---:, r/"%-_ - .# FINAL TECHNICAL REPORT t NASA GRANT NAGW-1247 "Cosmic Ray Positron Research and Silicon Track Detector Development" 1 Dec. 87 - 30 Nov. 90 Principal Investigators 1211187- 9/11188: W. Vernon Jones 9111/88 - 11130190: John P. Wefel Department of Physics and Astronomy Louisiana State University Baton Rouge, LA 70803-4001 Phone: 504-388-8696 Fax: 504-388-5855 E-Mail: WEFEL%[email protected] prepared for National Aeronautics and Space Administration Space Physics Division Code SS Washington, DC 20546 (_;,t':_-C_-] _!770o) C_?-,_[C <"hv _£[Ti<f]N 'x4_l-JqOqV ,:_ j{j_c.!l DNr_ qil+lC_.7 T,_.'tCK _'__T---clrr7 ", i o _7 - :'cD Joy. tvqu (Louisi,_n,D ;_a'_ Univ.) .7 , O",,..t o _:' 6 .31._ 3 Cosmic Ray Positron Research and Silicon Track Detector Development NAGW-1247 I. Introduction Grant NAGW-1247 was established at LSU in 1987 for the dual purposes of conducting research on: a) position sensing detector systems, particularly those based upon silicon detectors, for use in future balloon and satellite experiments, and b) positrons, electrons, proton, anti-protons and helium particles as measured by the NASA NMSU Balloon Borne Magnet Facility. The research was to be conducted, mainly, off-campus at the Goddard Space Flight Center (GSFC) under the direction of W. V. Jones, LSU, and J. F. Ormes, GSFC, with Prof. Jones spending a considerable fraction of his time in the Washington, DC area. The effort was composed of a postdoctoral associate working on detector development at GSFC, a part-time postdoctoral associate and other staff working at LSU and a graduate student working on the data from the Balloon Borne Magnet Facility (BBMF) both at GSFC and at LSU. Approximately half-way through the project, Prof. Jones retired from active status at LSU, becoming Professor Emeritus. With concurrence of NASA Headquarters, Prof. John P. Wefel replaced Prof. Jones as Principal Investigator to finish the work, particularly the analysis of the BBMF data with graduate student E. S. Seo. Also, due to hardware difficulties, the planned balloon flight schedule of the BBMF had to be altered. This shifted the focus of the data analysis away from e+e - in favor of p,t> and p, He spectra. This also caused a delay in the project plan which necessitated a one-year's extension to the term of the grant. Under grant NAGW-1247, we have made significant technical progress on a number of different position sensitive detector systems. Although no one system has been able to be completely developed for flight, the basic ground work has been established for the future development of such systems. In addition, a major advance has been made in the study of the rigidity spectra of the two most abundant cosmic ray species, Hydrogen and Helium, including analysis of re-acceleration effects in the Interstellar Medium. The next section gives the Bibliography to work support wholly or in part by this grant. This is followed by a Summary of the Research Accomplished in the two principal areas covered by the project. The attachments include articles for which preprints or reprints are currently available. 1I. Bibliography A. Journal Publications/Book Chapters: "Measurement of Cosmic Ray Proton and Helium Spectra During the 1987 Solar Minimum," E. S. Seo, J. F. Ormes, R. E. Streitmatter, S. J. Stochaj, W. V. Jones, S. A. Stephens and T. Bowen, Astrophysical Journal, Sept. 91 issue, in press (1991). "The Composition of the Cosmic Rays - An Update," John P. Wefel, in Cosmic Rays, Supernovae and the Interstellar Medium, eds. M. M. Shapiro, R. Silberberg and J. P. Wefel, NATO ASI Series C, Vol. 337, (Dordrecht, The Netherlands; 1991; Kluwer Academic Publishers), p. 29. "Central Collisions of 800 GeV Protons with Ag and Br Nuclei," Abduzhamilov, A., et al., (The Baton Rouge-Krakow-Moscow-Tashkent Collaboration), Physical Review, D39, 86, (1989). B. Articles in Preparation/Submitted: "Performance of a Balloon-Borne Magnet Spectrometer for Cosmic Ray Studies," R. L. Golden, C. Grimani, B. L. Kimbell, W. R. Webber, G. Basini, A. MorseUi, M. Ricci, J. F. Ormes, E. S. Seo, S. J. Stochaj, R. E. Streitmatter, P. Spillantini, A. Codino, M. Menichelli, M. P. DePascale, F. Bongiorno, and P. Picozza, Nucl. Instr. Meth. (1991). "Performance of focused Gas Cherenkov Detector Used for Cosmic Ray Studies," K L. Golden, C. Grimani, B. L. Kimbell, W. tL Webber, G. Basini, A. MorseUi, M. Ricci, J. F. Ormes, E. S. Seo, S. J. Stochaj, 1L E. Streitmatter, P. Spillantini, A. Codino, M. Menichelli, M. P. DePascale, F. Bongiorno, and P. Picozza, Nucl. Instr. Meth. (1991). C. Books: Cosmic Gamma Rays, Neutrinos and Related Astrophysics, eds. M. M. Shapiro and J. P. Wefel, NATO ASI Series C, Vol. 270, (Dordrecht, The Netherlands; 1989; Kluwer Academic Publishers), 692 p. D. Conference Proceedings: "Cosmic Ray Proton Spectra at Low Rigidities," E. S. Seo, J. F. Ormes, R. E. Streitmatter, J. Lloyd-Evans and W. V. fones, 21st Int. Cosmic Ray Conf. Papers (Adelaide), Vol. 3, p. 7, (1990). "Experimental Limit on Low Energy Antiprotons in the Cosmic Radiation," tL E. Streitmatter, S. J. Stochaj, J. F. Ormes, R. L. Golden, S. A. Stephens, T. Bowen, A. Moats, J. Lloyd-Evans, L. Barbier, E. S. Seo, 21st Int. GQ_mic Ray Conf. Papers (Adelaide), VoL 3, p. 277, (1990). "Positron Electron Magnet Spectrometer (POEMS) for the EOS Mission," P. Evenson, J. P. Wefel, S. Swordy, IL Streitmatter, M. Salamon, L. Barbier, T. G. Guzik, K. P. Magee-Sauer, J. W. Mitchell, J. F. Ormes, R. Ramaty and D. V. Reames, in Particle Astrophysics, eds. W. V. Jones, F. J. Kerr and J. F. Ormes, AIP Conference Proc. No. 203, (New York, 1990, American Institute of Physics), p. 58. E. Dissertation: "Measurement of Galactic Cosmic Ray Proton and Helium Spectra During the 1987 Solar Minimum" by Eun-Suk Seo, Louisiana State University (1991). Available from University Microfilms, Inc. F. Abstracts: "Micron Resolution Charged Particle Detectors," L. M. Barbier et al., Bull. Am. Phys. Soc., 33, (1988). "Cosmic Ray Proton Spectra," E. S. Seo, J. F. Ormes, R. E. Streitmatter, S. J. Stochaj, R. L. Golden, T. Bowen, A. Moats and J. Lloyd-Evans, Bull. Am. Phys. Soc., _ 1185 (1989). "Cosmic Ray Protons and Helium Spectra, Splash Albedo from Solar Minimum, 1989," E. S. Seo, J. F. Ormes, R. E. Streitmatter, S. J. Stochaj, R. L. Golden, S. A. Stephens, T. Bowen, A. Moats and J. Lloyd-Evans, Bull. Am. Phys. SOC., _ 1065 (1990). '_'he Deuterium to He 4 Ratio Obtained with a Balloon Borne Superconducting Magnetic Spectrometer," S. J. Stochaj, E. S. Seo, R. E. Streitmatter, J. F. Ormes, R. L. Golden, S. A. Stephens, T. Bowen, A. Moats and I. Lloyd-Evans, Bull. Am. Phys. Soc., _ 1065 (1990). '_pper Limits on 100-1200 MeV Anti-Protons in the Cosmic Rays," R. E. Streitmatter, S. J. Stochaj, L. Barbier, J. F. Ormes, E. S. Seo, R. L. Golden, S. A. Stephens, T. Bowen, A. Moats and J. Lloyd-Evans, Bull. Am. Phys. Soc., 35, 1066 (1990). Ill. Summary of the Research Observations over the past several years have produced significant and unexpected results on the fluxes of cosmic ray particles arriving at earth, while parallel theoretical developments have provided a new framework for understanding the particles' origin, acceleration, and propagation through the interstellar medium. Concurrently, profound astrophysical questions have been raised by the lack of definitive observations of either proton decay or free magnetic monopoles, both of which are expected within the framework of the Grand Unified Theories. Recognizing the importance of these rapid changes, the Astrophysics Division in 1985 chartered a team of scientists to define the goals and requirements for a superconducting magnet facility (now called Astromag) to be operated for several years on the Space Station or as a free flyer. This step was followed by a restructuring of the Particle Astrophysics Supporting Research and Technology Grants Program, in order to focus a large fraction of the community's research efforts toward the goals of Astromag. By conducting balloon flights at high geomagnetic latitudes (low geomagnetic cutoff) during solar minimum, some Astromag-type objectives are being realized, e.g. studies of antiprotons, positrons, and light-to-medium isotopes, albeit at energies substantially below those available with Astromag. The basis for several of the new investigations is the New Mexico State University (NMSU) baUoon-borne magnetic spectrometer which was flown during the summer of 1987 in an effort to measure the flux and energy spectra of antiprotons over the interval between about 200 MeV and 1 GeV. The LEAP experiment also obtained very high precision data on the rigidity spectrum of protons and helium. The next logical step following the LEAP experiment is a measurement of the positron flux. This experiment was conducted by a collaboration involving NMSU, the National Institute for Nuclear Physics, Frascatti, Italy (INFN), and the NASA Goddard Space Flight Center (GSFC), with Louisiana State University (LSU) participating through GSFC. The balloon flight apparatus, launched from Prince Albert, Canada, employed a new, tracking shower counter developed by the INFIX/ group, as well as the time-of-flight (TOF) counter developed by the GSFC group for the LEAP instrument. The flight of this experiment (MASS) took place in summer, 1989. Our effort under this grant was devoted to analyzing the LEAP spectral data and to developing the hardware for the MASS experiment. On-another-front, progress in the field of matter/anti-matter and isotope measurements in the cosmic rays depends upon improving the precision of the measurements of trajectories of particles through inhomogeneous magnetic fields. A possible solution here is the development of high resolution, solid state track detectors, that (i) can be employed in a strong magnetic field and (ii) can potentially be scaled up to the areas (~1 m2) required for Astromag. A position resolution of 50 - 100 microns is currently possible, but this resolution (e.g. with multi-wire proportional counters and scintillating optical fibers) will be difficult to improve to anything better than 25-30microns; this accomplishment would itself be a formidable task! Therefore, totally new types of detectors should be pursued, and the development of solid state (silicon) detectors is one possibility. Strong interest in developing solid state track detectors followed the Astromag Study Team's identification of improved track detectors as one of the two highest priority technical needs for Astromag (the other is an efficient, innovative design for the magnet proper). The realization of high quality position resolution over large areas for periods of more than a year will require detectors with excellent intrinsic spatial resolution and, of course, an adequate means to control/calibrate systematic effects that could degrade the measurements. For example, thermal control of the Astromag trajectory systems may be required to maintain dimensional stability, and accurate mapping of their responses will be needed for relating measured signals to absolute positions. Thus, this project involved two areas of research and development, established as a collaborative venture between LSU and GSFC, with LSU personnel spending extended periods at GSFC. The first area, position sensitive detector development, utilized the detector development laboratories and fadlities at GSFC, to investigate several types of detectors. The second area involved analysis of data from balloon experiments and has resulted in a Ph.D. being awarded to graduate student E. S. Seo. A. Detector Development Studies Several concepts for position sensitive detectors have been investigated including silicon drift chambers, scintillating optical fibers, silicon strip detectors and electron trapping materials. Results from each of these investigations are summarized below. In a Silicon Drift Chamber (SDC) device, the drift time of the liberated charge is directly proportional to the point of impact of the charged particle. Such a device is shown schematically in Figure 1. The SDC is a 300 micron, 1 cm 2, fully depleted device, p+ contacts on both surfaces provide a parabolic potential through which electrons are conducted by application of an additional voltage at a surface anode. Figure 2 shows an illustration of the potential configuration in the device. To evaluate the position sensing ability of an SDC, a device was borrowed from Brookhaven National Laboratory. Beta particles were used to scan the surface of the detector using the arrangement shown in Figure 3. A histogram of arrival times was made for each position of the source. The source could be positioned repeatably to 20 microns while the collimator opening was 100 microns. We found that we could detect 50 micron shifts in the source position. Presumably, with a better collimator, this could have been improved. Figure 4 shows that the drift time across the device (the plot actually covers 2.5 mm of travel) was extremely linear, and gave a drift velocity of approximately 6 microns per nanosecond. DRIFT FIELD ELECTRODES ANOOE DRIFT DISTANCE j Figure 1. Schematic diagram of a typical silicon drift chamber. The wafer is about 0.3 mm thick and has a front area of a few cm 2. The surface is covered by a strip array of p+n junctions which provides the depletion and the lateral drift field. (Only junctions at the extremes of the wafer are shown.) Electrons produced by the passage of a fast particle drift towards the anode, which is the only readout contact on the wafer. i:1 __5 :k._ ' ,do.o do.o _o.o ,oo.o X (_. ,oo.o ICro_8) T f.ICRON5_ Figure 2: The negative potential of a fully depleted semiconductor detector when an additional linear lateral field along the y-axis is superposed. The field stabilizes the full depletion by sweeping away all generated charges. A I ,- l l ! ' i ! i,i /i ' ,rcn¢ i ! ! i- L .. I. I [ [ i- ; I i - I t ; I b i I Figure 3: Schematic diagram of the testing configuration for the SDC. Radiation from a Sr90 [3source, collimated by the Pb shield (Sh), passes through the Si drift chamber (SDC) and reaches scintillator (Sc) where it creates a light pulse that is detected by the photomultiplier tube (PMT). The signal from the anode of the SDC is amplified, shaped and delayed (A) before passing to the time-to-pulse-height convertor (TPHC) where it supplies the stop signal. The start signal comes from the PMT. Pulse heights (proportional to drift time) are analyzed in a multichannel analyzer (MCA), and the data is also sent to a computer for further analysis. 7O 60 Time (ns) vs Channels 5O 40 "8 c 3O o t. I o =4 10 0 -10 ; ! | I I ! 0 IO0 2OO 3OO •_, (_,) Figure 4: Results from a calibration of the SDC device. Several attempts were made to fabricate SDC's in-house in the GSFC microelectronics lab. However this proved unsuccessful. The first batch of detectors all cracked during the processing. The devices made next all showed an unacceptably high noise level. This noise was caused by having exposed implanted regions on the surface and an improper annealing. A more critical problem, however, was the discovery that there is a strong temperature dependence to the electron drift time in this device. The high level of temperature control that would be necessary to maintain the position resolution would be extremely difficult in space, making the SDC's impractical in most applications. Other solid state position sensitive devices can provide equal resolution without this drawback. A second effort was initiated, in conjunction with the Washington University (St. Louis) group, to study the use of Scintillating Optical Fibers in the space environment. WU produces these fibers (down to 100 microns in diameter) and has demonstrated that they can be used to measure heavy charged particles. The open question for a space experiment is 'WV'hatis their stability in a changing thermal environment?" To answer this question, a series of measurements of the physical properties of the fibers (force to failure, tensile strength and Young's modulus) were undertaken, both for fiber strands and for fiber ribbons composed of many fibers mounted adjacent to one another on a substrate. Results of the mechanical tests are given in Tables 1 and 2. Table I Table of Force to Failure, Tensile Strength, and Young's Modulus for Fiber Strand Force to Tensile Young's Failure Strength Modulus Sample flbf) (psi) (psi) x 105 1 0.88 8,800 4.0 2 0.76 7,600 3.8 3 0.82 8,200 3.7 4 0.84 8,400 3.7 5 0.84 8.400 3.8 6 0.80 8,000 3.7 7 0.80 8,000 3.8 8 0.84 8,400 4.0 9 0.84 8,400 4.3 10 0.86 8,600 3.7 Average 0.83 ± .03 8,300 ± 300 3.9 ± 0.2 Table 2 Table of Force to Failure, Tensile Strength, and Young's Modulus for Fiber Ribbon Force to Tensile Young's Failure Strength Modulus Sample flbf) (psi) (psi) x 105 1 9.6 8,000 6.1 2 9.0 7,500 5.8 3 14.2 11,800 5.6 4 14.6 12,200 5.6 5 14.6 12,200 5.6 6 10.0 8,300 5.7 7 9.4 7,800 5.6 8 10.8 9,000 5.2 9 13.0 10,800 5.0 10 12.0 10,000 4.6 11 14.4 12,000 5.8 12 14.2 11,800 5.4 13 13.6 11,300 5.2 14 13.0 10,800 5.1 15 19.6 11_300 5.2 Average 12.4 ± 2.1 10,300 ± 1,700 5.4±0.4 Next, a mounting/testing apparatus for studying the change in the fibers as a function of temperature, was designed and constructed. The testing apparatus consisted of the following: (a) a rigid aluminum frame for mounting the fibers; (b) a thermally tight box in which we can control the temperature of the fiber array to +/- 1 degree. We then devised a method of testing the fibers by stimulating them with an ultraviolet lamp, focused down to a 50 micron spot size, through a fiber optic cable. The fiber array response was monitored for sensitivity and uniformity as a function of temperature by a CCD camera interfaced to a personal computer. Various temperature ranges typical of those encountered in space experiments were investigated. While thermal control will be necessary, the requirements on thermal design to maintain the stability and resolution of a fiber system are not stringent. Thus, fibers become an attractive position sensing option for certain types of experiments. (Such fibers have become a part of the LISA experiment which was selected for flight as one of the Astromag facility experiments.)