Drift properties of tabular drift chambers in high magnetic fields

NUCLEAR INSTRUMENTS

AND METHODS

169 ( 1 9 8 0 ) 4 1 3 - 4 2 1 ,

~)

NORTH-HOLLAND

PUBLISHING

CO

DRIFT PROPERTIES OF TUBULAR DRIFT CHAMBERS IN HIGH MAGNETIC FIELDS G BARANKO, J P GUILLAUD, H OGREN, D RUST, S EMS, S GRAY, B MARTIN and P SMITH

Phystcs Department Indtana Unwerstty, Bloommgton, Indtana 47405, USA Recewed 18 September 1979 We report measurements of the drift characterlsucs of a tubular drift chamber m magnetic fields up to 17 kG Dnft properttes

were measured for argon - CO2 (10%), argon - lsobutane (10%) - methylal (1%) and argon--ethane (50%) mixtures Drift properties are interpreted with a s~mple gas model

1. Introduction We have used a small drift chamber module to measure the drift properties of a new drift chamber configuraUon to be used in a PEP experiment - the HRS spectrometer Several properties of the cyhndrlcal cathode drift chambers were measured W e have determined the total drift time, the radius-drift Ume cahbratlon, and the drift veloclty for several gases as a function of magnetic field (between 0 and 17 kG) parallel to the sense w~res

2. Test module The small test module constructed for these tests is shown in fig 1 The drift chamber cathode tubes were short enough to allow the entire module with electronics to be inserted into an 8 inch gap dipole magnet Eight thin-walled (32 roll) tubes with an outside diameter of 1 0 inches were used for the cathodes Three of the tubes were brass and five BEAM

were aluminum No Important differences attributable to tube material were seen during the test The tubes were held at electrical ground High voltage ( + 2 1 0 0 to + 3 0 0 0 V ) was apphed to the central 5 0 ~ m gold-plated tungsten signal wires The wires were tensioned at 250 g The signal wire ends were positioned and supported by Bakelite stand-offs positioned in the aluminum endpleces These stand-offs held small brass cylinders to which the w~res were soldered The signals were coupled out through a blocking capacitor (600 pf), amplified, and then sent to the TDCs (fig 2) Due to high background noise the effective threshold was set to N 2 m V The clocked TDCs were run in a c o m m o n start mode for all 8 tubes I

i

'o BEAM

076 CM WALL ALUM - TUBE 2 54CM DIAMETER

IRE

(a) Fig 1 (a) The Isometric view of the test module (b) The end view of the chamber

1

(b)

414

G BARANKO et al +HV

6. Data reduction procedures 6 1 r VERSUSt CALIBRATIONCURVES

~M~ PRE~MP SIGNAL

150rns

WIRE

COMMON START

CLOCKED/ ] VERNIER DATA TDC LOGGING /~T 125 aS

+

Fig 2 Schematic of signal processing

3. Magnet - test beam The test module was pos~tioned m a 8" gap magnet with the field ahgned parallel to the signal wires The test b e a m m Beamhne 5 at the Argonne National Laboratory was used for all the measurem e n t s reported here The beam was 3 G e V negatwe p~ons, several inches m diameter centered on the module Typical rates during the test were 102 pmns s - 1 4. Trigger electronics The test module was set up near the center of the magnet 6 " x 6 " scintillators upstream and downstream of the magnet defined the incoming b e a m and m m a t e d the main electromc trigger A small 2 " x 2 " scintillator placed i m m e & a t e l y behind the module was tagged and used for on-hne efficiency determination The raw T D C counts for each of the e~ght tubes and the scintillator tag were logged onto magnetic tape for each event 5. Operating conditions The purpose of these tests was to measure the drift properties o f tubular drift cells at high magnet~c fields Three separate gases were used They were argon (90%) - CO2 (10%), argon ( 8 9 % ) - lsobutane (10%) - methylal (1%), and argon (50%) - ethane (50%) Several ddTicultles were noted m the operation o f the drift tubes, but none were considered serious With argon - CO2 a regeneration effect (multiple pulsing) and a resulting rapid increase m currefit ( > 2 / i A ) occurred at - 2 2 0 0 V This effect also occurred with the ~sobutane mixtures, but at a s o m e w h a t higher voltage This type of instability m a y be c o m m o n to all continuous cathode drift chambers, if, as we believe, ~t Is initiated by electrons that have been photo-emitted from the cathode W e are presently investigating gas ad&tlves that retard this discharge

The T D C s &gltlzed the time interval between a c o m m o n start signal and the signal from each drift chamber wire This time interval Is the s u m of offset time and a drift time The offset time is constant and Is the time between the start pulse and a signal from a particle passing through the wire The offset time was obtained for each wire from the &strlbutlon o f measured time intervals In each case the distribution rose from zero to a plateau in the space of a few nanoseconds The t~me corresponding to the half-height of the plateau was assumed to be the offset time This t~me was subtracted from the measured time intervals before determining the r versus t cahbrat~on The offset time v a n e d from wire to w~re and was different for &fferent gases and high voltage settings Th~s shows that s o m e of our m e a s u n n g uncertainty comes from a pulse height dependent time skewing For s o m e gases the pulse height was larger and consequently the offset t~me was shorter The m a g m t u d e of thin effect was of the order o f ___2 ns After the zero point ~s found we proceed to find the functional relationship between the distance of closest approach of the track and the time measurem e n t The flux of particles traversing the tube is d N / d x = K(x), where x is the coordinate perpen&cular to both the axis of the tube and the &rection of the particles N represents the n u m b e r of particles traversing the tube d u n n g one run s u m m e d over the coordinate parallel to the tube The n u m b e r of recorded tracks n will be related to the flux by an efficiency factor k(x) so that dn/

dx = K (x) k(x) The drift velocxty at each point x or ~ts corresponding point t is o = dx/dt so that dn/dt = v(t) K ( x ) k(x). Since we do not know x as a functton of t, th~s equation is useful only if K(x) and k(x) are constant T h e n d n / d t = a o ( t ) , showing that the distnbut~on of time m e a s u r e m e n t s from a tube is proportional to the dnft velocsty as a function of drift time T h u s --~ d t =

a

v(t)dt

= ax(tm)

415

DRIFT P R O P E R T I E S

or, in other words, the relationship of distance to time is proportional to the integral o f the distribution o f time m e a s u r e m e n t s if the efficiency and flux are uniform This uniformity constraint ts s o m e w h a t relaxed by the further observation that since we are sensitive to two sides o f the sense wire at once we are effectively averaging the flux on both sides o f the sense wire for this reason the condition is that the second spatial derivative o f the flux times efficiency must be zero over the width o f the tube for this method o f work The absolute cahbratlon requires that the constant o f proportionality by evaluated at one point This was done by looking at data from three hit tubes together for each event (see fig 1) The quantity

OkG

tOkG

lTkG

II

1211

###//P#'"

0

P m

<~

Ar- ISOBUTANE 2150 volts

fie

T, + T2 + k~ (7"1 - T2) 2 o

and

i

,oo

i

2oo

~

~o

~o

~

~o

~

~o ,do

TOTAL DRIFT TIME (NSEC)

T~ + T ~ - k ~ ( T 3 - T~)~ were plotted as a function o f T t - T2 and T 3 - T 2 respectively The subscripts refer to each of the 3 hit tubes, T2 being In the middle A horizontal band of good events was observed plus bad events scattered over the plot After cutting out most o f the bad events and averaging the good events, a line was drawn and extrapolated to T~ - T2 or T3 - T: = 0 where we obtain the ttmes T] + T 2 and T 3 + T 2 for the 5kG

IOkG

15kG

Fig 4 Radius of closest approach versus drift time for argon (89%), lsobutane (10%), methylal (1%) for magnetic fields of 0, 5, 10, 17 kG

middle point, that is, the point halfway between sense wires The numbers are different because the flux o f particles is at an angle to a line connecting the sense wires 1 and 3 but the average of Tl +T2 and T 3 + T 2 is the time m e a s u r e m e n t associated

17kG OkG

lOgG

~TkG

O

LJ.J Z
I,LI Z

E3

.-J


2150 volts

~' J
//

/

2 8 5 0 volts

<~

510

I

I

.

.

.

.

'

.

.

.

.

.

.

ioo 15o z~o 250 3OO 350 40O 4~0 50O 55O 600 650 1O0 750

TOTAL DRIFT TIME (NSEC) Fig 3 Radius of" closest approach versus dnf`t time for argon (90%) - CO 2 ( ] 0 % ) at 5, 10, ]5, 17 kG field parallel to stgnal wlrc

i

i00

2o0

i

i

i

I

i

i

30o

400

5oo

600

7o0

8OO

i

900

TOTAL DRIFT TIME (NSEC)

Fig 5 Radms of closest approach versus drift hme for argon (50%), ethane (50%) for fields at 0, 10, 17 kG

416

G

BARANKO

liO0

U

/,~Ar ETHANE A¢S I OBUTANE

I000

//

W or) Z

9001

bJ

800

J/

//

///// /

//./

I.-l'-h

7OO

l'r

6OO

--.I p.. 0 I"

500


//

.,/;

400

IE

At-C02

/f

2-$/~

I00

B-FIELD (kG)

Fig 6 1 2 cm drift time versus magnetic field for three different gas mixtures

with s = 1 27 cm, the spacing between the tube 1 or tube 3 and tube 2 The description o f the method ~s complete but w e d~d not k n o w that the flux was uniform W e could, however, check the assumption and m a k e small corrections for non-umform~ty in s o m e instances

~

One way o f reducing the dependence of the m e t h o d on flux umform~ty was to integrate and obtain the r versus t curve only up to the m~ddle point halfway between the w~re and the wall m tube 1 For each point m the curve we k n o w the correspondmg d~stance from the wire and the corresponding drift t~me m tube 2 Then assuming that tube 1 and tube 2 are ~dentlcal, these values determine the continuation of the r versus t curve found from tube 1 If the flux ,s uniform the contmuaUon o f the curve beyond the m~ddle point will be s m o o t h A curvature m the spatml flux d~stnbut~on will result m a kink at the middle point A precise cahbratlon of the tube would reqmre a truly uniform flux What we present here are accurate e n o u g h to show the shapes of r versus t for several gases The radms versus t~me cahbrauon curves are s h o w n m figs 3 - 5 In addmon w e have plotted the 1 2 cm drift t~me versus magnetic field for each of the gases Th,s is s h o w n m fig 6

6 2 VELOCITY VERSUS FIELD Once the r versus t cahbraUon has been determined for each gas, the radml drift velocity as a function of electric field can be easily found For a cyhndncal geometry the electric field is E _ _ _Vo

In(b/a)r

SWG

=

Vo ( V c m - 1 ) 6

17r

50

T/ / / i ~ / ~ [ ~ [ ~

3

et al

lTkG

45

45

I-

W 4C )

Ar -CO=

0 cJ 35 :E

--I-,o.o

I.d (/)

rr"

el," t,..)

35

7WG

~E 30 t..)

>- 25 I"0 ..J W

>I--

20

/

] . / -

Ar - ]SOBUTANE

zo

0 ._J lad

15

/

,4 I /

t! / / I i I

* 2

I

t 4

i 6

, 8

, i0

, 12

, 14

, 16

, $8

'

210"

212 24

* 26

' 28

• 30

E-FIELD (VOLTS/CM) (XIO 3) Fig 7 Radial drift velocity versus electric field for argon (89%) isobutane (1096), methylal (1%) for magnetic fields of 0, 10,

17 kG

I0

112

14

i1~

18

20

22

24

21~ 2B

30

E-FIELD (VOLTSICM) (XlO') Fig 8 Radial drift velocity versus electric field for argon (50%) ethane (50%) for magnetic fields of 0, 10, 17 kG

DRIFT PROPERTIES

I00

IOkG 50

~

///

417

Ar-ETHANE(50%)

+

17kG

17kG

:t

45

+

,}-~-

-I00

12)

I I00

"~"

A r - CO;t (10%)

30

17kG

A ~

E

25

lTD..O,,

'}

I
io

/

Ar-ISO (10%), M E T H Y L A L (1%) 17kG

I00

Ij

5

+ ~l~_l_4I~Tubw°l el

E I<1 E-FIELD (VOLTS/CM) (XlO s)

-Ioo

Fig 9 Radial drift velocity versus electric field for argon (9096)CO2(10%) for magnetic fields of 5, 10, 15, 17 kG

+

These are plotted in figs 7 - 9 6 3

DISPERSION RESOLUTION AND EFFICIENCY

The spatial resolution properties of the draft tubes have been studied using polynomml fits to the r versus t cahbratlon curves As can be seen from fig 1 the function

d

RI+R3

=---"-2---- + R2-R°

~s a measure of the 3 tube resolution, where R0 = 1 27 c m / c o s (0) The m e a n value of .4, .4, as a function of R2 is a measure of the quality of the r versus t fitting procedure As ~s shown m fig l0 for the A r - C O 2 cahbratlon curves, A is always less than 100/~m - completely satmfactory for these test results The standard deviation of A, a(`4) is a measure of the single wire resolution On average, we would expect that a(wlre) "-.

-

!

I0

RADIAL DISTANCE (CM) Fig 10 Average fitting residuals versus radms for 3 gases at

17 kG

W e have also used the spatial information to calculate the drift-tube efficiency The a r g o n - C O 2 , 1 7 k G tests results are shown in fig 11 The a r g o n - e t h a n e was of equal quality, however, the a r g o n - l s o b u t a n e data shows an average efficiency several percent lower

Ioo

>,

+÷

99

cIJ

-_-, ~

Ar-COz(lO%) 2150volts

98

17kG

a(A)/(1 5) 1/2 .

Due to the rather high thresholds used for these test a(wlre) appears to be dominated by rime .litter The average resolutions obtained should be considered upper hm~ts to that attainable w~th the actual HRS detectors, they are argon-CO2, 1 9 0 / z m , argon ethane, 173 # m , and argon - lsobutane - m e t h y lal, 160/.tm

I

5

Tube w o l l ~

97 I

I

I

I

I

5

I

I

RADIAL DISTANCE

I

I

I

I0

(cm)

Fig 11 Efficiency versus radius for argon - CO2(10°/o) at 17 kG

418

G BARANKO et al

7. Gas parameters in magnetic fields 7 1 A SIMPLEMODEL The motion of free electrons produced by lomzatlon of a gas by a charged particle, m presence of both electric and magnetic fields, been studied extensively m the hterature ~ 2) One can show that the drift velocity, W, of centroid of a group of electrons can be written W = -(4/3~z) (e/m)

fo

M -1 Ev 3

M

O.) z

-coz

=

--

C1

o

o

--

T

Ar -CO=

Y

(.Dj.

COt

. 0. 3. . . . . .0.4. . . . 0. 5. . . 0. 6. . . 0.7.0.8.0.9 1. 0. . . . . . . . . . . E-FIELD

;0 . . . .

;0 ....4

XIO ~ V/CM

O) r

12 K versus elecmc field for argon - CO2(10%) where Best fit for 5---17 kG fields

FIg

no =

•

rt

co,, col, co- are the three c o m p o n e n t s of the larmor frequency, co = ( e / m ) B , of the electron v is the effective time between colhsmns If n(x, t) is the density of electrons at p o s m o n x and time t, the n u m b e r of electrons m the group IS

•

)do,

-1 O.).L

•

the as

where E ~s the electric field vector and 0 is the electron velocity If we use a coordinate system attached to the centrold of the swarm m which the radml ax~s, ~, points toward the sense w~re, and z is parallel to the wire, as shown in fig 18, section 8, M is the matrix "C - - 1

"it OQ

the the has

.t~ Co

14/0=K(e/m)Er

then in the case of constant colllston time, K = 1 On the other hand, w h e n To is independent of o (constant length between c o l h s l o n s ) K = _2 3 Our purpose is to parametnze the drift velocity as a function of B m a c o n v e m e n t way W e will m a k e

n(x, t) dx

and the m e a n value of the distribution of the electron velocity over the whole group is f~0

5~ 50

fo* = ~o

n (x, t) fo (x, v, t) dx

with/o(x, o, t) being the first term of the expansion m legendre polynommls of the velocity dlsmbut~on funcUon /(x, u, t) In the absence of a B field the expression for the drift velocity turns out to be

~ ,., ,,, r_O ×

liP• O

40

Ar -CO~ s5

I---

1 eE ~1

+ 2 eE

Wo = 3 m

3m

~

~

the averages are clone over the f~0 d~stnbuuon, for example,

~°

/,

i

"co2 f ~ dp .

•..i

....

04

t ....

05

,....,~...|

06

07

i

08 09

E-FIELD

If we can write

Wo ~ K (e/m) E~,

i

. . . . . . . . .

I0

i°

2

XIO =

.

"

"

"

i

&O

....

,

40

V/CM

Fig 13 Average colhslon Ume versus electric field for argon CO2(10%) Best fit for 5--.17 kG fields

DRIFT P R O P E R T I E S

"1

6~

T °O

7C

t

1

•

'° t

•

I 9

o•

8c

ii

io

419

•

6 .=

t"

6C

5E

Ar-ETHANE

8

5C

< a..


< ",,e

EL

<

4oi 35,

Ar-ISOBUTANE

3oi 25 ZO I t5

,oi o~ •

oc

, , I

. . . .

04

I . . . .

05

I ....

06

h.,,I

07

I

08

09

E-FIELD t I .... 05

I' II I I'1''1''1'11'"1 I 04 05 06 07060910

I

=

I

E-FIELD

I

I t

' I 'l

I ZO

'

'

I t I l "l 3D

'I 40

J

.

.

.

.

.

.

.

.

.

.

I0

I

.

.

.

.

I

20

30

XIO 3 V/CM

Fng 16 K versus electric field for argon, ethane, (50%) Best fit for 0 4 1 7 kG

XIO 6 V/CM

Fig 14 K versus electric field for argon, lsobutane (10%), methylal (1%) Best fit for 0--,17 kG

85 80

60

•

.° I 75

75 • 70

7O 0 hi O3

65

0

60

65

60

Ar- ISOBUTANE _N

b

X

Ar- E THAN E

55

5o

X 55

45

~

50

4o 35 30 25

35 ZO 30

, I .... 03

I .... 04

h, 05

.111,,111..,I 06

1

07080910

E-FIELD

I

i

,

i1~1111,1

. . . . ZO

1 , 1,11 30

. . . .

40

XlO 5 V/CM

Fng 15 Average collnslon tnme for argon, isobutane (10%), Methylal (_1%) Best fit for 0--.17 kG

|

. . . .

05

1 ....

|..,,i

i

!

o~ 07 o8 09 ~o E-FIELD

•

i

XlO s

,

i

. . . . .

i

20

.

I

i

i

I

30

V/CM

Fig 17 Average colhslon time for argon, ethane (50%) Best fit for 0--, 17 kG

420

G

B A R A N K O et al

the assumption that the velocity d~sperslon ~s well enough behaved that we can replace 1" by ~ and furthermore we will assume that ~ ,s independent of B W~th these a s s u m p t m n s W, ~

W0

._[_O.)2.~2

,

~u

\\! ///

/I i

I

I

I

W,=Wo\ 1+co2~ 2 Wo ~ K 7 2

e m

I

i

\ I

I /

/i--..

\

\

\

\

\\

k\ \ \\

],

X

E'~.

I"

BEHAVIOR OF K AND l" PARAMETERS

For each gas we have used the r versus t cahbrat~on curves for the various B values to determine the best fit values for K and 17 These d e t e r m m a nons are shown m figs (12-17) As we expected K ts not strictly constant as a funct,on of E Stmflarly shows a rapid fall off with increasing E, as is well d o c u m e n t e d in the hterature 3)

(a)

.c_a

8. Drift tube properties The global constancy of K allows us to use the s~mple model to draw trajectories of the centrold of electrons drifting m various magnetic fields and m addition to parametnze the drift time dependence on small varmtions of the B field either parallel or perpendicular to the sense w~re 8 1 PARALLELCOMPONENTOF n Knowing the expression for the c o m p o n e n t s of the drift velocity, we can calculate the trajectory of the centro~d of the swarm of electrons This is shown in figs 18a and b Of course, as long as the magnetic field is along the sense wire, the equal time contours are circular Thin displays the mare advantage of the cylindrical design as compared wtth a square cell demgn T h e earhest arrival t~me ~s always assocmted w~th the point on a track at m i n i m u m radms, independent of entrance angle 8 2

\

TRANSVERSE COMPONENT OF

B

All drift chamber ceils in the HRS are poslUoned very close to the superconducting coils of the solenoid The tubes will be parallel to the magnet axm and near the magnet yoke At the tube ends we

f

f

/

///

\ \

/

N

/ /

/

I I

I

I

/

\

/

/ I

/

f

\ .

\ \

\

\

\

I /

I / /

I \

/

\

\

/

\ \

\

/

/ /

-/ /

/

/

/

/

/

/

/

Cb) Fig 18 a,b - Drift trajectories for a track through the drift tube (a) m i n i m u m radms 1 0 c m , (b) m i n i m u m r a d m s = 0 0 , for B = 17 kG and V = 2250 V Equal ttme contours are shown m dashed hnes

DRIFT PROPERTIES

B//= 17 KGauss

Br= 2 5 KGouss Ar-CO z ro = I 27cm p arh cle

910 / ¢J c

900 ~28n=

196Fcm

I-~ 890

880

421

expect that there will be fairly substantial transverse B field This will modify the s p a c e - t i m e relationships and could introduce a spread m the spatial resolution of the chambers For an example, we have calculated the variation m arrival time for various orientations o f the transverse magnet=c c o m p o n e n t w~th respect to the track direction This variation ~s s h o w n in fig 19 We conclude that fields transverse to the wire produce distortions m the drift time that are less than our resolution as long as these fields are less than 2 5 kG References

=

-~r/2

i

~

O

~r12

>

a (rodlons)

Fig 19 Vanatlon of m m l m u m drift tlme wlth the onentatlon of the transverse field wlth respect to the track

I) L G H Huxley and R W Crompton, The diffusion and drift of electrons m gases (John Wdey, New York, 1974) 2) j S Townsend, Proc Roy Soc 86 (1912) 571, G Schultz and J Gresser, Nucl Instr and Meth 151 (1978) 413, V Palladtno and B Sadoulet, Nucl lnstr and Meth 128 (1975) 323 3) A Breskm. G Charpak, F Sauh, M Atkmson and G Schultz, Nucl lnstr and Meth 124 (1975) 189