Commissioning of a new low energy π-μ at triumf

Nuclear Instruments and Methods 179 (1981) 95-103 © North-Holland Publishing Company

COMMISSIONING OF A NEW LOW ENERGY n-/a CHANNEL AT TRIUMF *

C.J. ORAM, J.B. WARREN, G.M. MARSHALL Physics Department, University of British Columbia, Vancouver, BC, Canada V6R 2A6 and J. DOORNBOS TRIUMF, Vancouver, B.C, Canada V6T 2A3 Received 7 July 1980

The TRIUMF high luminosity low energy n - ~ channel M13 is described and its performance compared with design values. Beam line momentum resolution measurements, using a solid state detector, are described. The measured effect on surface muon flux of the position of the primary beam on the production target is compared with a simple model.

1. Introduction

2. Alpha source measurements

The M13 beam line at TRIUMF is a very low energy ( 2 0 - 1 3 0 MeV/c) pion and muon channel [ 1]. It views the TRIUMP 1AT1 production target at 135 ° with respect to the primary 500 MeV proton beam. The beam line has two 60 ° bends, the first right, the second left, and is 9.4 m long (to F3). Fig. 1 shows the layout of the beam line and location of the three loci, the slits and the jaws. The beam line was initially tuned using a broad energy alpha source and a surface barrier detector, so as to minimize the requirement for primary beam time for tuning. Final tuning was achieved by using an uncooled 1.45 mm long pyrolitic graphite target at 1AT1. While the measurements in this paper were being made the maximum m o m e n t u m of the beam line was 100 MeV/c; however, with the new power supplies now installed the beam line operates over its designed range ( 2 0 - 1 3 0 MeV/c).

2.1. Apparatus A 244,~ 96 ~ m source with two lines at 5.81 and 5.77 MeV was used. The source had a thick window giving it a spectrum with a peak at 4.45 MeV and an 8% energy spread (fwhm). Since the alphas have charge +2, the equivalent momentum is 91 MeV/c for a singly charged particle. The source was mounted at the 1AT1 target position and could be traversed both horizontally, along the proton beam direction, and vertically. A surface barrier detector with resolution o f 30 keV (fwhm) was mounted at the focus being studied. It had a square aperture of either ~ in or ~ in and could be moved in the plane perpendicular to the beam axis. Using an alpha source for beam line tuning [2] has the advantage that the flux from the source is well determined and constant. Thus, using a surface barrier detector with high energy resolution and stability immediately determines the solid angle and momentum acceptance of the beam line between the source and detector. 2.2. Procedure used f o r beam line tuning

* Work supported in part by the Natural Sciences and Engineering Research Council of Canada and by the National Research Council of Canada.

Initially, the detector was placed at the first (dispersed) focus F1. The B1 setting was determined 95

96

C.J. Oram et al. / Commissioning o f a n e w low energy channel •vacuum valve beam blocker horizontal E~ vertical jaws

horizontal slit

absorber

beamline

t / vert.ical sli!

target X IATI

+

vertical horizontal

×F3 final focus

slit slit-

Imetre Icole

t J ' '1 '

0

'

I

', '

2

'

'I +

3 feet

Fig. 1. M13 beam line layout.

with Q1 and Q2 turned off. Then the currents of Q1 and Q2 were adjusted to produce a double focus at F1, where the magnification, dispersion and solid angle were measured. The detector was moved to the second (dispersed) focus F2 and the current settings of Q3, Q4 and Q5 were adjusted to produce a double focus there. The settings of Q3 and Q5 were kept identical as this symmetry is basic to the design of the beam line. Again at F2 the magnification, dispersion and solid angle were measured. The detector was then placed at the third (achromatic) focus and, with all quadrupoles off, the optimum setting of B2 was determined. The settings for Q6 and Q7 were obtained by maximising the flux into the ~ in aperture of the detector, with other magnets at their previously determined optimum values.

2.3. Results from ~ tuning The results of our measurements are compared with the design specification in table 1. There is good agreement between design and measured values, except for the solid angle. The magnet current settings for which this agreement was achieved differ somewhat from the predicted values (see table 2). The predicted currents for the hemispherical quadrupoles are all low. This is believed to be due to uncertainty in the magnetic field measurements on which the calculations were based. The Q1 magnet is very close to the iron shielding around the production target which might cause the discrepancy between its current value and the calculated value. 3. 91 MeV/c measurements with a 1.45 mm thick carbon target at IATI

3.1. Procedure

Table 1 Comparison of design and measured beam line parameters Experiment

Solid angle Dispersion Horizontal magnification Vertical magnification

Design specification F1 and F2

F1

F2

33 1.22

29 1.18

0.84

0.81

0.90

5.1

4.3

4.4

38 msr 1.26 cm/% Ap/p

An eight-inch-square multiwire proportional chamber with 2 mm wire spacing was positioned at 1=3 and was triggered by an 8 in wide square plastic scintillator placed just downstream of it. Pions were gated by TOF between the cyclotron RF signal and the plastic scintillator. Starting from the alpha source values for beam line components, small adjustments were made so as to produce maximum flux at F3. 3.2. Results It was found necessary to decrease the current in Q4, a vertically focusing element, by 9% to 301 A. This achieved 20% more pion flux than the alpha

C.J. Oram et al. / Commissioning o f a new low energy channel

97

Table 2 Comparison of calculated and measured magnet currents Magnet currents (A)

Q1 Q2 Q3 Q4 Q5 Q6 Q7 B1 B2

Type

Experiment

Calculation

% Difference

646 177 404 330 404 167 204 249 273

560 166 398 301 398 151 178 246 268

+15 +7 +1 +9 +1 +11 +13 +1 +2

tune. It can be seen in fig. 2 that the vertical beam envelope is smaller in B2 for the "pion tune" than the "alpha tune". The horizontal beam envelope is almost unaffected by this change in Q4. The B2 magnet vacuum box was acting as a limiting vertical aperture for pions when using the "alpha tune", as the pion production region is slightly larger than the alpha source. Ouadrupoles Q6 and Q7 were retuned to

i .=

Radiation hard rectangular Hemispherical Rectangular Hemispherical Rectangular Hemispherical Hemispherical

produce a focus at F3. All other elements were found to be in agreement with the alpha source tune. Quadrupole settings for foci further downstream than F3 were obtained by optimising the spot size at the chamber placed at the downstream position. Values so obtained are given in table 3. The electron, pion and cloud [3] muon beam spot sizes at F3 were measured. Beam spots obtained at F3 for electrons, pions and "cloud" muons are shown in fig. 3.

14 I I

4. Flux from 1.45 mm carbon target 1 8 - 1 0 0 MeV/c

I

4.1. A p p a r a t u s

9 8 7 6

4 5 2 I( O

I

2

3

4

5 Melres

6

7

8

9

3 4 5 6 7 8 9 I I

g

m

TUNE

L2 13 14 L5

Fig. 2. Calculated beam envelopes for "alpha source tune" and "pion tune".

Three methods were used to determine the flux over the entire energy range. (a) A surface barrier detector was placed in. the beam line at F3 such that there was no window between it and the 1AT1 target. This detector counted pions and muons over its active area, along with the protons and heavier ions. An energy spectrum at 45 MeV/c is shown in fig. 4. (b) A thin (0.015 in) plastic scintillator, placed after a 0.005 in mylar end window (3 in diameter), was used at F3 to detect the pions and muons. A thick scintillator behind it counted electrons. Those electrons from muon decays at F3 were removed by anticoincidence gating, and deadtime corrections were made. Pions, electrons and muons were distinguished when possible in the thin counter by pulse height and by timing against the machine R F signal. A spectrum illustrating R F timing from the thin plastic scintillator is shown in fig. 5. A multiwire proportional chamber placed between the thin and

C.J. Oram et al.

98

__

r

t

l

c

o

/

Commissioning o f a new low energy channel

Table 3 Values for Q6 and Q7 versus F3 focus position

l

Focus a)

Q6 (A)

Q7 (A)

-5-4-3-2-I O I 2 3 4 5 cm +

F3 F3 + 33 cm F3 + 66 cm

7T Horizontal

167 156 148

211 180 158

n beam spot size (slits and jaws open) fwhm (horizontal)

fwhm (vertical)

2.1 cm 2.3 cm 3.0 cm

1.3 cm 1.9 cm 2.6 cm

a) F3 is 87 cm from the yoke of Q7 (see fig. 1).

-8-7-6-5-4

~

-3-2

~

-I

O I

+

2

3 4

PROTONS and ALPHAS

5 6 7 cm

Vertical 3He+ +

-6-5-4-3-2-I

0

1 2 3 4

g g

I%?÷

cm

L 9Be4+

i,~ol

~l

4

xloo

Pulse Heigh,

Fig. 4. Typical surface barrier detector spectrum showing ion beams.

-9 -8 -7 -6 -5 -4 -3 -2 -I

0

I

2 3 4

the thick scintillators m o n i t o r e d the beam spot. (c) Using a 0.010 in end w i n d o w (6 in diameter) on the beam line, the n, /a and e rates were determ i n e d b y timing with an 8 in X 8 in X 0.25 in plastic scintillator against the cyclotron R F signal. A TOF spectrum is shown in fig. 6. The beam spot was monitored using a multiwire proportional chamber b e t w e e n the end window and the scintillator.

5

+ Vertical

4.2. R e s u l t s

The fluxes observed are plotted in figs. 7 and 8 for positive and negative particles, respectively, all slits

~o

-I1-10-9-8-7-6-5-4-3-2-I

0

I

2 3 4

5

6 7 8

cm I

Fig. 3. Pion, electron and cloud muon beam spots at F3 at

91 MeV/c.

3 ~..~,

4~ns Time of FHght

Fig. 5.33 MeV/c n, u and e TOF spectrum.

CJ. Oram et aL

/

99

Commissioning o f a new low energy channel 6,000 5,oo 0 4,0o0

,+

~

'

r

r

-

&ooo 2Doo

~

l,OOC 80C 60C

400

,4

e

X

~/~

3O0

E o

2OO

J

IO0 8O

~ 60 @ 4a /

I0

0

~

20

Time of

30 45ns

3C

40 o

Flight IC

8

Fig. 6.91 MeV/c TOF spectrum.

6 4

50,000 40,000 30,000 20,000

7/"

protons

+

6,000 4,000 3,oo0

+

u~ 2,000

o=

L)

,'0 2'0 3'0 4'0 ~0 6'0 7'0 do 9'0 i;o MeV/c

Fig. 8. Negative particle fluxes from 1.45 mm carbon target ( 1 8 - 1 0 0 MeV/c).

•

10"0oo 8,00o

u~

2

~ooo 800

and jaws fully open. It can be seen (fig. 7) that below 52 MeV/c we obtain a flux of e ÷ from ~t+ decays at the production target. This e ÷ flux is not time correlated with the cyclotron RF, unlike the e ÷ from pair production by gamma rays from 7r° decay. This can

e+

600

e+

400 300

e+

50 MeVlc

200 e+

I00 80 60

55 MeVI c .n.+

40 20

~

o

40

50 ~ 60

70

80 90

I00

MeV/c

Fig. 7. Positive particle fluxes from 1.45 mm carbon target ( 1 8 - 1 0 0 MeV/c).

15 30 14 45.4n.sec Time of

flight

45

0

15

L

30

L45

4314n see Time

of

fllcjh'(

Fig. 9. TOF spectrum for 50 and 55 MeV/c n ÷,/Z and e÷.

C.J. Oram et al. / Commissioning of a new low energy channel

100

clearly be seen in fig. 9 which shows the TOF spectra at 50 and 55 MeV/c. Pion fluxes were measured down to 3.8 MeV and positive muon fluxes down to 0.8 MeV. The absorber at F1 has proved useful for removing protons from the 100 MeV/c pion beam, without noticeably changing the final beam profile. However, attempts to use the absorber to reduce the electron contamination in the surface muon beam severely enlarged the final beamspot. This effect has also been observed at SIN [4].

5. Fluxes from various production targets

5.1. Comparison o f vanadium and copper targets Table 4 compares the fluxes obtained from 10 mm copper and 3 mm vanadium targets. While the flux for n÷ and ~÷ are about the same for copper and vanadium, the n - and tJ- rates are larger for vanadium than copper by about 1.4 at 100 MeV/c and 2.0 at 55 MeV/c. Hence for the same primary beam flux, an enhanced n - and p - flux can perhaps be obtained by using a vanadium rather than a copper target. Moreover, for the same primary beam current, vanadium (Z = 23) produces less primary beam spill than copper ( Z = 2 9 ) . The electron production is reasonably independent of material indicating roughly similar no production from copper and vanadium.

5.2. Comparison o f Be, C, Cu and V targets at 1AT1 for lr÷, ~* and e ÷ Table 5 shows the measured fluxes at 55, 91 and 100 MeV/c. The electron contamination is a function

Table 5 Comparison of Be, V, C and Cu targets, for positive particles 1ATI target

Beam line momentum (MeV/c)

Be V C V Cu C V Cu C

Rate per (g/cm2) target lr+ U+ e+ (Thousand events/,A protons s g/cm2)

91 91 91 100 100 100 55 55 55

30.3 33.5 62.0 37.7 35.4 96.0 1.46 1.52 4.90

2.6 2.2 4.9 2.7 2.6 6.3 0.83 0.72 2.08

1.2 9.2 2.6 5.4 5.4 2.6 9.1 8.3 4.4

of the size and shape of the target and so does not necessarily scale with thickness. Hence table 5 should be used with care when estimating electron rates from new targets. Measurements with a 2 mm and 1 cm carbon target at 1AT1 showed that rates for pions and muons scale with target length.

5.3. Neutron flux at F3 The neutron (1 10 MeV) flux was measured in the experimental area using a long BF3 counter [5] surrounded by wax with a 1 cm carbon production target inserted at 1AT1. The counter was calibrated using an A m - B e neutron source. The detector was mounted on a trolley at beam height. The neutron flux in the experimental area was 2 × 10 -3 neutrons/s /~A cm 2, and variations with position of the detector in the experimental area were less than 30%~

Table 4 Comparison of vanadium and copper targets 1AT1target

V Cu V Cu V Cu V Cu

1ATI target thickness (ram) 3 10 3 10 3 10 3 10

Rate per (g/cm2) target

Beam line

7r e (thousands events/taA protons s g/cm2)

Momentum

Polarity

15.0 10.6 0.89 0.43 37.4 35.4 1.47 1.51

100 100 55 55 100 100 55 55

+ + + +

7.04 7.86 9.47 7.03 5.36 5.44 9.17 8.23

0.88 0.68 0.32 0.17 2.68 2.62 0.83 0.72

10l

C.J. Oram et aL / Commissioning o f a new low energy channel 9

6. Momentum acceptance of the channel The momentum acceptance of the beam line was determined using the novel technique of measuring the energy spread of the protons and heavier ions present in the beam. A 40 mm diameter Si(Li) detector was placed at the final focus. In order that all the beam entered the detector the horizontal jaws (see fig. 1) were closed to 40 ram. A low primary beam current was maintained (20 nA) so that the rate in the detector was low enough to maintain the 24 keV resolution, and limit the radiation damage to the detector. The beam line was tuned for 55 MeV/c positive particles, and four peaks were observed in the pulse height spectrum corresponding to 3H*, 2H+, aHe 2* and a peak with both 1H+ and 4He 2+ (because of their similar charge squared to mass ratio). Pulse height spectra of protons, 4He2+ and 3He2÷ were recorded for a variety of slit settings. Spectra obtained are shown in figure 10 for slit settings between 20 and 100 ram. The aHe2÷ peak was used to calculate the momentum acceptance which is compared in fig. 1 l with calculated values [6]. The agreement between the measured and calculated values is good for slit settings between 20 and 80 ram. The disagreement at large horizontal slit settings is not understood. Slight adjustments, especially of Q2, might have improved the agreement at small slit settings substantially. PROTON AND ALPHA PEAK

2000

L 15oo

I000

o

500 3He++ PEAK

FI HORIZONTAL SLIT OPENING IOOmm

Ii/

4

i, '~'-~

~_

, 1"4

~2

, - L' ~ - ~ r 1'6

rn

40 m r_-_~ , - - ~ - - ~

('8

7

N6 5

/

I 4

~,/

5

x CALCULATED VALUE o MEASUREMENT WITH 2ram CARBON D MEASUREMENT WITH

2 I

/

/ / ,"( "Q/ x

I cm CARBON ,

] 5

i

i

,

i

I I0

i

i

FI HORIZONTAL SLIT WIDTH

i

cm

Fig. 11. Momentum acceptance of beam line (horizontaljaws opening 4 cm).

This technique of measuring protons and heavier ions in a solid state detector at the final focus of a secondary beam line should prove useful for: (1) calibrating the energy of a beam line setting; (2) measuring and monitoring beam line momentum bites.

7. Effect of the beam line slits and jaws

g

o

xS

8

2"0 ENERGY

2"2

~

20mrn

--

2"4

(MeV)

Fig. 10. Si(Li) pulse height spectra for various slit settings.

Figures 12 and 13 show the effect of horizontal slits and jaws on flux and horizontal beam spot, in terms of: full width at half maximum, fwhm, full width at quarter maximum, fwqm, full width at tenth maximum, fwtm. The horizontal jaws, before B1, limit the second order effects of the beam line, by decreasing the solid angle. The slits at F1 determine the momentum acceptance and limit chromatic second order effects. It was found that: (1) the jaws have a more drastic effect on the tails of the beam profile than the horizontal slits at F1; (2) the horizontal spot was not affected by the vertical jaws and slits; (3) the vertical spot was insensitive to the vertical jaws; (4) the effect of the silts at F2 was very similar to that of slits at F 1.

102

C.J. Oram et al. / Commissioning o f a new low energy channel 8

7

?r

4-

6

HORZ. BEAM SIZE ~ F W T M

5 E

u 4 3

~

FWQM

2 0

~

0 FWHM

0

25,000 < ~L20,OOC

15,ooc

+ +

J u IO,OOC F-

I.Smm production target)

5,ooc

o.

+

i

2.5 '

5'

715

IhO

I2' .5

15 ~

17.5 ~

22.5 ~

2LO

HORZ. dAWS FULL APERTURE

2'5

27'.5

3O

cm

Fig. 12. Effect on F3 horizontal beam spot and flux of horizontal jaws.

8. Effect on surface muon flux of steering the proton beam at 1 cm IATI production target The surface muon flux obtained at F3 has been measured as a function o f the position o f the beam

z~- FWTM

7 6

spot on the 1AT1 1 cm carbon target (see fig. 14). The target is 5 m m wide, and is supported from the side furthest from the beam line. The /a* measurements have been compared with a simple model. The model allows for the measured proton beam spot horizontal profile (1 mm fwhm) and assumes that the / l + come from decays of n ÷ stopped in a thin skin on the side of the target viewed by the beam line. The values from the model and the

~-+HORZ. BEAM SIZE

5 E 4 o 5 2 I

0 12

m8 ~_

ii

I

IO

~ 9

c5

~. PZ~

25/300

~=L20, 0 0 0

v

~

uJ I 5,000 _o. I 0,000

91MeV/c duction

a)

~.+ FI. . . . Iculoted ~'~® E from model o, c Measured

u_ J5 x 6

z ~XYO~flux

lu.÷ ~

'

mj =

target)

5,000 •'

,

!

,

2 5

5

75

I0

HORZ. SLITS

,

FULL OPEN

cm

I

o

-4

-~ -~ -I 0. . I . 2. . 3 4 5 6 7 8 9 I0 mm PRtMARY BEAM POSITION

Fig. 13. E f f e c t o n r a t e and h o r i z o n t a l b e a m spot (at F3) o f h o r i z o n t a l slits at F1.

Fig. 14. Surface muon flux as a function of horizontal proton beam position at 1AT1.

103

CJ. Oram et al. / Commissioning o f a n e w low energy channel

Table 6 Comparison of M9 and M13 n÷ fluxes Beam line

Momentum

Measured ratio

Calculated ratio

724, 317)

0.44

0.48

10001 141J

0.14

0.15

Rate

Production target

( 1 0 3 S- 1 b t A - 1 )

M9 M13

96 100

M9 M13

96 91

1 cm Cu 1 cm Cu 5 cm Be 2.54 cm Be

observed data have been arbitrarily normalized. The probability of a pion, coming from any point in the target, stopping in the skin of the target, is taken to be proportional to the product of the solid angle and length of the pion track in the thin skin. The associated positron flux is also shown in fig. 15; the smooth curve through this data is given to guide the eye. The ratio of the muon to electron flux is highly dependent on the horizontal primary beam position and width but is approximately unity with an optimum tune. The measurements show that it is possible to increase the ,u + flux and the #+ : e ÷ ratio while decreasing the target heating, by steering the proton beam to the edge of production target and actually allowing some of the primary beam to miss it. It is thus possible to deliver a beam of surface muons of 106 per second, from a 1 cm graphite target bombarded with a 100/JA proton beam, into an area

,6i

of 3 X2 cm with a unity.

e÷

:/a + ratio of approximately

9. Comparison of M13 with the M9 channel at

TRIUMF The M9 channel has a take-off angle of 135 °, as does M13; it is thus useful to compare the performance of the two beam lines. A comparison of measured M13 to M9 flux ratios for Be [8] and Cu [7] targets to the ratios as calculated using a REVMOC [6] Monte Carlo program, shows reasonable agreement (see table 6). The measured solid angle for M13 is less than predicted by REVMOC (see table 1) and the calculated flux ratio has b e e n adjusted to account for this difference. The authors would like to thank the Beam Lines, Beam Development and Magnet groups at TRIUMF for their assistance in making these measurements. We particularly wish to thank Dr. George Mackenzie and Mr. A1 Morgan for their efforts.

15

14

References

15 12

~LO' m

9

•~

B 7

d 6 5 4

i -'4-3-2-I

0 PRIMARY

I

2

.3 4

BEAM

5

6

7

B

L 9

, I0

mm

POSITION

Fig. 15. 29 MeV/c positron flux as a function of horizontal proton beam position at IAT1.

[1] J. Doornbos, M13 a new low energy pion channel at TRIUMF, Internal Design Report (April, 1978). [2] M.J.M. Saltmarsh and R.L. Burman, private communication. [3] K. Tanabe, Particle Accelerators 2 (1971) 211; C. Tschalar, LAMPF Internal Report LA-7222-MS (June, 1978). [4] M. Daum, SIN Newsletter 12 (Dec. 1979) p. 2. [5] T.L. MacFarlane, MSc Thesis, University of British Columbia (1968). [6] P. Kitching, TRIUMF Report TRI-71-2 (1971). [7] N.M.M.A1-Quazzaz, G.A. Beer, G.R. Mason, A. Olin, R.M. Pearce, D.A. Bryman, J.A. Macdonald, J.-M. Poutissou, P.A. Reeve, M.D. Hasinoff and T. Suzuki, submitted to Nucl. Instr. and Meth.; TRIUMF preprint TRIPP-80-4. [8] J.A. Macdonald, private communication.