The Neutron Walls

Signal cables connecting to the neutron walls. more

The neutron is a subatomic particle which is similar to the proton, but doesn't have electric charge. NSCL developed a special pair of detectors to observe neutrons. These neutrons are produced in reactions between atomic nuclei and they generally travel with about a quarter of the speed of light. Each detector looks like a wall—thus their name, the Neutron Walls. Among their other useful features, the Neutron Walls are able to determine a neutron's energy and a direction of motion.

Expanded Description

When a neutron strikes them, the Neutron Walls produce a short electrical signal. By analyzing the signals, one can determine the properties of the neutrons.

In each wall there are 25 horizontal glass tubes attached to electronic units. The tubes are 79 inches long and 3 inches high, and an aluminum framework hangs them one above the other. They are filled with a special liquid which has a peculiar feature. When a neutron interacts with the liquid, it produces a small amount of visible light. The interaction is simply a collision of the incoming neutron with a proton in the liquid, as when billiard balls collide. This mechanism works very well for neutrons in the velocity range of 10–40 percent of the speed of light. There are small devices similar to photocells at both ends of the long glass tubes to register the very short light flashes from the collisions and convert them into electric signals.

The properties of the signals tell us what type of particle hit our device—a neutron or a gamma-ray. The Neutron Walls measure the time that elapsed since the neutron was produced in the experiment. Electric circuits used with the Neutron Walls can determine the time to 1 billionth of a second. The most important property of the detected neutrons is their energy which is deduced from this elapsed time. By measuring the difference in time between the two ends of the glass tube firing, experimenters can determine—with a resolution of 3 inches—how far to the left or right of center of the tube the neutron interacted.

The Importance of the Neutron Walls

Some atomic nuclei contain surprisingly many neutrons, sometimes two or three times more than protons! The Neutron Walls extend our knowledge of the structure of these exotic nuclei.

Their other important use is to solve essential questions about how the elements were created in the cosmos. We know that there are more than a thousand isotopes on Earth. Although our current knowledge has not revealed the mechanism of the creation of these isotopes, we do know that neutrons were essential ingredients. The aim of the research is the experimental determination of the probability of each nuclear reaction where a neutron was involved.

Using the Neutron Walls we can determine the size and the structure of exotic nuclei by breaking them up and registering the outgoing neutrons. Here's an example. The nucleus of a helium atom is one of the most stable nuclear systems, consisting of two protons and two neutrons. Its exotic brother is the loosely bound helium isotope containing eight particles—two protons and six neutrons! A gentle impulse by an electromagnetic field can destabilize this system and four neutrons will suddenly run away carrying information about their location within the helium isotope.

A number of open questions in the field of nuclear astrophysics can be answered with the help of the Neutron Wall detectors. Neutrons played an important role in creating the elements during the Big Bang, and later on, when stars were formed.

The basic process is a capture of a neutron by a nucleus, thus creating a new heavier isotope of the same element. Several chains of these and other steps are responsible for developing the elements. Some key steps in these chains cannot be exactly reproduced in the laboratory because the capturing material lives for only milliseconds. We can use, instead, radioactive nuclear beams to investigate the time-reversed process and deduce the same information.

In the reverse process, after an absorption of a gamma-ray, the neutron-rich nucleus drops off a neutron. In this case the Neutron Walls can detect the neutron. The sweeper magnet will deflect the remaining nucleus to another detector. In this way, we safely execute in the laboratory exactly the opposite reaction from what took place under extremely hot conditions in the cosmos.

Technical Information

The neutron walls are two large-area (2 m x 2 m), high-efficiency, position-sensitive neutron detectors. Each wall consists of a stack of 25 glass cells filled with the scintillator liquid NE213, with which one can distinguish neutron from gamma-ray pulses by pulse shape analysis. Each cell is two meters long and has phototubes at its ends. Light from an interaction in the liquid reaches the phototubes via total internal reflection. Each wall has its own carriage and can be positioned independently of the other.

Status: Operational

Location: Extended N4 vault

Contact person: Michael Thoennessen

Reference:

A large-area, position-sensitive neutron detector with neutron/gamma-ray discrimination capabilities; P.D. Zecher, A. Galonsky, J.J. Kruse, S.J. Gaff, J. Ottarson, J. Wang, F. Deak, A. Horvath, A. Kiss, Z. Seres, K. Ieki, Y. Iwata, H. Schelin, Nucl. Instrum. and Meth. A 401 (1997) 329.