A method of determining charged-particle beam composition

NUCLEAR

INSTRUMENTS

AND METHODS

[2 4

(1975) I75-I83;

©

NORTH-HOLLAND

PUBLISHING

CO.

A M E T H O D OF D E T E R M I N I N G CHARGED-PARTICLE BEAM C O M P O S I T I O N *

C L E M E N S A. H E U S C H and A B R A H A M S E I D E N

University of California at Santa Cruz, California, U.S.A. Recewed 29 N o v e m b e r 1974 We describe a method which efficiently and precisely determines the relative content o f electrons/positrons, muons, and hadrons m a high-energy beam o f charged particles, over a wide m o m e n t u m range. There is no discrimination between hadrons o f a given charge and m o m e n t u m .

As an example, we cite the determmation o f electron and p]on contaminations in a 14 GeV SLAC m u o n beam, to a precision o f ~10-5.

1. Introduction

In conjunction with a series of experiments at the Stanford Linear Accelerator Center, where we studied minuscule cross sections (~0.1 #b) with muon and pion beams, we needed a particularly precise determination of the relative content of muons and pions in a 14 GeV beam, to a precision of ,-~ 1 part in 105. This level of accuracy is clearly needed in processes such as hadron production in muon-nucleon scatterings, since hadron production in pion-nucleon scattering occurs with a cross section ,-~I04 times larger. We developed a method that can rather generally be applied to similar problems with the specific purpose of determining:

As experimental inquiry into particle interactions proceeds to more and more ambitious schemes of isolating effects that are minuscule when compared with the total cross sections for easily available beams and targets, the determination of precise starting conditions assumes a greatly increased importance. For the study of hadron-hadron interactions, the use of Cherenkov counters and separators has long been able to successfully tell hadrons of different masses apart. The problem is considerably exacerbated when lepton-hadron separation is important in highenergy beams. This is particularly true when we restrict our attention to electrons, muons, and pions, all of which are closely relativistic in velocity at relatively low momenta, and can thereby not easily be separated by the techniques successfully applied on hadron mixtures.

/a, e admixtures in hadron beams ; e, rc admixtures in muon beams ; #, ~z admixtures in electron beams. The detection system uses basically the initiation of hadronic and electromagnetic cascades for a detailed

* W o r k stipported in part by the United States Atomic Energy Commission under contract AT(04-3)-34, P.A. 197.

SHOWER COUNTERS

2nd FOCUS

Pb COLLIMATOR

OF BEAMLIN[E]BEAM ~+,e~'m +

AT 14GeV/c MOMENTUM

0

0 SEAM

TRANSPORT SYSTEM

LARGE MAGNET

Pb SHIELD

FOR MUON REJECTION VETOFOR TWO PARTICLES HITTINGWITHIN ~40 ns OF EACH OTHER

Fig. 1. Schematic layout o f test downstream o f 2nd focus o f SLAC beamhne 23. (Elements are not to scale. Distance between magnet center and shower counters is ~ 10 m).

175

176

c.A.

HEUSCH

A N D A. SELDEN

MEASUREMENT APPARATUS FOR

"n//..L RATIO

SHONWTEE%s

I

LINE

I METER

I

Fig. 2. Diagnostic apparatus Including beam-defining scintillators, hadron degraders, shower counters, veto counters. beam analysis; it is described in some detail in sections 2 and 3. It was successfully used in the manner outlined in sections 4 and 5. The results of one particularly stringent test are discussed in section 6.

2. General description Fig. 1 shows the detection scheme in the framework of the downstream end of our muon beamline (SLAC beamline 23), starting from the second focusl). The incoming beam is approximately 1 cm × 1 cm in size at both the second and third focl; a typical beam angle is 2 m r a d in each plane. The beam halo is 1% in magnitude, and is composed principally of offmomentum muons which have passed through a modest amount of beamhne shielding (generally a few meters of iron or lead). The large SLAC streamerchamber magnet at the third focus serves to prevent essentially all of the halo from hitting our detection apparatus; the ensuing deflection of charged particles also eliminates all neutrals, principally photons, from the beam. (We did not measure the photon composition of the beam, although it would have been easy to do so.) The particle identification apparatus was well outside the magnetic field region to avoid the bending of any beam-associated spray into the apparatus. Fig. 2 shows the particle identification apparatus in more detail. It is composed of the following elements: (Note that all tests described here were performed in a beamline for positively charged particles, and we make specific reference to e +, p+, n +, ... ; most features are

equally applicable to negative beams, but hadrom~ absorption properties are clearly influenced by th~ beam sign.) (1) Beam defining counters $1, $2, $3. A beam par. ticle is defined by a triple coincidence of these counters $1 and S 2 intercept the whole beam, $3 about 1/2 ofth~ beam. SATURATION VALUE

BREMS 15

•

/

\

C,ONTRIBUTIO

m=/

/

y

~"

'

7/" + INTERACTIONS AND DEEPLY INELASTIC

~

\~_ ~

W,OTH - 2 o % MOSTLY

'~ /-~'~

DUE TO COUNTER RESOLUTION- (SEE FIG 4)

~,

3 m

~6 3

MINIMUM / IONIZING ~ MUONS ,/

~ J ~ ~

\ -/

e÷ INBEAM, e÷ FROM CAY

3 6 9 12 15 18 21 24 UNITS = (PULSE HEIGHT)/ (PULSE HEIGHT FOR MIN IONIZING) SH~

Fig. 3. Characteristics o f h a d r o n a n d lepton signatures ]n twodimensional space s p a n n e d by shower-counter pulse heights

SH1 and SH2.

DETERMINING

CHARGED-PARTICLE

(2) Hadron degraders (consisting of beryllium plus polyethylene) providing on the order of one hadronic interaction length, to initiate hadron showers. The use of these materials allows all positrons to be unambiguously seen in the first shower counter, since the number of radiation lengths ahead of it is small. Also, the use of low-A materials minimizes the probability of having a #+ initiate a hadron shower, which increases linearly with A, as compared to the hadroninitiated contribution, which goes like ,~A 2/3. (3) A special veto counter C, to eliminate triggers having a second beam particle passing through the apparatus within 40 ns before or after the triggering particle and thus confusing the pulse-height analysis. The use of this counter is discussed below. (4) Two shower counters SH1 and SHz 2), each filled with ~ 15 radiation lengths of lead. These are made of lead-lucite (SHI) and lead-scintillator (SH2) sandwiches, with each counter feeding into one 5" phototube. The shower counters are ~ 8" x 12" in area transverse to the beam direction. The total amount of material ahead of SH 2 provides nearly two hadronic interaction lengths, thus producing a maximum average particle multiplicity in S H 2 for pion-initiated showers. (5) The shower counters are followed by 1 m of lead and a veto counter measuring 2 4 " x 2 4 " × 1". This counter was used to veto penetrating muons which made up over 99% of the beam. The size of the veto counter was chosen to minimize the number of triggers due to deeply inelastic muon scattering in the hadron degrader and shower counters. To trigger, such a muon must either lose nearly all of its energy so it stops in the lead, or scatter through an angle >~ 15 °. We estimate that the veto size chosen reduces the trigger rate due to this source to ~< 1 x 10-5/incident muon. The pulse heights in SH~ and SH2 served as the principal particle identifiers. A beam particle which is potentiallly not a muon has the signature B.~¢ (with B - S I ' S 2 " S 3 ) . For each such event, we read out the pulse heights from SH t and SH2 using ADCs and a PDP-9 computer, and store them in a two-dimensional array with 256 x 256 channels. The expected signature for each particle type is then:

BEAM

177

COMPOSITION

3. Electronics The electronic logic is straightforward; it basically has to provide a gate for the shower-counter ADCs for all unvetoed B signals. The veto comes from a linear fan-in and is applied as a leading edge inhibit to a discriminator into which the B signal was fed as a trigger. The fan-in contains four inputs. They are: (1) The signal from the V discriminator, whose output is run on dc pass. This system proved to be extremely efficient. The number of muon-induced triggers, after analysis, can be accounted for entirely in terms of interactions in the detection apparatus and lead wall. The veto inefficiency is thus < 10 -5. (2) A signal from C, advanced in time by 45 ns compared to B, 40 ns wide. This signal vetoes triggers which have a second particle hitting C (and the shower counters) within 5 to 45 ns after the initial hit. This is necessary because the poor duty cycle at SLAC (1.6 #s pulses, 30 pps for this test) along with the high sensitivity desired forces us to run ,,~5 particles per pulse through the apparatus, to reach a reasonable statistical sample. This number gives a ~ 2 5 % probability to include the signal from a second stray particle within the 40-ns-wide A D C gate for the shower counters. (3) A second signal from C, retarded by 5 ns relative to B, and 40 ns wide, to eliminate triggers where a second beam particle hit within ,,~40 ns earlier than the triggering particle. (4) A self-inhibit, to veto the rest of the 1.6/~s SLAC pulse after a trigger. 18

SATURATION POINT~ 18

"/T+ - SOLID CURVE e+ - DASHED CURVE

-15

12SH2 SCALE NORMALIZED TO 9MINIMUM IONIZING 6"

-12 SPECTRUM PROJECTED ~ " ONTO SH= AXIS, ~ J NORMALIZATION ARBITRARY t -SPECTRUMPROJECTED ONTO SH= AXIS, ? t-, NORMALIZATION / "~ ARBITRARY / 7

/

: Small pulse height in both counters. e÷ : Very large pulse height in SH 1, small in SH2. rc+, or other hadron: Medium pulse heights in both.

,\

,

9

6

C~ ."i

/x +

The entire detection apparatus is quite compact, simple to set up and adapt for optimal performance.

g

~

I'z

15

18

zl

a4

SH~ SCALE NORMALIZED go MINIMUM IONIZING P i g . 4, P r o j e c t i o n s

of~

and electron distributions

onto

SH2 axes. (14 O e V ~+ o r e + i n c i d e n t . )

SH1

and

178

C.A. HEUSCH

4. Detailed pulse-height distributions Fig. 3 shows, in schematic form, the loci of the various particle components at given energy, in a twodimensional plot of pulse heights measured in the first 15 X ° (lead-lucite) versus the second 15 X ° (leadscintillator) shower sandwiches. The electrons are clearly closely lumped together at high pulse height for SH1; only little if any energy will be absorbed in the second shower counter. Pions (or other hadrons) are likely to interact with much lower multiplicity in the Be absorber or in one of the sandwiches, and will therefore give a very wide energy spectrum in both SH1 and S H 2. Muons, on the other hand, will be close to minimum ionizing throughout; they should cluster around one low pulse height for both SH~ and SH2 (with the exception of rare violent muon-nucleon interactions). 4.1. ELECTRONS Fig. 4 shows the projected spectra initially measured in SHI and S H 2 in the muon beam (labelled e + curve). As can be seen, they are entirely dominated by positrons in the beam. The number of counts in this run indicates a positron content of 3 x l0 -a. For later runs, a 1.5 radiation length radiator was inserted at the second focus to eliminate positrons. This reduced the positron content by more than a factor of 300, a change that can be seen in the two-dimensional spectrum in fig. 5. The data presented in the next sections are always taken with this radiator in position at the second focus. 2÷~ +4 2++ 33 +÷*2÷

SHz SATURATION

2 +

REGIONS DRAWN IN SAME AS IN FIGURE

*

3

+ + z4

~

12

z

+ + 2

+ + +

+

+ . ÷ ~ "=

6

+

÷

+

÷÷ .÷ +

2÷

÷

÷ +

+ ÷

BINS USED TO

+

~

÷2 t "1- i'

EVALUATE7/" CONTAMINATION IN BEAM

+

*

2++ 2÷ + ÷ ÷2 ÷ + + ~ ~ 2 ÷+ + ~ + ÷ 2 2 ÷4÷ ++÷ ÷4 + S ." ~2 + +2+ 3 343 33+<" * + ÷ V÷~ ÷2+42523243+47÷4÷+ I H | ?CA36 222532÷+ + + I I I I i 6 9 12 15

CUT LOCATION

IB

>

$HI

(MINIMUM IONIZING ~ BIN 6)

Fig. 5. T w o - d i m e n s i o n a l pulse-height plot o f m u o n b e a m , positrons have been removed f r o m b e a m by m e a n s o f Pb absorber at second focus. (No n+ attenuators.)

A N D A. S E I D E N

4.2. IMPORTANCE OF ELECTRON SPECTRUM IN S H 2 The calculation of the pion content in the beam requires a subtraction procedure to eliminate /2+Induced triggers. To do this, a run was made each with and without hadron attenuators at the second focus of the beamhne. The n + content is then obtained by subtraction of the two spectra. We implicitly assume that all triggering components are due either to /2+ interactions or to pions. This procedure doesn't work for triggers whose rate changes with the insertion of attenuators, e.g. e +'s in the beam,/2 + decay ahead of the apparatus, and beam spray which may change somewhat with beam shape. These triggers all involve electromagnetically showering particles. The S H E spectrum for 14 GeV/c e +'s shows that most showers produce less than 3 x minimum-ionizing pulse heights in S H 2. This will be even more so for lower-energy showering particles. Therefore, by considering only triggers with S H 2 > 3 x minimum, we effectively eliminate the electromagnetically showering component (provided it is small to start with, as in our case). This cut will be applied later in the subtraction procedure. For measuring n + contamination at a smaller level than in our case, it would probably be better to use ~ 2 0 radiation lengths in the first shower counter, so the SH2 cut would be even more efficient at eliminating showering particles. 4.3. MUONS By removing the veto counter from the trigger, we took the pulse-height spectrum for muons. Here are the important results of this procedure: 99.5% of the muons give < 3 x minimum-ionizing signals in SH1. 98.8% of the muons give < 3 x minimum in SH 2. 98.6% give less than 3 x minimum in SH~ andSH2. For muons which trigger because of interactions in the lead wall upstream of the veto counter, we should get very little inflow into bins relevant to the n + measurement. These bins are constrained to have SH E> 3 minimum for reasons discussed in the previous section. 4.4. PIONS The beamline used in the test is flexible and can be equally well used to produce a n + beam of nearly identical phase-space properties to the p+ beam1). Using such a n + beam, the apparatus could be directly calibrated for pions. The trigger rate was found to be 70% for a n + beam. The reduced trigger rate is due to hadron penetration of the lead to the veto counter, as

179

D E T E R M I N I N G C H A R G E D - P A R T I C L E BEAM C O M P O S I T I O N TABLE 1 ~z cahbration run - pulse-height breakdown for tmggers. Number of Events a

SH2

SHI

SHz < 3 3 < SH~ < 6 6 < SH2 < 9 9 < S H z < 12

12 < SH2 < 15 15 < S H z < 18 18 < S H 2

Totals Totals excluding SH~ < 3

SH1 < 3

3
6
9
12
15
Totals

713 106 96 72 57 62 30 1136

201 80 56 28 8 8 1 382

125 65 15 10 6 2 1 224

71 38 11 3 4 0 0 127

56 15 11 4 3 2 0 91

36 10 2 1 1 0 0 50

1202 314 191 118 79 74 32 2010

423

181

99

56

35

14

808

a All pulse heights normalized to minimum ionizing.

well as the f a c t t h a t t h e l e a d w a l l was s h o r t e r vertically t h a n t h e v e t o c o u n t e r , so t h a t s o m e s e c o n d a r y particles w e r e a b l e to hit the v e t o c o u n t e r w i t h o u t h a v i n g to p e n e t r a t e t h e lead. T h i s l a t t e r loss was u n n e c e s s a r y , a n d w o u l d h a v e b e e n r e m e d i e d h a d we h a d t i m e to r e p e a t t h e test. F o r t h e p i o n s w h i c h trigger, t h e p u l s e - h e i g h t s p e c t r a are s h o w n in fig. 4. T a b l e 1 s h o w s a b r e a k d o w n o f t h e spectra, n o r m a l i z e d to m i n i m u m - i o n i z i n g . A n o t h e r interesting distribution obtains from the summing of t h e t w o p u l s e heights, a g a i n n o r m a l i z e d to m i n i m u m i o n i z i n g . T h i s is g i v e n in t a b l e 2. T h e a b s o r b e r , u s e d at t h e s e c o n d f o c u s to a t t e n u a t e pions, was m a d e o f b e r y l l i u m a n d lucite. Its effect was c a l i b r a t e d in the zc+ b e a m . T h e a t t e n u a t i o n f a c t o r was 54%.

5. M u o n - r e l a t e d e v e n t s other than d e e p l y i n e l a s t i c scattering 5.1. MUON DECAY T h e p r o b a b i l i t y f o r a 14.2 G e V m u o n to d e c a y in the a p p a r a t u s is ,-~0.6 x 1 0 - 2 . I n a d d i t i o n , a b o u t ten t i m e s as m a n y triggers c a n be e x p e c t e d f r o m m u o n s w h i c h decay upstream of the shower counters. These produce

SH1"2 +

+ 15.

12

TABLE 2 cahbration run - summed pulse heights, a ST ~ SH1 + S H 2 .

't "l

+ \ l l + ++

SATURATION l l l

l ++ + ll ~ + ~ ~ I ~ v " ~ I+

+

SH=

SH=

1

+

(UNITS SAME AS 9 FOR

SH,)

++

.

+~ J

Counts

Counts if S H 2 > 3 required as well

6

~ ++

+

ST< 3 3 18 Totals

598 280 284 299 210 196 143 2010

0 50 135 195 156 147 125 808

a Pulse heights are normalized to mimmum lomzlng.

3

4+

++

•

+

t

++l +

RES~

\

+

÷\

+

+

I

÷\

~+

+++

,

7i-+

~~ t - - - ~ ' ' ~

÷ ,f÷~÷÷ ST range

MUON BREMSSTRAHLUNG REGION

I

.

+++t

\

I

.

,SH2-3

I.

SH I

UNITS • (PULSE HEIGHT)/(PULSE HEIGHT FOR MINIMUM IONIZING}

Fig. 6. Two-dimensional pulse-he;ght plot after apphcatmn oi cuts. Absorber at second focus of muon beam. S H 1 > 2 , 3 < S H 2 < 18. Muon bremsstrahlung events and ~z interactions now dominate the spectrum.

180

c . A . H E U S C H AND A. SELDEN TABLE 3

Pulse heights for/, run - no z~ absorber at second focus. Data for run: number of hve muons = 8.24 x 106; tnggers/mu = 1.45 x 10-4 numbers in parentheses are calculated number of plon-lnduced events m each bin, resulting from final analysis.

\ SH2

SH1

\

SHI<3

SHz < 3 3 < SH2 < 6 6 < SH~ < 9 9 < SH2 < 12 12 < SH~ < 15 15 < SH2 < 18

SH~ > 18 Totals Totals SH2 < 3 excluded

(73) 556 (10.8) 23 (9.8) 11 (7.3) 17 (5.8) 12 (6.3) 18 (3.1) 39 676 120

3
6
(20.5) 163 (8.2) 12 (5.7) 11 (2.9) 9 (1.0) 1 (1.0) 2

(12.7) 130 (6.6) I1 (1.5) 4 (1.0) 1

/~+ e -

(7.2) 75 (3.9) 9 (1.1) 6

12
15
(5.7) 47 (1.5) 6 (1.1) 1

(3.7) 3 (1.0) 3

Totals

974 64

2

35

2

1

0

30

1

4

0

0

18

3

2

0

0

25

10 208

2 152

0 98

0 55

0 8

51 1197

45

22

23

8

5

223

p o s i t r o n s w h i c h will give a b r o a d s p e c t r u m i n S H ~ associated with very small pulse heights in SH2. The n u m b e r o f s u c h t r i g g e r s is b e a m - s h a p e d e p e n d e n t , a n d a (SH 2> 3 x minimum) cut can remove most of them. 5.2.

9
SCATTERING (6 RAY PRODUCTION)

multiple #+escatterings with very large energy transfer in each event. These processes have very small c r o s s s e c t i o n s , so t h a t t h e p r o b a b i l i t y o f s u c h e v e n t s is negligible. 5.3. MUON BREMSSTRAHLUNG

For/~+ e- scattering, the muon can only transfer a m a x i m u m f r a c t i o n ( ~ ½) o f its e n e r g y t o t h e e l e c t r o n . T o t r i g g e r , t h e m u o n m u s t lose n e a r l y all o f its e n e r g y so it s t o p s i n t h e l e a d wall. T h i s w o u l d t h e r e f o r e r e q u i r e

I n t h i s c a s e n e a r l y all t h e m u o n e n e r g y c a n b e t r a n s ferred in one collision. For photons radiated with e n e r g y k ' ~ E v , t h e t o t a l c r o s s s e c t i o n f o r t h i s p r o c e s s is g i v e n by3):

TABLE 4 Pulse heights for # run - absorber in. Data for run: number o f h v e muons = 7.44 x 10 6, trlggers/mu = 1.26 x 10-4.

I

SH1

SH,, SH2 < 3 3 < SH2 < 6 6 18 Totals Same for SHz > 3

SH1 < 3 503 12 11 9 8 6 43 592 89

3
6
108 9 3 0 1 2 7 130 22

88 9 3 1 0 4 1 106 18

9
12
15
Totals 777 45 25 14 15 12 51 939 162

DETERMINING CHARGED-PARTICLE BEAM COMPOSITION 2 a 3 Z 2 f f dkdl z ( k , l ) ~'B . . . .

= ~ 4 - ' - ~ , J.~ tt~ - -

(1+1) 2

'

where I=(OkE,/M~) z and g(k,l) is a m o m e n t u m transfer integral over the lead nuclear f o r m factor. F o r k varying between 12.6 to 14 GeV (so the m u o n will stop in the lead wall), the integral a m o u n t s to 0.18. This gives for lead: O'B. . . .

=

3.53 x 10 -29 cm2/nucleus.

The m u o n bremsstrahlung events occur r a n d o m l y along the shower counter lengths and have some simple characteristics. F o r events occurring in the shower counters, but not t o o near the end o f SH2, the p h o t o n loses all o f its energy in the counters. These events thus lie approximately along a straight line in the twodimensional pulse-height space (cf. Fig. 3). This line, furthermore, goes t h r o u g h the locus for positrons in the beam, corresponding to a bremsstrahlung q u a n t u m o f m a x i m u m energy (Eu). Fig. 6 shows a clear m u o n bremsstrahlung signal: these events lie near a line SH~ + 0 . 3 SH2,~, 15. W h e n we plot all the data in terms of the summed pulse heights, this signal stands out as a peak at large values (cf. table 5). In addition, this interaction produces a n u m b e r o f events where the counter SH 2 does not contain the full shower energy. SH2 then spreads over a wide range, while SH 1 remains minimum-ionizing, since the bremsstrahlung process occurs only in the second shower array. Such events have to be separated from the pion events by a subtraction procedure. The m u o n bremsstrahlung events provide a uniquely useful signal. First, their contribution is exactly calculable, so that it gives us a check on our normalization, and on our a p p r o a c h in general. Furthermore, the slope and intercept o f the line on which these events lie, provide a long-term m o n i t o r on the gain of the two shower counters. The m u o n bremsstrahlung processes occurring in the lead wall downstream o f SH~ and SH2 also provide a n u m b e r o f minimum-ionizing muonic triggers. The rate for these is expected to be ,-~3 times the rate o f triggers originating in the shower counters. 6 . R e s u l t s for m u o n b e a m

6.1. RAW DATA Tables 3 and 4 show the breakdown o f our data for pion absorbers present or absent at the second focus. In parentheses are the n u m b e r o f pion events in each bin, as determined below. Table 5 shows the data binned in terms o f the summed pulse heights. Fig. 5 shows a two-dimensional spectrum for 40% o f the data, without

181

h a d r o n absorbers in place. The trigger rate for the apparatus was , , ~ l . 4 x l 0 - 4 / i n c i d e n t particle and ,~ 107 particles were run through. 6.2. PION CONTAMINATIONIN BEAM The rc+ contamination can be calculated from any subset o f the bins in the plot. Taking 5 = a b s o r b e r a t t e n u a t i o n = 0 . 5 4 , C ~ = c o u n t i n g rate without absorber, C 2 = r a t e with absorber in, and e=efficiency (measured in zc calibration run, and reduced by a few percent due to the lead positron attenuator at the second focus) for seeing pions in those bins, we have: A 7~

#+

= C1 - C

2

e(1 - ~ )

Using this formula we can get an upper limit on the ~+ contamination by using all bins. The result is (4.7 _+ 1.5) x 10- 5 (e = 0.65). To be insensitive to e + 's we can delete S H 2 < 3 x min., as discussed previously. Trying this as well as other cuts gives:

Cuts

Result

n+//~+ = (3.8-4-1.5) x 10-5 (e = 0.26) zt+//~+ =(4.2-I-2.0) x 10-s (6 = 0.13) SH2 > 3, SH1 > 3: SH~> 3, SHI+SH2 < 18: z~+//~+ = (3.8 + 1.5) x 10-s (e = 0.22)

SHz > 3 × min."

The latter cut is motivated by the desire to remove the muon-brems.-sensitive bins. Using table 5 we see TABLE 5 Summed pulse heights:/~ runs. Number in parentheses is predicted contribution from plon-lnduced events based on zt calibration run and final analysis. Notme large muon bremsstrahlung contribution in last bin. Result of final analysis: the 223 events contain 107 muon brems, events, 83 plon-induced events, and 33 events due to deeply inelastic muon scattering.

SH2 > 3 ST Range

Total number of muons for run 3 18 Totals Result leaving out last bin

Number of events, no absorber at second focus

Number of events with absorber at second focus

8.24 × 106 6(5.1) 25 (13.8) 27 (20.0) 42 (15.9) 28 (15.0) 95 (12.7) 223

7.44 x 106 3 12 21 25 21 80 162

128

82

182

C . A . H E U S C H A N D A. S E I D E N

that ,-~(30__+12)% of the data after this cut is not pioninduced. This is consistent with our expectations based on muon scattering. 6.3. MUON BREMSSTRAHLUNG Using the last entry in table 5, we get 82 muon bremsstrahlung events with summed shower outputs > 18 x minimum ionizing. Using the pulse-height line on which these types of events lie, we can estimate that ~ first 18 radiation lengths of the total 30 in SH1 + S H 2 contribute to this region. The 18 radiation lengths give a predicted 90 events in this region, which is in excellent agreement with our data. We can further examine the data for SH ~~ minimum ionizing, S H E < 18 × minimum for those muon bremsstrahlung events where the shower begins in the last 12 radiation lengths of S H 2. Using the SH~ <3, 3 3 imposed cut (which eliminates events occurring in the last few radiation lengths of the counter). 6.4. BREAKDOWNOF REMAINDEROF DATA For 80% of the data, S H 2 < 3 x minimum. This includes a large peak with S H t < 3 x minimum as well. For this region, subtracting the 7t+ contribution leaves a number of events consistent with calculations based on bremsstrahlung plus inelastic scattering in the lead wall. The remainder of the data for S H 2 < 3 × minimum is consistent with the number expected from muon decay. Thus, very few of the triggers come from uncalculable sources such as veto inefficiencies or beam spray. The final results of our specific application yielded a pion contamination in the positive muon beam of

~z+//~+ = (4___1.5)x 10 -5 This number constitutes ~ 17% of the triggers where our analysis system detects something other than a passing muon. A detailed breakdown of all triggers yields these fractional components: 7z+ interactions = 17% /~÷ minimum-ionizing (event in lead wall) =40% /~+ bremsstrahlung in SH 1 or SH2 = 9% /~+ inelastic scattering in detector = 5% e + in beam < 4 % ) e ÷ from muon decay ~ 2 5 % ~ Total =29% We believe the technique described here could be used to measure pion contaminations in muon beams down to ~ 1 0 -s. For such a small contamination, it would probably be useful to decrease the thickness of the lead wall (perhaps by ~ 50%) in order to reduce the number of non-pionic triggers; also adding ,-~5 radiation lengths to SH 1 would enhance that signal significantly and make e ÷ separation even more unambiguous. 7. C o n c l u s i o n : u s e f u l n e s s o f m e t h o d

The method described here was designed principally to measure 7~and electron admixtures in a muon beam of 14 GeV momentum. From the detailed distributions of pulse heights in figs. 4 and 5, and the trigger rates encountered in the test runs, we can determine the accuracy of measurement of relative contents of lepton and hadron beams as laid out in the introduction. The numbers given in table 6 are for the configuration as described. With added effort, the method can be improved to measure individual admixtures other than 7~/# more accurately than claimed here. This work has benefited greatly from interactions with many members of the University of California/ Santa Cruz - SLAC Group D collaboration. In

TAaLE 6 Characteristics for the configuration as described. Beam particle

Contaminant

Accuracy of separation

Defining this level process

/z

g e

~ 10 -5 ~ 1 0 -5

dmcussed m section 4-6 # - e decay;/z bremsstrahlung

:t

~ 10-5

limited only by beam-hne quahty and statistics

< 10-6 7t

/z e

10-5 ~ 10-3

mostly deep inelastic/t scattering high-energy hadron cascades, level depending on statistics

D E T E R M I N I N G C H A R G E D - P A R T I C L E BEAM C O M P O S I T I O N

particular we thank G. Luxton and L. Wang for their help in reading the data into the PDP-9 computer, and H. Meyer and T. Schalk for their assistance during the test runs. We also acknowledge the excellent work of W. Nilsson and D. Patterson, who helped to assemble the detection apparatus.

183

References 1) S. Flatt6, C. Heusch, A. Seiden, Nucl. Instr. and Meth. 119 (1974) 333. 2) The shower counters used are described in" C. A. Heusch and C. Y. Prescott, IEEE Trans. Nucl. Scl. NS-12, no. 3 (1965). a) K. W. Kim and Y. S. Tsal, Phys. Rev. D8 (1973) 3109.