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Coincidence laser spectroscopy: A new ultrasensitive technique for fast ionic or atomic beams
Volume 60, number 5
OPTICS COMMUNICATIONS
1 December 1986
C O I N C I D E N C E LASER S P E C T R O S C O P Y : A N E W U L T R A S E N S I T I V E T E C H N I Q U E FOR FAST IONIC OR A T O M I C B E A M S D.A. E A S T H A M , P.M. W A L K E R , J.R.H. S M I T H SERC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
J.A.R. G R I F F I T H , D.E. EVANS, S.A. W E L L S Department of Physics, University of Birmingham, Birmingham B15 2TT, UK
M.J. F A W C E T ' F and I.S. G R A N T Department of Physics, University of Manchester, Manchester M13 9PL, UK
Received 16 July 1986
A new technique for laser spectroscopy of fast ionic or atomic beams is described. This involves measuring coincidences between resonantly scattered photons and ions (or atoms) in the fast beam. Measurements on strontium ions have shown that Doppler-free spectroscopy is possible with fewer than 100 ions s-1.
Doppler-free spectroscopy using the collinear beam [1 ] method is now a well-established technique for measurement of atomic isotope shifts and hyperFme structure. In this technique light from a tunable dye laser is scattered from a fast collinear beam. Measurements based on this technique [2] have most often used the scattered radiation to signal the resonance position. On resonance, the large cross-section for scattering (of the order 2rrX2) means that high sensitivity can be achieved. For example, measurements on beam intensities in the range 104 to 105 atoms s -1 have been reported [3,4]. The limiting sensitivity in these experiments is determined by the non-resonant component of the scattering. This arises mainly from scattering off collimators and slit edges together with dark current pulses from the photomultiplier. In the techniques reported here the sensitivity is increased by several orders of magnitude by counting both scattered photons and ions (atoms), and recording only coincident ion-photon events. The general arrangement for these experiments is shown in fig. 1 '. Ionic beams up to 100 keV in energy of a given mass collide with a laser beam. Resonantly scattered light is gathered with an array of mirrors and Fresnel lenses 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
and focussed on to the photocathodes of a series of photomultiplier tubes. Downstream from the light collector the ions are deflected electrostatically into a single-channel electron multiplier (channeltron). The system for use with atoms is shown in the lower part of the figure. Here the atoms are detected from the secondary electrons emitted when they impinge on a metallic mirror. The system was tested using stable and radioactive beams of strontium ions. The radioactive beams were produced using the nuclear reactions 54, 56Fe (32S, a2p)80, 82Sr at 160 MeV whilst the stable beams were produced by placing a small quantity of SrCO 3 in the ion source. Laser light from a Spectra Physics 380D dye laser, using stillbene-3 dye, was used to excite the strong 5s 2S1/2 to 5p 2P1/2 (at 421.552 nm) in SrlI. In these tests a single cell of the array illustrated was used. The mirror-lens cell was arranged to image a 30 mm section of the interaction region on to the photocathode of a cooled phototube (EMI 9883B). During the experiments the laserfrequency was locked to a known atomic reference. Scanning across the collinear resonance was achieved by altering the electrostatic potential of the complete 293
Volume 60, number 5
OPTICS COMMUNICATIONS
1 December 1986
Scan voltage Deflector
c"/ Mirrors Ion beam from separator
k A J, J. J. J.
(~(~(~(~'//¢'~ \
Laser beam
1
~t'~ Fresne' lenses
Channeltron
v Photomultipliers (a) Ions
Charge exchange cell
Scan voltage
Secondary electrons "~
7 \
v Photomultipliers
Metallic
ChanneltronP~'
mirror
/ Laser beam (b) Atoms ( plan view)
Fig. 1. The arrangement for coincidence laser spectroscopy with a beam of fast (a) ions (b) atoms. The atomic system is shown in plan view to illustrate the rays in the light collector.
right collection assembly. At an ion energy of 30 keV the Doppler-shifted transition in strontium is conveniently close to the 421.932 nm transition in atomic samarium. Part of the laser beam was sprit off and passed through an atomic samarium beam. Fluorescent right from this transition in 152Sm was then used to maintain the laser beam at a fixed frequency for long periods of time. Details of this active locking technique are given elsewhere [5]. Fig. ~ shows the results. The top left-hand spectrum is a singles fluorescent spectrum for 84Sr at 2 × 105 ion s -1 and 1 mW of laser power whilst the spectrum below it is a spectrum with photon-ion coincidences. The peak has a width of approximately 75 MHz which can be compared with the natural linewidth of 21 MHz for this transition. Coincidence spectra from the radioactive isotopes 82Sr at ~ 3 × 103 ions s -1 and 80Sr at 103 ions s -1 are also shown. On resonance the true rate for coincidences was approximately one per second for 82Sr. These results can be compared with those calculated from the resonance fluorescence cross-sections. The 294
Sr84 (Singlesphotons)
Sr82(Coincidences)
1000-
OI 244
290
0
1013
Sr84 (Coincidences) 500 ~
o.
1040
Sr80 (Coincidences) 50
290 o
~6~ ° " "-'v-~19
Scan voltage
Fig. 2. Spectra showing the number of detected photons as a function of the voltage on the light collector assembly for a separator potential of 31504 V and 1 mW laser power. The 84Sr spectra are for 2 × 105 ions s-1 whilst the 82Sr and 8°Sr are for approximately 3 × 103 and 103 ions s-1, respectively.
Volume 60, number 5
OPTICS COMMUNICATIONS
probability for absorption of a p h o t o n (in the interaction region) for a Doppler-broadened transition at linecentre can be written Pa = A R / R = n o a / h
(1)
and
o a = 27r3/2 (In 2) 1/2 "AZ/k,
(2)
where &R = rate of absorption of photons, R = incident photon rate in the laser beam, n = instantaneous number of ions (atoms) in the interaction region in the lower atomic level, k = ratio of the Doppler-broadened width to the natural linewidth, and A = ionic (atomic) beam area. Here it is assumed that the system is inhomogeneously broadened with a linewidth much greater than the natural linewidth. However, the numerical factors are not too much different if the broadening is completely homogeneous. Also, it is assumed that the laser beam is of smaller lateral extent than the ionic beam. To calculate the maximum signal-to-background we assume that n = n o = number of ions or atoms in the interaction region. This is equivalent to the condition of no saturation or optical pumping. (Optical pumping is not a problem for the ionic arrangement, where scanning is done by adjusting the light collector potential, since the ions are only on resonance during the flight through the collector). The number of detected photons depends on the spontaneous emission lifetime. If this is much'less than the time, t, it takes for an ion to traverse the interaction region, then the rate of detecting photons is D s = AReq,
(3)
where q is the quantum efficiency of the photomultiplier and e is the total light collection efficiency. For the experiments in question, n = 10 - 4 for 1000 particles s - 1 at 30 keV and the beam areaA is approximately 20 m m 2. The geometrical efficiency for a single cell is about 9% and the quantum efficiency is 30% at
1 December 1986
this wavelength. This gives a total yield of approximately 4 × 10 - 4 photons s - 1 per ion s - 1 per mW of laser power, close to that observed in the experiment. Since the efficiency of detecting ions in the channeltron is ~ 9 5 - 1 0 0 % , the coincidence counting rate will be approximately D s. The number of random ionphoton coincidences for a resolving time equal to the transit time, t, is D R = 2DpD I t,
(4)
where D I is the ion beam rate and Dp is the photon rate off resonance. This gives D R = 0.12 s - 1 for t = 10 - 7 s , D I = 3000 s - 1 a n d D p = 200 s -1. The true to background rate is therefore 8:1, close to that observed in fig. 2 for 82Sr and 84Sr. The improvement in signalto-background is therefore a factor of 1600. Notice that this improvement factor increases with correspond. ing reduction in the ion beam rate since the true to background remains constant as long as the ion beam is composed solely of strontium. In some mass regions contaminant beams from the ion source may be unavoidable and hence the sensitivity will be reduced. However, for favourable species, where the atomic transition is strong, measurements with fewer than 100 ions s-1 are clearly possible.
References [1] S.L. Kaufmann, Optics Comm. 17 (1976) 309. [2] R. Neugart, Nucl. Instrum. Meth. 186 (1981) 165. [3] J. Eberg, U. Dinger, T. Horiguchi, G. Huber, H. Lochmann, R. Menges, R. Kirchner, D. Klepper, T. Ktihl, D. Marx, E. Roeckl, D. Schardt and G. Ulm, Z. Phys. A323 (1986) 119. [4] R. Neugart, E.W. Otten, K. Wendt, C. Ekstrom, S.A. Ahmed and W. Klempt, Laser Spectroscopy VI, Proc. VI Intern. Conf. Interlaken, Switzerland, 1983 (Springer Verlag, Berlin, 1984) p. 206. [5] J.A.R. Griffith, G.R. Isaak, R. New, M.P. Rails and C.P. van Zyl, J. Phys. B10 (1977) L91.
295
OPTICS COMMUNICATIONS
1 December 1986
C O I N C I D E N C E LASER S P E C T R O S C O P Y : A N E W U L T R A S E N S I T I V E T E C H N I Q U E FOR FAST IONIC OR A T O M I C B E A M S D.A. E A S T H A M , P.M. W A L K E R , J.R.H. S M I T H SERC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
J.A.R. G R I F F I T H , D.E. EVANS, S.A. W E L L S Department of Physics, University of Birmingham, Birmingham B15 2TT, UK
M.J. F A W C E T ' F and I.S. G R A N T Department of Physics, University of Manchester, Manchester M13 9PL, UK
Received 16 July 1986
A new technique for laser spectroscopy of fast ionic or atomic beams is described. This involves measuring coincidences between resonantly scattered photons and ions (or atoms) in the fast beam. Measurements on strontium ions have shown that Doppler-free spectroscopy is possible with fewer than 100 ions s-1.
Doppler-free spectroscopy using the collinear beam [1 ] method is now a well-established technique for measurement of atomic isotope shifts and hyperFme structure. In this technique light from a tunable dye laser is scattered from a fast collinear beam. Measurements based on this technique [2] have most often used the scattered radiation to signal the resonance position. On resonance, the large cross-section for scattering (of the order 2rrX2) means that high sensitivity can be achieved. For example, measurements on beam intensities in the range 104 to 105 atoms s -1 have been reported [3,4]. The limiting sensitivity in these experiments is determined by the non-resonant component of the scattering. This arises mainly from scattering off collimators and slit edges together with dark current pulses from the photomultiplier. In the techniques reported here the sensitivity is increased by several orders of magnitude by counting both scattered photons and ions (atoms), and recording only coincident ion-photon events. The general arrangement for these experiments is shown in fig. 1 '. Ionic beams up to 100 keV in energy of a given mass collide with a laser beam. Resonantly scattered light is gathered with an array of mirrors and Fresnel lenses 0 030-401/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
and focussed on to the photocathodes of a series of photomultiplier tubes. Downstream from the light collector the ions are deflected electrostatically into a single-channel electron multiplier (channeltron). The system for use with atoms is shown in the lower part of the figure. Here the atoms are detected from the secondary electrons emitted when they impinge on a metallic mirror. The system was tested using stable and radioactive beams of strontium ions. The radioactive beams were produced using the nuclear reactions 54, 56Fe (32S, a2p)80, 82Sr at 160 MeV whilst the stable beams were produced by placing a small quantity of SrCO 3 in the ion source. Laser light from a Spectra Physics 380D dye laser, using stillbene-3 dye, was used to excite the strong 5s 2S1/2 to 5p 2P1/2 (at 421.552 nm) in SrlI. In these tests a single cell of the array illustrated was used. The mirror-lens cell was arranged to image a 30 mm section of the interaction region on to the photocathode of a cooled phototube (EMI 9883B). During the experiments the laserfrequency was locked to a known atomic reference. Scanning across the collinear resonance was achieved by altering the electrostatic potential of the complete 293
Volume 60, number 5
OPTICS COMMUNICATIONS
1 December 1986
Scan voltage Deflector
c"/ Mirrors Ion beam from separator
k A J, J. J. J.
(~(~(~(~'//¢'~ \
Laser beam
1
~t'~ Fresne' lenses
Channeltron
v Photomultipliers (a) Ions
Charge exchange cell
Scan voltage
Secondary electrons "~
7 \
v Photomultipliers
Metallic
ChanneltronP~'
mirror
/ Laser beam (b) Atoms ( plan view)
Fig. 1. The arrangement for coincidence laser spectroscopy with a beam of fast (a) ions (b) atoms. The atomic system is shown in plan view to illustrate the rays in the light collector.
right collection assembly. At an ion energy of 30 keV the Doppler-shifted transition in strontium is conveniently close to the 421.932 nm transition in atomic samarium. Part of the laser beam was sprit off and passed through an atomic samarium beam. Fluorescent right from this transition in 152Sm was then used to maintain the laser beam at a fixed frequency for long periods of time. Details of this active locking technique are given elsewhere [5]. Fig. ~ shows the results. The top left-hand spectrum is a singles fluorescent spectrum for 84Sr at 2 × 105 ion s -1 and 1 mW of laser power whilst the spectrum below it is a spectrum with photon-ion coincidences. The peak has a width of approximately 75 MHz which can be compared with the natural linewidth of 21 MHz for this transition. Coincidence spectra from the radioactive isotopes 82Sr at ~ 3 × 103 ions s -1 and 80Sr at 103 ions s -1 are also shown. On resonance the true rate for coincidences was approximately one per second for 82Sr. These results can be compared with those calculated from the resonance fluorescence cross-sections. The 294
Sr84 (Singlesphotons)
Sr82(Coincidences)
1000-
OI 244
290
0
1013
Sr84 (Coincidences) 500 ~
o.
1040
Sr80 (Coincidences) 50
290 o
~6~ ° " "-'v-~19
Scan voltage
Fig. 2. Spectra showing the number of detected photons as a function of the voltage on the light collector assembly for a separator potential of 31504 V and 1 mW laser power. The 84Sr spectra are for 2 × 105 ions s-1 whilst the 82Sr and 8°Sr are for approximately 3 × 103 and 103 ions s-1, respectively.
Volume 60, number 5
OPTICS COMMUNICATIONS
probability for absorption of a p h o t o n (in the interaction region) for a Doppler-broadened transition at linecentre can be written Pa = A R / R = n o a / h
(1)
and
o a = 27r3/2 (In 2) 1/2 "AZ/k,
(2)
where &R = rate of absorption of photons, R = incident photon rate in the laser beam, n = instantaneous number of ions (atoms) in the interaction region in the lower atomic level, k = ratio of the Doppler-broadened width to the natural linewidth, and A = ionic (atomic) beam area. Here it is assumed that the system is inhomogeneously broadened with a linewidth much greater than the natural linewidth. However, the numerical factors are not too much different if the broadening is completely homogeneous. Also, it is assumed that the laser beam is of smaller lateral extent than the ionic beam. To calculate the maximum signal-to-background we assume that n = n o = number of ions or atoms in the interaction region. This is equivalent to the condition of no saturation or optical pumping. (Optical pumping is not a problem for the ionic arrangement, where scanning is done by adjusting the light collector potential, since the ions are only on resonance during the flight through the collector). The number of detected photons depends on the spontaneous emission lifetime. If this is much'less than the time, t, it takes for an ion to traverse the interaction region, then the rate of detecting photons is D s = AReq,
(3)
where q is the quantum efficiency of the photomultiplier and e is the total light collection efficiency. For the experiments in question, n = 10 - 4 for 1000 particles s - 1 at 30 keV and the beam areaA is approximately 20 m m 2. The geometrical efficiency for a single cell is about 9% and the quantum efficiency is 30% at
1 December 1986
this wavelength. This gives a total yield of approximately 4 × 10 - 4 photons s - 1 per ion s - 1 per mW of laser power, close to that observed in the experiment. Since the efficiency of detecting ions in the channeltron is ~ 9 5 - 1 0 0 % , the coincidence counting rate will be approximately D s. The number of random ionphoton coincidences for a resolving time equal to the transit time, t, is D R = 2DpD I t,
(4)
where D I is the ion beam rate and Dp is the photon rate off resonance. This gives D R = 0.12 s - 1 for t = 10 - 7 s , D I = 3000 s - 1 a n d D p = 200 s -1. The true to background rate is therefore 8:1, close to that observed in fig. 2 for 82Sr and 84Sr. The improvement in signalto-background is therefore a factor of 1600. Notice that this improvement factor increases with correspond. ing reduction in the ion beam rate since the true to background remains constant as long as the ion beam is composed solely of strontium. In some mass regions contaminant beams from the ion source may be unavoidable and hence the sensitivity will be reduced. However, for favourable species, where the atomic transition is strong, measurements with fewer than 100 ions s-1 are clearly possible.
References [1] S.L. Kaufmann, Optics Comm. 17 (1976) 309. [2] R. Neugart, Nucl. Instrum. Meth. 186 (1981) 165. [3] J. Eberg, U. Dinger, T. Horiguchi, G. Huber, H. Lochmann, R. Menges, R. Kirchner, D. Klepper, T. Ktihl, D. Marx, E. Roeckl, D. Schardt and G. Ulm, Z. Phys. A323 (1986) 119. [4] R. Neugart, E.W. Otten, K. Wendt, C. Ekstrom, S.A. Ahmed and W. Klempt, Laser Spectroscopy VI, Proc. VI Intern. Conf. Interlaken, Switzerland, 1983 (Springer Verlag, Berlin, 1984) p. 206. [5] J.A.R. Griffith, G.R. Isaak, R. New, M.P. Rails and C.P. van Zyl, J. Phys. B10 (1977) L91.
295