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M13 beam line tuning Oram, C. J.; Warren, J. B.; Marshall, G.; Doornbos, J.; Ottewell, D. May 31, 1980

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T R I U M FMl 3 BEAM LINE TUNINGC.J. Oram, J.B. Warren and G. MarshallDepartment of Physics University of British ColumbiaJ. Doornbos and D. Ottewell TRIUMFMESON F A C I L I T Y  OF:U N I V E R S I T Y  OF ALBERTA  S IMON FRASER  U N I V E R S I T Y  U N I V E R S I T Y  OF V I C T O R I A  U N I V E R S I T Y  OF B R I T I S H  COLUMBIA T R I -80 -1TRI-80-ITRIUMFMl 3 BEAM M l 3 B E A 3 l 3 LC.J. Oram, J.B. Warren and G. MarshallDepartment of Physics University of British ColumbiaJ. Doornbos and D. Ottewell TRIUMFPostal address:TRIUMF4004 Wesbrook Mall Vancouver, B.C.Canada V6T 2A3 May 1980C O N T E N T SPage1. INTRODUCTION 12. ALPHA SOURCE MEASUREMENTS 12.1 Apparatus 12.2 Procedure for beam line tuning 12.3 Results from alpha tuning 23. 91 MeV/c MEASUREMENTS WITH A 1.45 mm CARBON TARGET AT 1AT1 33.1 Procedure 33.2 Results 34. FLUX FROM 1.45 mm CARBON TARGET 18-100 MeV/c 34.1 Apparatus 34.1.1 Solid state detector in beam line vacuum 44.1.2 Thin plastic scintillator 44.1.3 MWPC and thick scintillator 44.2 Results 45. FLUXES FROM VARIOUS TARGETS AT 1 ATI 45.1 Comparison of vanadium and copper targets 45.2 Comparison of Be, C, Cu, and V targets at 1AT1 for tr+.y+and e+ 56. MOMENTUM ACCEPTANCE OF CHANNEL 66.1 From time-of-f1ight width 66.2 From SiLi detector pulse height spectrum 67. NEGATIVE MUON FLUX 26 TO 32 MeV/c 78. MAXIMUM ENERGY OF SURFACE y+ 79. EFFECT OF THE BEAM LINE SLITS AND JAWS 810. NEUTRON FLUX AT F3 811. EFFECT ON SURFACE MUON FLUX OF STEERING PROTON BEAM AT 1 cm 1AT1 TARGET 812. ACKNOWLEDGEMENTS 8References 9LIST OF TABLESPageI Comparison of design and measured beam line parameters 2II Comparison of calculated and measured magnet currentsIII Values for Q6 and Q7 vs F3 focus position23IV Comparison of vanadium and copper targets 5V Comparison of Be, V, C and Cu targets, for positive particles 5VI Comparison of fluxes with those calculated from cross section measurements of Bryman et at. at 100 MeV/c 6VII Comparison of M9 and M13 fluxes 6VIII Calculated momentum acceptances 7LIST OF FIGURES1. Ml 3 beam line layout.2. Calculated beam envelopes for "alpha source tune" and "pion tune".3. Pion, electron and cloud muon beam spots at F3 at 91 MeV/c.A. Typical surface barrier detector spectrum showing ion beams.5. 33 MeV/c 7T, y and e TOF spectrum.6. 91 MeV/c TOF spectrum.7. Positive particle fluxes from 1.45 mm carbon target (18-100 MeV/c).8. Negative particle fluxes from 1.45 mm carbon target (18-100 MeV/c).9. TOF spectrum for 50 and 55 MeV/c EEI N p+ and e+ .10. Momentum acceptance of beam line (horizontal jaws opening 4 cm).11. Negative muon flux into 25 mm2 surface barrier detector.12. Effect on F3 horizontal beam spot and flux of horizontal jaws.13- Effect on rate and horizontal beam spot (at F3) of horizontal slits at FI.14. Effect of jaws and FI slits (horizontal) on horizontal beam spot atF3 + 33 cm.15- Effect of jaws and FI slits (horizontal) on horizontal beam spot at F3 + 66 cm.16. Surface muon flux as a function of horizontal proton beam position at 1AT1.17- 29 MeV/c positron flux as a function of horizontal proton beam position at1 ATI .i v1 . INTRODUCTIONThe Ml 3 beam line is a low energy (20-130 MeV/c) pion and muon channel.1 It views the TRIUMF 1AT1 target at 135° with respect to the primary proton beam. Thebeam line has two 60° bends, the first right, the second left, and is 9-4 m long (toF3)• Figure 1 shows the layout of the beam line and location of the three foci, the s 1i ts 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 using an uncooled 1.45 mm pyrolitic graphite tar­get at 1AT1.The beam line elements can be controlled by computer through the REMCON system at TRIUMF. Slits, jaws and absorber wheel are also controlled through the same com­puter. While the measurements in this report were being made the maximum momentum ofthe beam line was 100 MeV/c; however, with the new power supplies now installed thebeam line operates over its designed range (20-130 MeV/c).2. ALPHA SOURCE MEASUREMENTS2.1 ApparatusA 2ggCm source with two lines at 5-81 and 5-77 MeV was used. The source had a thick window giving it a spectrum witha peak at 4.45 MeV and an 81 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 of 30 keV (FWHM) was mounted at the focus being studied. It had a square aperture of either 3/8 in. or 3/16 in. and could be moved in the plane perpendicular to the beam axis.Use of an alpha source in tuning the beam line 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 for beam line tuningThe detector was first placed at FI. The B1 setting was determined with Q] and Q2 turned off. Then the currents of Q1 and Q2 were adjusted to produce a double focus at FI, where the magnification, dispersion and solid angle were measured. The detector was moved to 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 F3 and with all quadrupoles off, the optimum setting of B2 determined. The settings for Q6-  2 -and Q7 were obtained by maximising the flux into the 3/8 in. aperture of the detector, with other magnets at their previously determined optimum values.2.3 Results from a tuningThe results of our measurements are compared with the design specification in Table I. There is good agreement between design and measured values, except for the solid angle. The values of the magnet current settings for which this agreement was achieved differ somewhat from the predicted values (see Table II). The predicted currents for the hemispherical quadrupoles are all low and 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.Table I. Comparison of design and measured beam line parameters.Exper i mentFI F2Design Specification FI & F2Solid angle 33 29 38 msrDi spersion 1.22 1.18 1.26 cm/% Ap/pHori zontalmagni fication 0.8b 0.81 0.90Verticalmagn i fi cat ion 5.1 A.3b.bTable II. Comparison of calculated and measured magnet currents.Exper imentMagnet currents (amperes) Calculation % Di fference TypeQT 61(6 560 + 15Radiation hard rectangularQ2 177 166 +7 Hemi spher i cal03 hob 398 + 1 RectangularQ1* 330 301 +9 HemisphericalQ5 bob 398 + 1 RectangularQ6 167 151 + 11 Hemi spheri ca 10.7 20b 178 + 13 Hemi spher ica 1B 1 2b3 2b8 + 1 —B2 273 268 +2 —-  3 -3. 91 MeV/c MEASUREMENTS WITH A 1.45 mm CARBON TARGET AT 1AT13 . 1 P rocedureAn eight-inch-square multiwire proportional chamber with 2 mm wire spacing and delay line readout was positioned at F3. The chamber was triggered by an 8 in. wide square plastic scintillator placed just downstream of it. Pions were gated by TOF between the machine 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 Resu1tsIt was found necessary to decrease the current in Q4, a vertically focusing element, by 3Z to 301 A. This achieved 20% more pion flux than the alpha tune. Itcan be seen in Fig. 2 that the vertical beam envelope is smaller in B2 for the "piontune" than the "alpha tune". The horizontal beam envelope is almost unaffected bythis 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. Quadrupoles Q6 and Q7 were retuned to produce a focusat F3. All other elements were found to be in agreement with the alpha source tune.Tunes for foci further downstream than F3 were obtained by optimising the spotsize at the chamber placed at the downstream position. Values so obtained are giveni n Tab 1e III.Table III. Values for Q6 and Q7 versus F3 focus position.Focus Q6 amperes Q7 amperes lE Beam spot s i ze FWHM (horizontal)(slits and jaws open) FWHM (vertical)F3 167 21 1 2.1 cm 1 .3 cmF3 + 33 cm 156 180 2.3 cm 1 .9 cmF3 + 66 cm 1U8 158 3.0 cm 2.6 cm[F3 is 87 cm from the yoke of Q7 (see Fig. 1)].The electron, pion and cloud muon beam spot sizes at F3 were measured. Beam spots obtained at F3 for electrons, pions and cloud muons are shown in Fig. 3-k. FLUX FROM 1.45 mm CARBON TARGET 18-100 MeV/c k . 1 ApparatusThree methods were used to determine the flux over the entire energy range.-  4 -4.1.1 Solid state detector in the beam line vacuum (18-60 MeV/c)A surface barrier detector was placed in the beam line at F3 so 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. 4.4.1.2 Thin (0.015 in.) plastic scintillator (21-50 MeV/c)A thin plastic scintillator placed after a 0.005 in. mylar end window (3 in. diam) was used at F3 to detect the pions and muons with a thick scintillator behind it to count 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 RF signal. A spectrum illustrating RF timing is shown in Fig. 5- A multiwire proportional chamber placed between the thin and the thick scintillators monitored the beam spot.4.1.3 MWPC and thick scintillator (50-100 MeV/c)Using a 0.010 in. end window (6 in. diam) on the beam line, the lEN y and e rateswere determined by timing with an 8 in. x 8 in. x 0.25 in. plastic scintillatoragainst the cyclotron RF signal. A TOF spectrum is shown in Fig. 6. The beam spot was monitored using a multiwire proportional chamber between the end window and the scinti1lator.4.2 ResultsThe fluxes observed are plotted in Figs. 7 and 8 for positive and negative particles, respectively, all slits and jaws fully open. It is worth noting that below 52 MeV/c we obtain a flux of e+ from y+ decays at the production target. This e+ flux is not time correlated with the cyclotron RF, unlike the e+ from pair produc­tion from gamma rays from lE0 decay. This can 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.5. FLUXES FROM VARIOUS TARGETS AT 1AT15.1 Comparison of vanadium and copper targetsTable IV compares the fluxes obtained from copper and vanadium targets. While the flux for EEI and y+ are about the same for copper and vanadium, the EET and y” ratesare 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 ir" and y~ flux can be obtained by using a vanadium rather than a copper target. Moreover, for the same beam current vanadium (Z = 23) produces less spill than copper (Z = 29). The electron production is reasonably independent of material indicating roughly similar lE0 production from copper and vanadium.-  5 -Table IV. Comparison of vanadium and copper targets.1 ATI Target1AT1 Target Rate per (g/cm2) Thickness * e(mm) Thousands events/yA protons/targetsec per (g/cm2)Beam Momen turn1 i nePolari tyV 3 15.0 7.04 0.88 100 -Cu 10 10.6 7.86 0.68 100 -V 3 0.89 9.47 0.32 55 -Cu 10 0.43 7-03 0.17 55 -V 3 37.4 5-36 2.68 100 +Cu 10 35.4 5.44 2.62 100 +V 3 1.47 9-17 0.83 55 +Cu 10 1.51 8.23 0.72 55 +5.2tami na sari 1y electrCompari son of Be, C, Cu and V targets at 1ATT for ir+ , y+ and e+ron con- neces- t i mat i ngTable V shows the measured fluxes at 55, 91 and 100 MeV/c. The elect tion is a function of the size and shape of the target and so does not scale with thickness. Hence Table V should be used with care when es on fluxes from new targets.Table V. Comparison of Be, V, C and Cu targets, for positive particleBeam 1i ne Rate per (g/cm2) target1 ATI momentum + +7T y e+Ta rget (MeV/c) Thousand events/yA protons/sec per (g/cm2)Be 91 30.0 2.6 1.2V 91 33.5 2.2 9.2C 91 62.0 . 4.9 2.6V 100 37-7 2.7 5.4Cu 100 35.4 2.6 5.4C 100 96.0 6.3 2.6V 55 1.46 0.83 9.1Cu 55 1.52 0.72 8.3C 55 4.90 2.08 4.4Table VI compares the fluxes with the values calculated from differential cross-section measurements. The agreement is rather poor with carbon giving more flux than expected and Cu and Be less. However, comparison of measured Ml 3 to M9 flux ratios for Be and Cu targets to the ratios as calculated using the REVMOC program3-  6 -shows reasonable agreement (see Table VII). The measured solid angle for Ml 3 is lessthan that predicted from REVMOC (see Table i) and the calculated flux ratio has beenadjusted to account for this difference.Measurements with a 2 mm and 1 cm carbon target at 1AT1 showed that rates forpions and muons scale with target length.Table VI. Comparison of fluxes with those calculated from cross section measurements of Bryman et at.2 at 100 MeV/c.Element Calculated rate M 13 rate RatioCu 48.6 ± 1.95 35.4 I .38 ± 0.04C 73-6 ± 2.7 92.8 0.76 ± 0.03Be 65.8 ± 2.2 45.0a 1 .46 ± 0.05[units: thousands events/pA/sec (cm2/g) target]a Estimated from measured value at 91 MeV/c of 30.0 K.Table VII. Comparison of M9 and M 13 w+ fluxes.Beam 1i ne MomentumMeV/cProductiontargetRate Measured (103 sec-1 pA_1) ratioCalculated rat ioM9 96 1 cm Cu 687 0.46 0.48M13 100 1 cm Cu 317M9 96 5 cm Be 10,0 0.14 0.15M13 91 2.54 Be 141MOMENTUM ACCEPTANCE OF CHANNEL 6.1 From time-of-f1ight widthThe momentum acceptance can be estimated by measuring the time- of-f1i ght wi dthof e+ and lE+ at 70 MeV/c. By Monte Carlo simulation using the REVMOC program3 weobtain a conversion factor between TOF width and momentum spread. We obtain a value of:Ap/p = 6.4 ± 0.4%with slits and jaws open.6.2 From SiLi detector pulse height spectrumPlacing a large (4 cm diam) SiLi detector at F3 we measure a pulse height spec­trum. Protons, alphas, deuterium, tritium, and (He3)++ peaks are observed. The method relies on all the beam entering the detector, so the measurements were made-  7 -with horizontal jaws* closed to b cm. The width of the proton and (He3)++ peaks was measured, with the beam line tuned to 55 MeV/c, for a variety of FI slit settings.In all cases it was assumed that the flux of (He3)++ was independent of momentum in the range observed. Results obtained are compared with theoretical values in Fig. 10. The agreement between the measured and calculated values is good for slit settings between 2 and 8 cm. The disagreement at large horizontal slit settings is not understood. At slit settings below 2 cm the discrepancy is probably due to a com­bination of factors, for instance:a) Slight mistune of primary beamspot focus on 1AT1b) Slight mistune of Q1 and Q2c) Slight instabilities in current in beam line elementsTable VIII shows the calculated momentum acceptance of the beam line for a variety ofcond i t i o n s .Table VIII. Calculated momentum acceptances.Beam line statusFI horizontal slit open i ng cmAp/p %1 0.7b cm jaws 1 2 1 .60.2 cm target b 3.2open 9.11 0.8Jaws ful1y open 2 1 .60.2 cm target b 3.3open 8.31 0.9Jaws fully open 2 1 .61 cm target b 3.2open 8.57. NEGATIVE MUON FLUX 26 TO 32 MeV/cThe negative muon flux shows a 23% enhancement in flux at 28.7 MeV/c, as com­pared with the monotonica1ly increasing flux as a function of energy (see Fig. 11).This peak may be due to the slow EEU in the "cloud" around the target. These slow pions are in the "cloud" viewed by the channel longer than faster EEU N and can only give p" of about 28 MeV/c when they decay.8. MAXIMUM ENERGY OF SURFACE p+The maximum energy of surface y+ was measured with a surface barrier detectorat F3. An energy spectrum was taken with the beam line tuned to 28 MeV/c. The*See Fig. 1 .resulting spectrum is complicated by the muon decaying in the detector, within the 2 ysec shaping time of the amplifier, causing more than the maximum p+ energy to be deposited. The resulting spectrum shows a sharp cutoff at an energy of A.11 ± 0.01 MeV/c.9. EFFECT OF THE BEAM LINE SLITS AND JAWSFigures 12 to 15 show the effect of horizontal slits and jaws on flux and hor­izontal beam spot, in terms of:Full width at half maximum FWHMFull width at quarter maximum FWQMFull width at tenth maximum FWTMThe horizontal jaws, before Bl, limit the second order effects of the beam line, bydecreasing the solid angle. The slits at FI determine the momentum acceptance andlimit 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 FI2) The horizontal spot was not affected by the vertical jaws and slits3) The vertical spot was insensitive to the vertical jawsA) The effect of the slits at F2 was very similar to that of slits at FI10. NEUTRON FLUX AT F3The neutron (1-10 HeV) flux was measured in the experimental area using a longBF3 counter surrounded by wax with a 1 cm carbon target inserted at 1AT1 . The counterwas calibrated using a Am-Be neutron source. The detector was mounted on a trolley at beam height. With one foot of wax and three feet of concrete shielding the entrance to the 1A tunnel, the neutron flux in the experimental area was 2 x 10-3 neutrons/sec/ uA cm2 , and variations with position of the detector in the experimental area were less than 30%.11. EFFECT ON SURFACE MUON FLUX OF STEERING PROTON BEAM AT 1 cm 1AT1 TARGETThe surface muon flux obtained at F3 has been measured as a function of the position of the beam spot on the 1AT1 1 cm carbon target (see Fig. 16). The target is 5 mm wide, and is supported from the side furthest from the beam line.The p+ 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 y+ come from decays of ir+ stopped in a thin skin on the side of the target viewed by the beam line. The values from the model and the 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. 17; the smooth curve through this data is to guide the eye.-  8 --  9 -The measurements show that it is possible to increase the y+ flux and the y+ :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 the target.12. ACKNOWLEDGEMENTSThe 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 assistance.REFERENCES1. J. Doornbos, M13, A new low energy pion channel at TRIUMF, internal design report (April 1978).2. P.W. James, D.A. Bryman, G.R. Mason, L.P. Robertson, J.S. Vincent and T.R. Witten, Low energy, large angle pion production by 580 MeV proton bombardment of various nuclei, internal report VPN-75~1 (1975).3. P. Kitching, REVMOC, A Monte Carlo program for calculating charged particie transmissions through spectrometers and beam lines, TRIUMF report TRI — 71 — 2 (1971)-A. T.L. MacFarlane, Efficiency calibration of a neutron long counter, M.Sc. thesis,University of British Columbia (1968)-  10  -se t vcw s c *  lm  ccmonbjts pcw to*  mA STRAIGHT L M .Fig.  1 . Ml 3 beam line layout.7r Vertical-  1 1Fig. 3. Pion, electron and cloud muon beam spots at F3 at 91 MeV/c.-  1 2  -PROTONFig. 4. Typical surface barrier detector spectrum showing ion beams.Fig. 2. Calculated beam envelopes for "alpha source tune" and "pion tune".Fig. 5- 33 MeV/c EEN p and e TOF spectrum. Fig. 6. 91 MeV/c TOF spectrum.-  13 -MeV/c6,0005.00 04 .0003.000 zpoo1.000 8 0 0  6 004 0 03001008 06 0<0  10 2 0  3 0  4 0  50  6 0  7 0  8 0  9 0  100MeV/cFig. 7. Positive particle fluxes from Fig. 8. Negative particle fluxes from1.45 mm carbon target (18-100 MeV/c). 1.45 mm carbon target (18-100 MeV/c).Fig. 9. TOF spectrum for 50 and 55 MeV/c EEI N Fig. 10. Momentum acceptance ofy+ and e+ . beam line (horizontal jaws opening4 cm) .PART (CLES A SEC. ' ^-/sec/^A-  \ k  ~HORZ. JAWS FULL APERTURE cmFig. 11. Negative muon flux into 25 mm2 surface barrier detector.Fig. 12. Effect on F3 horizontal beam spot and flux of horizontal jaws.0 2-5 5 7-5 10 12-5Horz. Slits F I Full Aperture cmFig. 14. Effect of jaws and FI slits (horizontal) on horizontal beam spot at F3 + 33 cm.OPENHORZ. S L IT S  cmFig. 13- Effect on rate and horizontal beam spot (at F3) of horizontal slits at FI.0 2-5 5 7-5 10 12-5Horz. S lit s  Fi Full Aperture cmFig. 15- Effect of jaws and FI slits (horizontal) on horizontal beam spot at F3 + 66 cm.- 15 -Fig. 16. Surface muon flux as a function of horizontal proton beam position at 1 AT 1.Fig. 17. 29 MeV/c positron flux as a func­tion of horizontal proton beam position at 1 ATI .

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