Iridium Spectrometer

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.

Physics Nobel Prize laureate Luis Alvarez and Frank Asaro in front of the Luis-W.-Alvarez Iridium-Coincidence-Spectrometer at the Lawrence Berkeley National Laboratory. This is the first iridium coincidence spectrometer built based on an idea of Luis Alvarez (photo from 1987). In the photo Frank Asaro hands over the one dollar he lost when betting that the instrument would never work.
Nuclear Physics professor Per Kristiansson and Frank Asaro in front of the Luis-W.-Alvarez Iridium Coincidence Spectrometer at the Lawrence Berkeley National Laboratory (photo from 2011). The Berkeley spectrometer served as an inspiration to the construction of the Lund Triple Coincidence Spectrometer.
Schematic overview of the Ir Triple Coincidence Spectrometer. The sample (yellow) is located in the middle, surrounded by the 14 LaBr3 detectors (red and grey). Outside the LaBr3 detectors six plastic scintillators (grey) are positioned. These are used as a suppression of Compton scattered gamma rays.
The Lund Ir Triple Coincidence Spectrometer. The spectrometer is connected to an automatic sample exchange system, allowing uninterrupted analyses without supervision of a large set of samples.


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.