LLNL EBIT Atomic lifetimes

Atomic physics - how long does an atom "live"?

Our investigations of ions with long-lived excited states encompass the

Production of specific ion species inside the electron beam ion trap

Spectroscopy (visible light, EUV, soft-x-ray) with links to astrophysics and plasma diagnostics

Identification of long-lived ion species that are of particular interest in plasma diagnostic applications

Accurate determination of transition rates, testing atomic structure calculations as well as the conditions inside the electron beam ion trap

The time-resolved x-ray measurements use an IGLET-type solid state detector. Lifetimes in the range from microseconds to milliseconds have been studied using this option.

Time resolved measurements in the visible employ the "stovepipe" device and a low-noise photomultiplier. This detection system has been used for lifetime measurements in the millisecond range.

Time resolved observations in the EUV / soft-x-ray range and x-ray ranges are feasible with the XRS microcalorimeter, for timing features with millisecond lifetimes and above.

The "stovepipe" instrument for optical observations

Schematics View of the stovepipe arrangement

When the electron beam in the EBIT is switched on, ions of a given charge state are produced and then radiate (increasing part of the excitation curve). When the electron beam is switched off, excitation ends, but the ions remain trapped ("magnetic trapping mode" as in a Penning ion trap) and continue to emit light. The decay time constant can be extracted after subtraction of the background signal from the excitation curve, and it then reveals the upper level lifetime (after correction for ion losses from the trap).

Examples of data
obtained with the "stovepipe" instrument

Excitation curve of Cl¹²⁺
at 5742 Å Decay curve of the "green coronal line"
of Fe¹³⁺ at 5303 Å

Examples of data
obtained with the x-ray detector

Excitation curves of Ne⁸⁺
In these measurements, the electron beam
has been switched off from t=0.18 ms to t=1.2 ms.
When the electron beam energy lies just above the
production threshold for Ne⁸⁺ ions (say, at 1 keV),
a clean single-exponential decay curve is being observed.

With an electron beam energy set well above the
production threshold of the He-like ion, a higher
electron beam current can be used, maximizing the
x-ray signal. Under these conditions the decay curve
shows a tail that relates to the recombination of more
highly charged Ne ions. This also causes a systematic
error in the lifetime data evaluation.

Examples of data
obtained with the x-ray detector
	Excitation curves of Ne⁸⁺
In these measurements, the electron beam has been switched off from t=0.18 ms to t=1.2 ms. When the electron beam energy lies just above the production threshold for Ne⁸⁺ ions (say, at 1 keV), a clean single-exponential decay curve is being observed. With an electron beam energy set well above the production threshold of the He-like ion, a higher electron beam current can be used, maximizing the x-ray signal. Under these conditions the decay curve shows a tail that relates to the recombination of more highly charged Ne ions. This also causes a systematic error in the lifetime data evaluation.