NASA/Marshall Astronomy: MIXE2 instrument
A Pinhole Camera for X-ray Astronomy
In any movie involving radiation, someone picks up a Geiger counter and waves it around while the electronics click away in response to how radioactive the area is. Astronomers have used a similar technique to measure not just radiation in space, but to make pictures of stars emitting X-rays.
Now, scientists from Marshall Space Flight Center are working on a sophisticated version - the Marshall Imaging X-ray Experiment (MIXE2) - that is to take its first test flight aboard a balloon now scheduled for launch from Fort Sumner, N.M.
|MIXE2 is mounted in the Harvard College Observatory carrier (which includes a pointer) and then covered with thermal insulation.|
"Marshall Space Flight Center is leading the scientific community in the development of these microstrip detectors for astronomy," said Dr. Thomas Parnell, chief of the astrophysics branch at Marshall's Space Sciences Laboratory. "MIXE2's most significant advantages are its high spatial resolution and low background," meaning it can see finer details in weaker objects. It also can tell the difference between X-rays from the target and stray radiation that might accidentally set off the circuits.
MIXE2 works in a section of the electromagnetic spectrum - 25,000 to 100,000 electron volts (25 to 100 keV) - which is above the range that can be focused by telescopes like the Marshall-managed Advanced X-ray Astrophysics Facility, explained Dr. Brian Ramsey, the principal investigator at Marshall. That requires the use of sophisticated pinhole cameras with special detectors behind them.
"These detectors have three distinctive characteristics," Parnell said, "high spatial resolution, high energy resolution, and high background rejection." It's somewhat like the difference between color TV now and in the 1960s: the pictures are sharper, the colors better defined, and there's less "snow" or noise.
Marshall scientists did this by sort of flattening a Geiger counter tube onto a glass plate about 30 cm (12 inches) on a side to provide finer image resolution, and putting the whole thing inside a pinhole camera to complete the "telescope."
A Geiger counter clicks when radiation zips through a gas and ionizes it. That completes an electric current that runs from the wall of the Geiger tube to a wire down the middle. It's simple, effective, and not terribly precise. It detects just about anything that passes through and has no sense of direction or energy.
In the 1960s, astronomers expanded the technique with multiwire proportional counters. In place of the tube and one wire they set up two arrays of wires, one positive (anode) and one negative (cathode) in a grid at the bottom of a container of gas that would interact with X rays or gamma rays.
In front of this box was a collimator, metal tubes blocking the field of view (you get the same effect by looking through a cluster of straws). An X-ray passing down the tube ionizes the gas, and the circuit is completed to the wires. The electronics detect where on the X-Y grid it hit, and one more spot is painted on the growing portrait of a star or galaxy.
|After the complete system is integrated inside the NASA hangar, it is attached to the launch truck and connect to the balloon and parachute risers.|
This kind of telescope helped open the field of X-ray astronomy, but it's coarse because the wires have to stand off from each other in order to work. That limits how detailed a view of the sky, or of the X-ray energies ("colors," if you will) can be detected.
MIXE2 provides better resolution by collapsing the wire array into an ultrathin set of chromium strips (microstrips) coated on a plate of glass. The strips form a pattern somewhat like placing your hands flat with the fingers laid between each other. The cathodes on one hand are 676 microns wide, and the anodes on the other hand are 10 microns wide; another 657 microns of open glass lies between the two so they don't make electrical contact. The fingers are 300 mm (almost 12 inches) long. Another set of electrodes runs in the opposite direction beneath the glass to fix the exact position of each hit.
MIXE2 sits inside a box filled with xenon gas which not only ionizes when X-rays hit it, it fluoresces. The effect is much like minerals glowing in a distinctive color under a fluorescent lamp. This means that every X-ray arriving from the target star will be detected twice: once when it ionizes the gas, a second time when the gas emits an X-ray with a unique energy.
Computer programs on the ground sift out any hit that occurs only once - it probably was a cosmic ray or even a gamma ray penetrating the box. Only the double events are counted.
The microstrip technology, made possible by recent advances in photolithography (used to make electronic circuits), lets MIXE2 paint a more detailed picture of the stars than previous balloon-borne X-ray telescopes.
The objective end of this telescope is a tungsten plate, 50 x 52 cm (20 x 21 in.) in size and 2 meters (6.6 feet.) in front of MIXE2. Tungsten is a dense metal that will absorb all of the X-rays - except for those passing through an array of holes in a special pseudo-random pattern, arranged so that false patterns don't show up in the images. The base pseudo-random pattern has 31 x 33 holes and is repeated in a 3 x 3 array across the plate.
The holes are 4 mm (1/12th inch) squares, pretty big for a "pinhole," but just right to admit enough X-rays to make a picture during a short balloon flight.
MIXE2 has a 3.6 degree. field of view and a resolution of 6.8 arc minutes. That means it sees a region of sky about 7.2 times the apparent diameter of the moon, and sees details about 1/4th the apparent diameter of the moon. That's much coarser than optical telescopes can see, but is more than adequate to clearly separate individual cosmic sources in its energy band.
MIXE2 is flying alongside, and co-aligned with, a Harvard College Observatory high-energy X-ray scintillator that can detect X-rays all the way to the 511 keV gamma-ray line emitted by electrons and positrons (anti-electrons) annihilating each other. Although the Harvard instrument (EXITE2) sees more of the spectrum, its angular and energy resolution are not as good as MIXE2, so the two complement each other.
And what will they be observing on this expedition?
"The target list changes almost daily," explained Dr. Brian
Ramsey, the principal investigator at Marshall. "We have to look at
bright objects. In a 1-day balloon flight, with just a few hours on each
source, they have to be bright galactic sources or bright galaxies."
Candidate Galactic Targets
|Crab nebula & pulsar||Supernova remnant and X-ray pulsar|
|Cygnus X-1||Most probable black hole candidate|
|Cygnus X-3||Binary system with suspected Wolf-Rayet companion|
|Hercules X-1||Accretion powered neutron star with strong magnetic field|
|GRS1915+105||Black hole candidate with superluminal radio jets|
|Scorpius X-1||Binary system with weak field non-pulsed neutron star|
|GS1843+00||Transient X-ray pulsar|
|EXO1846-031||Black hole candidate|
Candidate extragalactic targets
If you really want to dig into the subject, check:
"A large-area microstrip-gas-counter for X-ray astronomy."
B.D. Ramsey, et al. Nuclear Instruments and Methods in Physics Research
A383 (1996) 424-430.
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