- 01 Overview
- 02 How It Works
Used in conjunction with the TIGRESS gamma ray spectrometer, TIP (the TIGRESS Integrated Plunger) is a tool for precisely measuring the picosecond, trillionths-of-a-second, lifetimes of nuclear excited states. The TIP project was led by Simon Fraser University. TIP was developed in a close collaboration between SFU and TRIUMF, with contributions from St. Mary’s University.
02 How It Works
To measure the picosecond lifetime of a nuclear excited state, TIP uses the Recoil Distance Measurement technique. This process relies on the Doppler shift phenomenon, the fact that an object’s speed and direction of movement in relation to an observer changes the frequency of the sound or light from the object detected by the observer (see TIGRESS).
Mounted inside TIGRESS, TIP’s core is a target chamber, called a plunger device (hence the name, TIGRESS Integrated Plunger). Inside the plunger are two closely positioned parallel foils. The first foil is a reaction foil. Radioactive rare isotope beams of interest are directed onto this first. This induces one of a variety of nuclear reactions (Coulomb, transfer, or fusion) producing a shower of scattered nuclei in an excited state that exit the backside of the target foil. Foils are typically around 2 micrometers thick; they need to be thin enough for the reaction products to escape.
The second foil, two-to-20 micrometers thick is a degrader foil. It acts as a brake, reducing the velocity of the excited nuclei which pass through it(if the degrader foil is thick enough, the excited nucleus will simply come fully to rest).
In these experiments we know ahead of time what the energy of the gamma ray should be if it is emitted from a nucleus at rest. Based on the reaction mechanism, we can calculate the expected speed of the excited nucleus, and from that, how much the energy of the gamma ray will be Doppler-shifted. Any gamma rays observed at this “full shift” energy were emitted before the nucleus reached the degrader foil. Gamma rays emitted with a lower (or no) shift were emitted after the nucleus passed through (or stopped in) the degrader foil.
TIP scientists position the degrader foil at about the distance the excited nucleus will cover over its lifetime, from just ten micrometers, to several millimeters. Measurements are made for a series of different target-to-degrader foil distances, thus zeroing in on the lifetime of an excited state. The challenge is controlling the separation of the foils; making sure that they are uniform thickness and flat (not wrinkled), making sure they are parallel, and keeping them at a constant, well-known distance for the duration of a measurement. The latter is complicated by the thermal expansion and contraction of TIP: over the course of a 24-hour period in the summer, the day-night temperature variation in the experimental hall has been observed to give a 3 micrometer change in separation. To compensate, the distance between the foils is automatically adjusted by highly precise servo motors guided by an electrical signal coming from the foils and related to their separation.
By combining the speed of the nuclei, the distance between the foils and the ratios between the different energy levels of the Doppler-shifted gamma rays detected by TIGRESS, TRIUMF scientists can measure the lifetimes of excited nuclear states as short as a picosecond.
For more information on TIP, please see here.