Laser Based Detection of Radiocarbon,14 C

Daniel Murnick Department of Physics, Rutgers University Newark NJ 07102 USA [email protected]

Why Lasers?

Current Radiocarbon Analysis has Limitations Requiring Analytical Advances  Higher sensitivity (smaller samples; less radioactivity)  Higher speed (more samples, faster analysis)  Lower cost (high volume use)  Ease of use (simple sample preparation)

LASER techniques are being developed to meet these needs

Carbon-14 Quantitation today, 2015  Decay Counting High sensitivity and precision for β decay Laboratory sized instrument Low sensitivity for 14C Requires count rates greater than ~10 dpm for routine use

Radioactive decay measures only ~1 in ~5,000,000,000 14C atoms present

 Accelerator Mass Spectrometry High sensitivity for 14C Central Facility Instrument AMS counts 14C ions 1 at a time

 Laser Based Instruments High sensitivity for 14C Laboratory sized Instrument

Lasers interrogate 14CO2 molecules1 molecule interacts with 1000’s of photons per second! Image of single 85Rb atom scattering resonant photons T. Grünzweig, A. Hilliard, M. McGovern and M. F. Andersen, "Near-deterministic preparation of a single atom in an optical microtrap", Nature Physics. 6, 951–954 (2010)

Prototype 14C ICOGS system in Newark

Commercial version of 1990s instrument for 13C breath (50,000 ppm CO2 ) analysis developed for Urea Breath Test for H-Pylori infection

SPECTROSCOPY provides SPECIFICITY ISOTOPE IDENTIFICATION requires HIGH RESOLUTION

Extremely high resolution laser absorption spectrum near 4.49276m of highly enriched 14CO2

Spectroscopy of the 14C16O2 n3 band Courtesy of Davide Mazzotti from: Mol. Phys. 109, 2267 (2011)

Dates in Laser based 14C spectroscopy 1977 Tunable diode laser spectroscopy of 14CO2 : absorption coefficients and analytical applications M. Wahlen, R. S. Eng, and K. W. Nill, Applied Optics, 16, 2350 (1977) +34 YEARS (waiting for technology) 2011 SCAR “Molecular Gas Sensing Below Parts Per Trillion: Radiocarbon-Dioxide Optical Detection” I. Galli, S. Bartalini, S. Borri, P. Cancio, D. Mazzotti, P. De Natale, and G. Giusfredi, Physical Review Letters 107, 270802

1980 14CO2 laser “Absolute Frequencies of Lasing Transitions in Seven CO2 Isotopic Species” Charles Freed, Lee C. Bradley, and R. G. O'Donnell, IEEE J. of Quantum Electronics, QE-16, 1195 +28YEARS (waiting for ideas) 2008 ICOGS “Intracavity Optogalvanic Spectroscopy. An Analytical Technique for 14C Analysis with Subattomole Sensitivity” Daniel E. Murnick, Ozgur Dogru, and Erhan Ilkmen, Anal. Chem. 80, 4820

Narrow Band Stabilized lasers must be used for 14CO2 spectroscopy

Tunable DFG Ti-Sapphire laser system

Intrinsic line width ~10 Hz Courtesy of Davide Mazzotti from: Opt. Lett. 35, 3616 (2010)

Narrow Band Stabilized lasers must be used for 14CO2 spectroscopy

Fixed frequency 14CO2 gas laser

Intrinsic line width ~10 Hz

SPECIFICITY is insufficient at 14C concentrationsEnhanced SENSITIVITY is also required.

SCAR external passive ringdown cavity; Absorption path-length Leff ~ 10 km

SPECIFICITY is insufficient at 14C concentrationsEnhanced SENSITIVITY is also required.

14C

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laser tube

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DAQ Board

To PC

To Pump

ICOGS internal active cavity optogalvanic cell; interaction path-length Leff~ 100 km

ENHANCED SENSITIVITY also enhances BACKGROUND SCAR chooses spectral region near 4.5 m with “lowest” possible background Simulation from Mazzotti et.al.

Courtesy of Davide Mazzotti from: Phys. Rev. Lett. 107, 270802 (2011)

Experimental Results show good discrimination for 14CO2 Limitations (esp. for small samples and dating ): Large samples ~70 mg Long time per sample ~3 hours Improvements are possible

CAVITY RINGDOWN SPECTROSCOPY WITH A QUANTUM CASCADE LASER FOR NUCLEAR POWER PLANT MONITORING

5 Minute spectra for enriched ~1000x, 100x and ambient CO2 samples

G. Genoud, M. Vainio, H. Phillips, J. Dean, and M. Merimaa OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015

Laser resonances in CO2 are Isotope Dependent Lincoln Laboratory Journal, 3, 491 (1990)

ICOGS chooses fixed frequency laser transitions14CO P(20) at 2 11.767726 m and harnesses the Optogalvanic effect to minimize and measure backgrounds

Laser transitions are between excited states, have large isotope shifts and can have enhanced populations in well designed gas discharges Nitrogen-CO2 Laser levels 2000

The CO2 laser is based on energy transfer from vibrationally excited Nitrogen to the upper laser level in an electrical discharge.

SENSITIVITY further enhanced through THE OPTOGALVANIC EFFECT (OGE) Laser radiation changes distribution of various species within an electrical discharge which changes the electron energy distribution function. This leads to an easily measurable impedance change of the system .

Advantages •Electrical signal in phase with laser •No optical Background •Integrates over space

Ref: Moffatt, S. and Smith, A.L.S Temperature Perturbation Model of the Optogalvanic effect in CO2 laser discharges J. Phys: Appl. Phys., 17(1984) 59-70

0.4

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However, Laser Resonances are not zero width! Lorentzian tails from more Abundant Isotopes Contribute to signal 0.3

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10M 1ml sample

LASER P(20) transition (Gaussian line shape) is in far-far wings (Lorentzian tail) of 13CO2 “Hot Band” P(68) transition

(1/  ) v 0 S (v )  2 2 (v  v 0)   v0

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collision rate 0  2

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At 1 M,