SOME COMMENTS AND QUESTIONS ABOUT THE Ft.;TURE OF NEUTRINO PHYSICS AT NAL. M. L. Stevenson I.awrence Berkeley Laboratory

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1 55-73/220 SOME COMMENTS AND QUESTIONS ABOUT THE Ft.;TURE OF NEUTRINO PHYSICS AT NAL M. L. Stevenson I.awrence Berkeley Laboratory 1. Is Another Neutrino Facility Required for a 1000-GeV Accelerator? The purpose of this note is to study the possible gains to be made by lengthening the present target to detector distance from the present 1400 meters to the maximum of 3700 meters and to understand how these gains depend upon detector size. In order to do this I examine the fraction of mesons of various momenta that decay in such a way that their neutrinos pass through a detector of radius R. I assume some ideal focusing system always points the mesons at the detector. Let S = Shield thickness D 7 Length of the decay tube I, = Oetector to decay point distance T) = P/MoC for the meson. The fraction of mesons decaying (with meanlife T) via f1 + v that cause their neutrinos to pass through the detector of radius R is f x y fr 2. e -x (1) X o where f1 + v branching ratio, 1,0 for pions, 0.63 for kaons r = R/CT, S/ T)CT X o x = (D + S)! "let y L/T/CT. I shall refer to Y as the "neutrino yield" per produced meson of momentum I?M C' The o "canonical" mesons 1 of the accelerator are those that have nearly the same velocity as the protons that produce them. Since kaons are responf;ible for producing the most energetic neutrinos (they can have almost the same energy as the kaon}, most attention will be paid to them, I shall assume at the present 400-GeV mode of operation, too- and ZOO-GeV kaons will be copiously produced. Figure 1 shows how the integrand of equation (1) fa!" R = 1 meter (r = ) varies with y and clearly shows that the greatest contribution to Y comes from small values of y 0.3. For 200-GeV kaons the present neutrino facility with 0 + S = m (x = , X eo 0.6(6) does o not allow contributions in this ideal region. The shaded area of Fig. t. (which represents the present facility), when multiplied by the factor /e-x gives Y/f = showing that (0.63) X 3.32% of the 200-GeV kaons produced would put neutrinos within a 1 meter radius detector. -147

2 It is evident from Fig. f that the greatest gains in neutrino yield will come from decreasing the length of the!:;hield. Figure 2 (3) displays the dependence of YIf on the shield thickness for 200-GeV kaons (pions). Figure 2 shows the variation of vir VB S for too GeV kaons. Figure 4 ' is a similar curve for 600-GeV kaons, showing that only modest gains can be made for them by shortening the shield. Figure 5 gives the gain factor for various energy kaons over the present 1000 meter shield by shortening it. For the present 400-GeV mode of operation very substantial gains in neutrino flux can be made by shortening the shield as much as possible. What can be done for the 600-GeV kaons that may be copiously produced by TeV proton beams of the future? I shall assume that a 600-meter shield will be the shortest possible one for a future facility. The longest possible beam will be one coming from the "Q" area where D + S 3700 meter. Figure 6 shows V/f VI' target to detector (D + 5) difltance for various momenta mesons for a 600 m shield. So long- as the transverse si".e of the detectors remains at R '" f m trivial gains in yields will result. Also shown are the V's if the detector is increased to its canonical transverse size (= CT = 3.7f mi. One should note that at these canonical decay lengths and detector sizes almost.half the ooo-gev kaom; produced would yield neutrinos through the detector, 2, The Need for a Bigger Neutrino Bubble Chamber Perhaps this is the moment to mention that a 3.7 m radius sphere i!1 very close to the size of the spherical 2.5 foot hydrogen bubble chamber that several of us proposed during the 1969 NAL Summer StUdy, 2 Taking the increased path length of the neutrinos through this detector, the increased number of neutrinos and the linearly rising cross nection with E one could obtain at lea5t a factor of 20 increase in event rates from the canonical kaomi. One should not underrate at this time the capahilities of bubble chambers of this iiize to do neutrino physicf'. Such a chamber filled with hydrogen (neon) is a (500) ton detector. my experience with bubble chambers over the year>! I have developed a workin rule of thumb From regarding electronic detectors that try to manquerade as bubble chambers and compete with them. In order to do so they must be able to handle beam intensities 1000 times greater than the bubble chamber. In neutrino physics both devices share the same beam, To compete with the 25 ton hydrogen Lest I be misunderstood, 1 do not classify Experiment fa or Experiment 2.1 in this categ-ory, 3, The Need for Dichromatic Beams 4 The need for dichromatic beamf?with large aperture focusing devices similar to the Nezrick monohorn is one of the underrated subjects of this summer study. They have been given only lip service in spite of the following qualities: 1. At lower energies they can produce neutrinos of well defined energy that can greatly improve the analysis of neutrino events in the hydrogen-filled i5-foot bubble chamber. 2. They are more easily monitored and the flux can be more easily determined, 3, They can provide a very useful "fall back position" in case clever ideas about reducing shield thickness fail. They represent an excellent way of eliminating high energy (as well as low energy) mesons whose decay muons can later cause trouble. bubble chamber the electronic detector would have to have the equivalent of 2.7,000 tons of hydrogen. -148

3 4. For neon filled bubble chambers it is by far the best way to "turn down the neutrino flux" so as not to have the chamber swamped with tracks and energetic electron showers. 5. They will reduce the number of "regenerated muons" from the muon shield and the bub hie chamber coils. With all these nice things said about dichromatic beams I wish to examine how the dichromatic spectrum is affected when shield thickness and decay length changes are made. These calculations shall deal with an "idealized" system where the focusing device points all mesons of p - P + ll.p at the center of the detector and whose meson beam transverse dimension is negligible. The energy of the neutrino that is "decayed" at an angle 8 is E : 21') q/(t + 1') 28,\ where q is the momentum of the neutrino in the meson rest frame and 71 is the usual laboratory meson momentum divided by the meson rest mass. If the neutrino event is observed at a distance R' off the axis of the detector there will result an uncertainty in the angle 8 caused by lack of knowledge of where the meson decayed in the decay pipe; i. e. (8":: R'/S + D) < 8 < (8' =R'/S). This uncertainty leads to a fractional neut rino energy unce rtainty. 71 (_E_"_=_1_+_1'/_2...:...;( :-=-+.;,...R''::'D--=-)_2-:),...-_q_(_E_' _t_+_l'j_:--,(,"",_' :..,.)_2_1.1,_+_---=-t.,, L l'j2(s'd)2 where, E" (EI) is the neutrino energy if the kaon decayed at the target (start of the shield). Figure 7 summarizes this fractional neutrino energy uncertainty for R' = t m for 200 GeV and 600-GeV kaons as one varies the shield thickness keeping the target to detector fixed at 1400 m. This figure shows how the "energy accuracy" is compromised as one decreases the shield length in order to gain in neutrino yield. One should remember that the energy accuracy is only one of five virtues of the dichromatic beam. Figure 8 summarizes 6E/E for various target to detector distances holding the shield thickness fixed at 600 meters. 4, 25 Foot Diameter Hybrid Hydrogen (Neon) Bubble Chamber {or Neutrino Physics a. TST's Become Large Enough for Neutrino Physics The large volume allows reasonably large track sensitive targets (TST\, unlike the present 15 foot chamber, to he placed inside the 25 foot diameter sphere. b. Quantameters Quantameters can be placed outside the bubble chamber sphere with pure hydrogen filling. Even with this increased wall thickness of the chamber body ( 3 radiation lengths). accurate position and energy of '( ray or electron induced showers can be made; i. e. ox = ± 3 mm and 6E/E'{ '" O. t 5/.JE (GeV), It is much more desirable to measu re the showers externally rather y than internally where the shower may cause some confusion (especially in pure neon). -149

4 c, External Muon Identifier The superconducting coils of this chamber, which will be designed to give good magnetic field uniformity in the region of the metallic expansion bellows (no pistons allowed in this design!), will also provide for hadron absorbing material for a muon identifier (EMU placed extel'nally to the vacuum system. Figure 9 shows schematic elevation and plan views of such a chamber. d, The Intel'nal Material as a Particle Identifier The large volume of liquid will cause a reasonable fraction of the hadronic particles of the neutrino interaction to interact before leaving the chamber, thus possibly causing them to reveal their identity. The nuclear mean free path is about 4 (0.6) meters of hydrogen (neon), References i See M. L. Stevenson, NAL 1968 Summer Study, The Neutrino Facility at NAL, p LR Palmer, R, Huson, M. L. Stevenson, C. Baltay, B. Roe, and H. Wachsmuth, Reconsideration of a Spherical t Bubble Chamber in the Light of Track Sensitive Targets. NAL Summer Study, 2, 225, 3R. Stefnski et a1. A Sign-Selected Dichromatic Neutrino Beam, National Accelerator Laboratory Report NAL-Pub-73/66-EXP. 4 F. A. Nezrick, A Monoenergetic Neutrino Beam Using Current Sheet Focusing Elements, Nuclear Science NS!,!. 759 (i 971), submitted to Particle Accelerator Conferenc e on Accelerator Engineering and Technology. -i 50

5 KAON NEUTRINOS FROM 200 GeV MESONS THAT PASS WITHIN IMETER OF THE AXIS OF THE DETECTOR..., \1l..., -8 '" + > 6 ;:. 4U I E J w E 0 E <t 0 <t...j w :r w U> C> c:: r'///////////////r E 0 0 N I <t I llj (!) a::: <t I o y FIG.I

6 NEUTRINO YIELDS FROM MONOENERGETIC KAONS TARGET TO DETECTOR =1400m 200GeV K I 6E..., t dnii deli E 2 E 1 Eo E2 I.. I 0.15 YII' AE I ="1)2 Eo 1+ "1 2 {s!o)2 fle 1]2(t}2 = Eo I+1]2(f} 2 6E EO EO$:::S 2"1Q O---JI...--.JI-..---L_-----.L_----L-----L._--.L SOD SHIELD (m) FIG.2 ""'" -152

7 NEUTRINO YIELD FROM 100 GeV KAONS TARGET TO DETECTOR =1400m 0.05 YK/ f t 53

8 NEUTRINO YIELDS FROM MONOENERGETIC PIONS 200GeV." TARGET TO DETECTOR =1400m o SHIELD(m) FIG.3-154

9 NEUTRINO YIELD FROM 600 GeV KAONS TARGET TO DETECTOR =1400m 0.20 o -.J W s: en... z IJ.I en!

10 GAINS TO BE MADE BY SHORTER SHIELD TARGET TO DETECTOR: 1400 METERS DETECTOR RADIUS =I METER 0 6 -J IJJ >= 5 => IJJ z 4 a:: g 0 rl z a::: to -156

11 NEUTRINO YIELDS vs TARGET TO DETECTOR DISTANCE S=600m I ij' -.J I GeV K(R_:::.:.:.;lm:.:.J)_ GeV K[R= 1m) ZOOGeV K CR= 1m) (S +0)= TARGET TO DETECTOR (m) FIG.6

12 FRACTIONAL I' ENERGY UNCERTAINTY IN IDEALIZED DICHROMATIC BEAM FOR KAON NEUTRINO SOO SHIELD, S(m) FIG.7 -i58

13 FRACTIONAL NEUTRINO ENERGY UNCERTAINTY IN IDEALIZED DICHROMATIC BEAM FOR KAON NEUTRINOS _a_ 0.4 I BE E--E' E 2."q I... I II> -.D I (5+0) a TARGET TO DETECTOR (m) FIG.8

14 SUPER CONDUCTING COILS VACUUM TANK BELLOWS t1quantameter" II, I III 'I I.. " EXTERNAL MUON IDENTIFIER (1m 2 MODULES) ELEVATION VIEW COILS QUANTAMETER PLAN VIEW FIG.9-160