Friday, May 16, 2008

When acronyms go bad

For anyone in my line of work (I realize, of course, that this does not constitute a large fraction of the population), particle detection is of supreme importance. We need to know what reaction products we've made and where they went and with what energy. Specific to our lab, we run many experiments with beams of very heavy particles (like tin) striking targets of much lighter particles (like hydrogen). Typically, beams of light particles will be sent into much heavier targets, and that's what's called normal or forward kinematics. What we do, then, is referred to as inverse kinematics. It solves quite a few of the problems inherent with running in normal kinematics (like how to prevent a heavy but radioactive target from decaying away before the experiment is over), but it creates some of its own. Particle detection becomes somewhat more difficult, for one. In the case of (d,p) reactions like we often do here - taking a heavy beam, striking a target of deuterons (that's a hydrogen nucleus with an extra neutron), and detecting the outgoing proton (that's just a hydrogen nucleus) - we need to cover as much space around the reaction as possible with detectors, and they need to have very specific properties to determine important experimental conclusions. Hence, ORRUBA was born.

ORRUBA (the Oak Ridge Rutgers University Barrel Array) is, as the name suggests, in fact, not a resort town in Mexico, but instead an array of particle detectors, in the shape of a barrel around the reaction of interest. We set them up inside a vacuum chamber. We use them to detect the protons from these (d,p) reactions. The detectors look similar to the one shown below.

On a more technical note, the detectors, fabricated by Micron, are silicon wafers with either four position-sensitive resistive strips, eight non-resistive strips, or one large active area (for residual energy detection in a dE-E telescope), in thicknesses of 65, 500 or 1000 micrometers. The design is such that a position resolution of fractions of millimeters is not out of line, and an energy resolution on par with any silicon strip detector, ~50 keV, is to be expected. An early implementation was used here recently in several radioactive ion beam experiments. Another important aspect of the array is the number of electronics channels, which is small enough to be instrumented using conventional electronics. The array itself is designed as two rings, one forward of 90 degrees in the lab, and one backward; a target can easily be manipulated between the two rings. Together, the barrel covers 80% of the total solid angle.

Though the topic of particle detection in nuclear astrophysics may be near and dear to my heart, I think that certain things can be appreciated by nearly everyone. Like the way ORRUBA looks once every detector is in place. It's very sci-fi. It's very cool. Like GAMMASPHERE in The Incredible Hulk. Science isn't untouchable. There's certainly a little art in it. And yes, the first author is actually Dr. Pain. Who wouldn't appreciate that?

PAIN, S., CIZEWSKI, J., HATARIK, R., JONES, K., THOMAS, J., BARDAYAN, D., BLACKMON, J., NESARAJA, C., SMITH, M., KOZUB, R. (2007). Development of a high solid-angle silicon detector array for measurement of transfer reactions in inverse kinematics. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 261(1-2), 1122-1125. DOI: 10.1016/j.nimb.2007.04.289

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