Tohoku 1.4m holographic bubble chamber

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PROCEEDINGS SUPPLEMENTS

Nuclear Physics B (Proc. Suppl.) 36 (1994)491-494 North-Holland

Tohoku 1.4m Holographic Bubble Chamber T. Kitagakia, E.L.Hartb, and E745,782 Collaborationc a. Bubbble Chamber Physics Laboratory, Tohoku University,Aramaki, Sendal, 980 J a p a n b. Department of Physics, University of Tennessee, Knoxville, TN.37909, USA c. BrownUniv.,IndianaUniv.,IHEP,Beijing,MIT,SugiyamaJyogakuenUniv., Univ.Tennessee,Tohoku Univ.,Tohoku Gakuin Univ.

The Tohoku Bubble Chamber has the distinctions of being the last built, and of having carried out the last bubble chamber exposure in the western world (1990). It was designed as a large volume freon chamber with high resolution holography throughout, and carried out two successful neutrino exposures in 1985 and 1987. We present here a brief description on this chamber.

1. I n t r o d u c t i o n The Tohoku 1.4m Freon Bubble Chamber system was designed and built for a beam dump experiment at the TEVATRON. The chamber was to be located 80 m from the dump target. A 1 ton mass and l x l x l meter volume were chosen as optimum. D e v e l o p m e n t a n d c o n s t r u c t u i o n of t h e chamber and its optics were begun five years before the TEVATRON commenced operation in 1985. The chamber was tested in Tokyo, and installed at FNAL in 1984. Between then and 1990 it performed two experiments, a twor u n 800 GeV n e u t r i n o s e x p o s u r e w i t h holography in 1985 and 1987, and a 200 GeV muon exposure without holography in 1990. 2. C h a m b e r Fig.1 shows the Toholu bubble chamber and optics [Ref.1] as seen looking along the beam line. It is a piston driven freon chamber; the visible volume is 830 I. The filling is a mix of R l l 5 ( C 2 C 1 F 5) and R l l 6 ( C 2 F 6) with a density of 1.15 gm/cc and radiation length of 90 cm under the normal operating condition of 23 kg/cm 2 at 25°C. R u b y l a s e r l i g h t is s p l i t to t h e illumination and reference b e a m s and sent from the left in the figure. The illumination beam is diverged by a nonspherical lens to fill

the c h a m b e r and u n i f y the s t r o n g radial dependence of the intensity of light scattered b a c k to t h e h o l o g r a p h i c c a m e r a s . The scattered light and reference light are s u p e r p o s e d by a glass wedge in front of camera. The triplet of conventional cameras and their illuminating flash lamps are mounted on a b o a r d t h a t also holds the h o l o g r a p h i c camera. The inside wall of the chamber is covered by green Scotchlite. Also, successive green Scotchlite covered disks with holes to admit the illumination laser light act as the retrodirectors for the conventional cameras. 3. L a r g e v o l u m e h o l o g r a p h y Holography was required to obtain a high resolution over the entire cubic m e t e r chamber volume needed in order to detect the short tau decay. Extensive tests showed that the basic difficulties involved are a. the large object distance, b. wide field angle and c. the large number of extraneous light scattering objects in the field. a. A distance, L, gives a spatial resolution, R,

R=1.2 2L/d, where diameter of emulsion, d, is practically limitted by the c o m m e r c i a l film width of 70mm.

0920-5632/94/$07.00 © 1994 - Elsevier Science B." All rights reserved. SSDI 0920-5632(94)00564-C

T. Kitagaki et al./Tohoku 1.4 m holographic bubble chamber

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Fig.1 Tohoku Bubble Chamber . 1 aria Optics Neutrino beam perpendicular to the sheet Piston Membrane

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b. The spatial frequency of interference :~ given by f = ( sin0sc + sin0ref) / 2, where 6sc and 6ref are the angles formed by the scattered and reference light to the plane of hologram. For an object this large, the incident angle of the light can be more than 16 degrees. We had to make a compromise between the distance and field angle. A distance L = 1.5 - 2.9 m was chosen for the maximum diameter of 1.1m, as seen in Fig.1. It was decided after many trials that the optimum system is two beam, dark field holography. In laboratory test, using AGFA Holotest 10E75 film with liquid immersion, we o b t a i n e d a s p a t i a l r e s o l u t i o n t h a t fluctuated over a range of 35 - 70/~m under conditions of no turbulance, compared with a theoretical 35/~m. The main reason for this loss of resolution is the reduction of usable diameter, d, due to poor local area quality of the film. Finally, in order to compensate v a r i o u s i m p e r f e c t i o n s in h o l o g r a m , w~,

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:,uppressed the maximum spacial frequency of i n t e r f e r e n c e to 700 l i n e / m m f r o m a commercial value of 2000 line/mm by the design of half mirror superposing system. c. A large object volume of this n a t u r e contains a great m a n y e x t r a n e o u s l i g h t scattering objects which act as additional interference sources and clutter the main interfernce pattern. The overall quality of the hologram, and resolution are both harmed by this noises. We minimized the extraneous reflected light by using green Scotchlite for the conventional photograph bright field retrodirectors, and also by careful masking. The holographic view is quite dark, and bubbles are seen as stars in the dark sky ( Fig. 2 of Bubble chamber activities in Japan in this proceeding). A 10 J Ruby laser, operating at only 1.5 J provided adequate illumination. Since events occur randomely over the beam ping width of 2.5 ms, we developed an anytimeready scheme where the excitation flash pulse

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width is stretched to 1.5 ms with a Q switch usable time of 1.2 msec [Ref.2]. In order to suppress a potential boiling problem in the liquid as the laser beam passed through, the laser pulse width was stretched to 5/~s by a feed back technique• [Ref.3] The hologram was taken approximately 130/~s after an event (bubble size about 55 /Jm) and the normal picture after 2.5 ms. A r o t a t i n g s h u t t e r s y n c h r o n i z e d with the neutrino beam pings masked the holographic cameras from the conventional picture flash

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4. Reconstruction projector We built five real image hologram reconstruction projectors, each having two direction orthogonal goniometers and depth searcher. The full size real image is directly observed by TV cameras• This system worked satisfactorily. 5. Some results In the 1987 neutrino run, E745, 5366 holograms were taken and analyzed• ( about 55 % of the events seen in conventional pictures) Of these approximately 91% of the

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event images are of high q u a l i t y , i.e. the reconstructed bubbles appear as separated circles of less than 100 ~m diameter. We believe that this is not a bad figure for a hologram taken of such a large volume. The resolution in the run was about 35 100 zm, slightly worse than the test bench results. Reasons include local area variation in the film q u a l i t y , c h a m b e r t u r b u l a n c e , variation in the laser operation, and film development in real time, all of which can cause a deterioration in quality. However, the position distribution of holographic events in the chamber shows that turbulance was not the main reason of the drop in resolution. Fig.2 shows examples of charm events from the n e u t r i n o r u n ( s e c t i o n of t h r e e dimensional holographic image). Fig.2(a) is an opposite sign di-muon event. Fig.2(b) is a D°--*4 prong event. We frequently magnify the image in the direction perpendicular to the incoming beam to look for any anomaly. This is done by making use of a memory type copy machine on hologram of high quality. I ,~ this picture y is magnified by 4 times.

Fig.3 shows a candidate t a u neutrino event (holographic picture). v~ p ~ r- + 6 charges, r- -~ i f - ~ v~ The if-, the l a s t t r a c k in t h e c l o c k w i s e ditrection (24.7 GeV/c), was identified by the down stream hybrid spectrometer. It shows a kink; / = 1 2 . 2 _ + l . 6 m m and t h e i m p a c t parameter 158 +45/~m. The high r e s o l u t i o n h o l o g r a p h y is useful not only for the short decay search. It is also used to find scattering of outgoing prongs close to the p r i m a r y event vertex, and in looking for very short recoil protons from nucleus. To be able to see 10MeV protons simultaneously with 100 GeV particles is one of the unique feature of high resolution bubble chambers. References 1. T.Kitagaki et al., Nucl.Inst. and Meth. A281 (1989) 81 2. T.Kitagaki et al., Nucl.Inst. and Meth. A265 (1988) 461 3. G. Harigel et al., Apl.Opt. 25 (1986) 4102 4. J . F e r r y of SLAC, Private communication (1983)