The Lund Iridium Triple Coincidence Spectrometer is an instrument optimised built at the Nuclear Physics Division in Lund for measuring small quantities of iridium.
The principle behind analysis is the following:
- The sample is sent to a nuclear reactor with a high neutron flux.
- Some of the Ir nuclei in the sample absorbs a neutron and become radioactive. This probability (cross-section) is well known.
- The half-life of iridium is 73.8 days which makes it ideal to measure in this kind of two-step analysis.
- The sample is returned and positioned in the spectrometer.
- In the spectrometer a “fingerprint” of three gamma rays, from the decaying iridium nucleus is measured using 14 LaBr3 crystals arranged in a close to 4pi geometry.
- The “fingerprint is based both on energy and time information of the three gamma rays.
- The number of “Ir-fingerprint” detections per measuring time, together with the information about the sample mass and the neutron irradiation time in the reactor is used to quantify the iridium concentration in the sample.
- The detection limit for the spectrometer is below 20 pg/g in geological samples, which is roughly the same mass ratio as a penny and an aircraft carrier.
- The system is fully automatic and is capable of measuring 24 hour per day 7 days a week.
The instrument has been built by Linus Ros and Per Kristiansson with contributions by Mikael Elfman and Bengt Martinsson, all at the Nuclear Physics Division at Lund University. We are grateful to Bernhard Peucker-Ehrenbrink at the Woods Hole Oceanographic Institution for his support.
Figure showing the Iridium anomaly and pancake spherules (Monte Vaccaro Section, Italy) related to the Popigai impact. First Iridium result produced with The Lund Iridium Triple Coincidence Spectrometer. Figure from Doctoral thesis by Samuele Boschi.