INTRODUCTION

We report here the progress of the NASA funded Cosmic Ray Electron Synchrotron Telescope (CREST) project. CREST, a balloon-borne instrument, is designed to measure the flux of primary cosmic ray electrons with multi-TeV energies by detecting their synchrotron photons which are emitted as they pass through the earth’s magnetic field. Analogous to ground-based air-shower arrays, the primary need not (and in general, won’t) pass through the detector, allowing for a much larger effective detection area than the actual detector array size. The project has two parts: CREST-1 is a smaller array of inorganic crystals for verifying the technique, particularly in regards to showing that the background photons — including atmospheric cosmic ray interactions — can be rejected. CREST-1 only needs a short duration flight in the continental US. CREST-2 is the full experiment with an array more than an order of magnitude larger in size, to be flown multiple times on Long Duration Balloons (LDBs) with circumpolar trajectories.

CREST Institutional Responsibilities

  • Indiana University: A) Program oversight and management; B) Payload Integration; C) Front-end electronics; D) Calibration system.
  • University of Chicago: A) Mechanical design and engineering; B) Trigger electronics; C) (joint with U. Michigan) photomultiplier & crystal assembly verification and testing.
  • University of Michigan: A) Barium Fluoride crystal implementation R&D; B) photomultiplier high voltage control design; C) photomultiplier design and selection; D) (joint with U. Chicago) photomultiplier & crystal assembly verification and testing.
  • University of Minnesota: A) onboard power system; B) flight computer hardware and software; C) ground station computer hardware and software.
  • Penn State University and Northern Kentucky University: Veto system design, procurement, testing, and implementation.

All institutions share the responsibilities of executing the balloon flight and analyzing data.

Subsystem Status

Crystals and Photomultipliers

An array of 96 inorganic crystal scintillators will be flown, each attached to its own photomultiplier (PM). Figure 1 shows the arrangement of the crystals in the array. 80 of the crystals are Bismuth Germanate (BGO), chosen for its high gamma ray detection efficiency. 16 Barium Fluoride (BaF2) crystals will also be a part of the array. Although less efficient at detecting gammas, BaF2 has excellent timing resolution as shown by our measurements in figure 2. The ability to reject background photons from their asynchronous arrival times could be a critical factor for CREST. The fast timing characteristics of BaF2 compared to BGO could result in its choice as the scintillator for CREST-2.

Four photomultiplier manufacturers made bids for CREST and two (Burle and Photonis) proved optimal in a cost vs. performance analysis. For diversification and future delivery reliability comparisons we’ve chosen both manufacturers as suppliers for CREST-1, each delivering half the required quantity.

We have done R&D studies on the use of waveshifter in conjuntion with the BaF2 crystals since the desired fast decay component of BaF2 has its peak emission around 220 nm, below the cutoff of standard PM windows. We continue to improve on this front and current results can be found in the attached documents. An alternate scheme uses PMs with fused silica windows — more costly than PMs with standard glass windows.

All BGO crystals have arrived at U. Michigan. BaF2 crystals are on order. Burle PM’s began arriving in mid-December and Photonis PM shipments first arrived in early January. Mating of crystals and PM’s and their characterization and testing have begun at both U. Michigan and U. Chicago. The completion date for all PM testing is projected for mid-March. The high voltage control electronics design is complete and parts are on order.

Front End Electronics

The amplification, analog to digital conversion, time to digital conversion, and pre-mainframe signal processing are being performed by a modification of electronics designed, built, and currently in use by the DoE/Fermilab Main Injector Neutrino Oscillation Search (MINOS) experiment. All components are already in hand at Indiana University and testing has begun, as can be seen in the photo of figure 3. A Xilinx based trigger processor designed by U. Chicago will be integrated into the front-end system. The parts for this trigger are on order.

Onboard and Ground station Computers

As is the case with the front-end electronics, we have taken advantage of knowledge gained in our involvement with other experiments (NASA funded CREAM and ANITA) and copied the successful computer systems of these projects. Space qualified Intel-based hardware has been combined with LINUX-based software to control the payload, record the data onboard, and transmit some of the data to the ground in real time (including two-way housekeeping monitoring and commanding). The onboard computer is currently at the U. of Minnesota being programmed and tested. The ground station is presently being configured.

Calibration system

We also use a modification of MINOS’s calibration system, consisting of a computer controlled pulse generator, light emitting diode (led), and fiber optic distribution network. Each front-end electronics board (servicing 16 PM’s) holds a pin-photodiode(PD) for reference. Each PM in the CREST array has its own optical fiber. One ultra bright Agilent blue led feeds all PMs and pin-PDs, simplifying the timing cross-calibration. In addition to timing calibration, this system provides pulse height (PM and front-end-electronics) cross-calibration. Figure 4 shows some of the components of the fiber calibration system.

Veto System

5 mm thick plastic scintillator will completely surround the top and sides of the crystal array. A fourth piece of scintillator is located below the array, but in order to allow cable runs and fiber access to the array, the bottom sheet of scintillator when combined with the other veto components does not completely enclose the array. Each piece of scintillator is viewed by two photomultipliers positioned at opposite ends. We will use the same PMs and electronics for the veto as are being used for the crystal array.

The veto counter was designed by us this past summer and Bicron / St. Gobain has been contracted for its manufacture. There have been some delays at Bicron and the current expected delivery date is late January. The projected installation date is mid-March, following assembly extensive bench testing at both Penn State University and Northern Kentucky University.

Onboard Power

Balloon flight standard lithium batteries will provide power, feeding an onboard conversion system already built at U. Minnesota and delivered to Indiana U. This power system (without batteries) is shown in figure 5. All distribution will be at low voltages. Each PM has its own low power high voltage system, designed and installed by the PM providers per our specifications.

Gondola Design and Engineering

The gondola design and engineering is better than 80% complete. Figure 6 is a photo of the assembled frame. The honeycomb support platform for the array is currently being machined and will be ready in mid-January.

Payload Integration and System Test

We expect to begin assembly of the crystal array in late January as component testing of crystal/PM assemblies progresses. In parallel the power, electronics, and calibration systems will be installed in the gondola frame. With the arrival of the veto system in mid-March, the assembly will be completed and system testing will be performed during early April. In late April the gondola will be shipped to Palestine for environmental testing in the NSBF vacuum chamber. Early May has been set aside for the environmental testing, leaving most of the remainder of the month for contingency in fixing any problems found.

Monte Carlo Simulation

A GEANT-based Monte Carlo has been written to aid in understanding the source of backgrounds. In combination with the results of CREST-1, possible design enhancements for CREST-2 are anticipated.

Balloon Flight and Data Analysis

The payload will remain in Palestine, awaiting the summer balloon campaign there. We anticipate a 6-10 hour flight peaking above 130,000 foot altitude sometime in June. The remainder of the summer allows time for data analysis and status report writing for the continuation of the project (CREST-2).

Figure 1: CAD drawing showing the 96 crystals and the side veto counters.

Figure 2: One standard deviation timing resolution spectra for BGO (upper curve) and BaF2 (lower curve) as a function of -ray energy.

Figure 3: Front end electronics ADC card and readout Control board.

Figure 4: Calibration system components, including ultrabright blue led, sample clad fiber, and distribution fiber holder.

Figure 5: Housekeeping module and low voltage power conversion and distribution system.

Figure 6: Assembled gondola frame with meter stick providing scale.