics, Stanford 1982, SLAC-250, UC-34d. p. 34; D. Mattern, A.H. Walenta, J. Fehlmann, H. Hofer, J. Paradiso and G. Viertel, to be published. [19] V. Palladino and B. Sadoulet, Application of the Classical Theorie of Electrons in Gases to Multiwire Proportional and Drift Chambers, LBL-3013 (1974). [20] A.H. Walenta, Nucl. Instr. and Meth. 151 (1978) 461. [21] J.E. Bateman and J.F. Connolly, Nucl. Instr. and Meth. 156 (1978) 27.