Gamma-ray Spectrometers

Atomic nuclei can emit light called gamma rays when they are excited. Gamma rays have a much higher energy than can be seen with our eyes. Special equipment called a gamma-ray spectrometer allows study of these rays and peering into the internal structure of the nucleus. Studying gamma rays reveals what makes each nucleus special. For example, some nuclei are spherical like a basketball, while others are oblong like a football.

Expanded Description

The light emitted by de-exciting atomic nuclei is called gamma radiation. Gamma rays are similar to X-rays in that they cannot be seen with our eyes.

The NSCL has several detectors designed to "see" gamma rays. These detectors have names based on the material from which they are made. One is called Sodium Iodide, usually abbreviated to its chemical symbols: NaI. The NaI detector converts the gamma rays into visible light. This visible light is then converted to an electronic signal, which is then digitized and recorded on a computer. This detector is effective at "seeing" gamma rays, but is has a poor resolution. In other words, it has somewhat blurry pictures.

To get significantly better resolution, another type of detector has been developed. It is called a germanium detector, also sometimes abbreviated by its chemical symbol: Ge. Germanium is an element that is chemically similar to silicon—the material used in computer chips. The germanium material in the detector acts as an insulator. In other words, no current flows through the germanium when a voltage is applied. When a gamma ray hits the germanium detector, a small current is made. This current can be detected with sensitive electronics equipment and, similar to the NaI signals, digitized and stored on a computer. By using these detectors separately or in combination, we can identify the gamma rays from a particular de-excitation of an atomic nucleus. Each type of nucleus has a unique signature of gamma rays.

The Importance of Gamma-ray Spectrometers

Nuclei are the fundamental building blocks of matter. Understanding the structure of atomic nuclei leads to understanding the nature of the world in more detail than was ever thought possible. Study of high-energy photons is an effective means to understand the internal structure of these nuclei because it is a relatively gentle process compared to other methods. From these studies comes not only a deeper understanding of the world, but also direct benefits. For example, many modern medical scanners, such as MRI or PET devices, are the direct result of nuclear structure studies.

While the ultimate goal of studying the structure of atomic nuclei is to understand the underlying nature of the world and derive a benefit from it, scientists must first ask the "gee-whiz" questions like "Why is magnesium-32 deformed?" By first asking the simple, but seemingly uninteresting questions and then answering them, a foundation of knowledge is built that is critical to advancing to more difficult questions.

There are many ways to study nuclei to try to answer fundamental questions. The NSCL has a technique to make nuclei that no longer exist on earth; however, the nuclei also must be studied. One of the most popular methods to study these rare and short-lived species is to gently excite them by pinging them with another nucleus. After the nucleus is excited, it will often de-excite by emitting energy in the form of high-energy light—gamma rays. With arrays of gamma-ray detectors, this light can be collected to learn about the properties of that rare nucleus. One of the most challenging problems is that the nuclei typically are traveling very fast—about half the speed of light. Similar to the different pitch a train horn sounds when traveling towards or away from you, the energy of a gamma ray will change depending on the perspective. The detectors at the NSCL are unique for detecting gamma rays from very rare nuclei that travel at very high velocities.

Technical Information on SeGA

The segmented germanium array (SeGA) allows “high-resolution” in-beam γ-ray spectroscopy of intermediate-energy beams from the Coupled Cyclotrons. Each of the eighteen detectors in the array is a single-crystal 75% relative-efficiency germanium counter with the outer surface electronically divided into 32 segments. By using the segment information, the interaction of the γ-ray can be localized within the detector, therefore reducing the uncertainty in the Doppler correction due to the finite opening angle of the detector. A detector frame is available and allows the detectors to be placed at several distances, so the experimentalist can decide on the compromise between efficiency and resolution for their particular needs. The standard configuration is 18 detectors at 20 cm, which gives an approximate 3% photo peak efficiency at 1.3 MeV with about 2% in-beam energy resolution. The detectors are also available for stopped beam experiments such as β-delayed γ-ray decay studies.

Status: Operational

Location: N2 vault, S2 vault, S3 vault

Contact person: Dirk Weisshaar

Funding acknowledgement: The National Science Foundation through Major Research Initiative grant PHY-9724299 supported the acquisition of the SeGA array.

Reference:

W. F. Mueller, J.A Church, T.Glasmacher, D. Gutknecht, G. Hackman, P.G Hansen, Z. Hu, K.L Miller, P. Quirin, Nucl. Instr. and Meth. A 466 (2001) 492.
doi: 10.1016/S0168-9002(01)00257-1

Technical Information on CAESAR

The scintillator array CAESAR (CAESium iodide ARray) is optimized for high gamma-ray detection efficiency. It consists of 192 CsI(Na) scintillation crystals of two geometries: 2"x 2"x 4" (144 pcs) and 3"x 3"x 3" (48 pcs). The intrinsic energy resolution of the detectors is better than 8% FWHM at 662 keV. The rectangular crystal shapes allow for a close-packed geometry around the target, yielding high solid angle coverage. A frame is currently being constructed for in-beam spectroscopy experiments in conjunction with the S800 spectrograph. The array will provide a full energy peak efficiency of 40% at 1 MeV. The intrinsic energy resolution of the detector units and the geometry of the array will result in an in-beam energy resolution of 10% (FWHM) at 1 MeV. The array was commissioned in May 2009.

Status: Operational

Location: S3 vault

Contact persons: Alexandra Gade and Dirk Weisshaar

Funding acknowledgement: The National Science Foundation through Major Research Initiative grant PHY-0722822.