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Position sensitive detection of photons in ultrasensitive fluorescent spectroscopy
Volume 82, number 1,2
OPTICS COMMUNICATIONS
1 April 1991
Position sensitive detection of photons in ultrasensitive fluorescent spectroscopy D.A. Eastham, A. Gilda, D.D. W a r n e r SERC DaresburyLaboratory, Daresbury, Warrington WA4 4AD, UK
D.E. Evans, J.A.R. Griffiths Department of Physics, Universityof Birmingham, Birmingham B I 5 2T~, UK
J. Billowes, M.P. D a n c e y a n d I.S. G r a n t Department of Physics, Universityof Manchester, ManchesterM I 3 9PL, UK
Received 9 October 1990
The use of a position sensitive detector for ultrasensitive laser spectroscopyis described. The detector has been used in conjunction with coincidencelaser spectroscopyof fast ionic/atomic beams to produce significantimprovementsin sensitivity.
In a previous communication [ 1 ] we reported on the use of a coincidence technique to improve the sensitivity of detection of fast radioactive beams. This method was a development of collinear laser spectroscopy [ 2 ] in which the background of non-resonant scattered radiation could be considerably reduced by recording only photons which were associated (coincident) with the ions/atoms in the beam. For favorable species, where the atomic transition is strong, this technique now allows measurements to be made with fewer than 100 ions per second [3 ]. The level of discrimination achieved is clearly determined by the coincidence timing resolution since the main cause of background is random coincidences between photons and ions. These may be either ions which did not give a photon count in the detector or they may be ions of the same mass but a different atomic number than the desired one. (The latter is the most important source of background when studying weak radioactive beams from a hot source which often produces a much stronger beam of a stable isotope with the same atomic mass. ) The main physical factor determining the coincident time resolution is the differing path length of the ions from the point of emission of the photon to
the final detector. This has two contributions, stemming from the differing trajectories of ions and the differing points of emission along the intersection region for the photons. The latter is by far the largest contribution. It cannot be reduced by decreasing the interaction region since to do so would reduce prorata the overall coincidence rate and hence preserve the same signal-to-noise ratio. However, if the position from which the photon is emitted is measured using an imaging photomultiplier, then an appropriate time correction can be applied to compensate for the ion path length. Fig. 1 shows the general layout of the system for use with an ion beam. The method for atomic beams is given in the earlier paper. Part of the overlap of the laser and the ion beam is imaged onto the photocathode of the multiplier using Fresnel lenses and a retro-reflector (comer cube) as shown. With this arrangement there is unit magnification and rays collected in the reflector are each projected back through their point of origin so that the light collection efficiency is increased by about 50%, after losses in the optics and solid angles are considered. The proximity-imaging photon detector (manufactured by ITL, St-Leonards-on-Sea, East Sussex, U K ) con-
0030-4018/91/$03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )
23
Volume 82, number 1,2
OPTICS COMMUNICATIONS
1 April 1991
__-II. . . . . . A .,¢" .;:" " [ I I I
Corner cube Ions
Single Fresnel lens
_~__
Doublet Fresne[ lens
~ h
Position sensitive
PM
f--~
, , L 90n~ J
Deflector
Laser ~"~ ' t
f~ old cathode
--j
Channel plate detector
Coinc
Tac
Ddny
I X,Y ~
90nS
t Fig. 1. Experimental layout for position sensitive detection of ions in coUineargeometry.
sisted of a series of channel electron multipliers placed behind a photocathode. Electrons from the photocathode are amplified in the channel plated before being collected on a resistive anode. Four conducting strips in the form of a square are embedded into the outside of this anode with each strip being connected to a separate charge sensitive amplifier. The charge measured on each strip can be used to determine the position of the incident photon in the x, y plane in the normal manner. For fast timing a signal was coupled via a capacitor from the last anode of the channel plate assembly into a charge-sensitive preamplifier. The system was tested using a stable beam of 134Ba accelerated to 28.573 keV ( _+ 1 eV) using the Daresbury on-line isotope separator DOLIS [4]. The ions were made to interact with a counter propagating laser beam tuned to resonate with the 2=455.4 nm (6s 2S1/2-6p =P3/2) line in the singly ionized BalI spectrum. The frequency of the dye laser output was locked to an absorption line in tellurium and the resonance of Ba ions was achieved by altering the voltage on the light collector region (Doppler tuning). 24
Delayed coincidence events were recorded using a TAC (time-to-amplitude converter) whose start pulse was taken from the photomultiplier (suitably delayed to account for the ion transit time) and whose stop pulse was taken from the channel plate ion detector (fig. l ). Each TAC output pulse representing the time difference of each "coincidence" was associated with signals representing the x, y position. An "event" consisting of four numbers x, y, t and V (the voltage on the light collector) was recorded on disc via a CAMAC based data acquisition system for each output of the TAC. To measure the improvement in the time resolution events were recorded with the voltage set at the peak of the fluorescence resonance. The x, y distribution consists of a background of counts spread uniformly over the circular photocathode area superimposed with a strip along the x direction for the resonantly scattered photons. By selecting only events within this strip the dark counts and non-resonant scattering could be kept to a minimum. The x, t plot of events in this strip is shown in fig. 2a and consists of a line whose slope represents the velocity of the
Volume 82, number 1,2
OPTICS COMMUNICATIONS
1 April 1991
/,000
(b}
3000
2~
90nS
1
OV-
oo
?~. %
0
~0
80
120
160 200
CHANNELNUMBER
O
m ~0 W
(el
z
3~
2000
-
J
20nS
I000i 'o
(a)
'~s
'3=
:w
'6,
~0
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'~la
TIME
0
/.0
80
120
160 200
CHANNEL NUMBER
Fig. 2. (a) The position versus time plot where the slope represents the velocity of the ions. (b) The projection of the plot on the time axis. (c) The projection when the time has been corrected for position. In all cases one channel along the time axis represents 2 ns.
1S4Ba ions (2 × 10s m s - ~). The photocathode is 18 m m in diameter and the projection on the t-axis of the events gives an uncorrected time resolution of 90 ns as shown in fig. 2b. This corresponds to the transit time of the Ba ions across the face of the photocathode. To correct for the differing transit times, the time coordinate of each event is modified using its measured position, viz tc = t - x / s where s is the slope dx/ dt determined from the x, t plot of fig. 2a. The resuiting corrected time spectrum is shown in fig. 2c for comparison. The full width at half height is reduced to 20 ns so that the improvement in peak to background is about a factor of 5. The corrected time resolution here is still significantly larger than what could be achieved with these optics. The limit to time resolution is determined principally by the finite
depth of field needed to image the laser beam. For a beam of about 1 m m this should be around 5 ns so the limiting resolution will be somewhat larger than this. Ultimately the system should be capable of attaining sensitivities of a few ions (or atoms ) per second in collinear laser spectroscopy. In the meantime, however, the system has already been used to measure the hyperfine structure of a previously unknown nuclear isomer in '27Ba [5].
References [ 1 ] D.A. Eastham, P.M. Walker, J.R.H. Smith, J.A.R.Griffith, D.E.Evans, S.A. Wells, M.J. Fawcett and I.S. Grant, Optics Comm. 60 (1986) 293.
25
Volume 82, number 1,2
OPTICS COMMUNICATIONS
[2] S.L. Kaufmann, Optics Comm. 17 (1976) 309. [3]D.A. Eastham, P.M.Walker, J.R.H.Smith, D.D.Warner, J.A.R.Griffith, D.E.Evans, S.A. Wells, M.J.Fawcett and I.S. Grant, Phys. Rev. C 36 (1987) 1583.
26
1 April 1991
[4]I.S.Grant, D.A. Eastman, J.Groves, D.W.L. Tolfree, P.M.Walker, V.R.Green, J.Rikovska, N.J.Stone and W.D. Hamilton, Nucl. Instr. and Meth. B 26 (1987) 95. [5] J.A.R. Griffith, D.E. Evans, D.A. Eastham, J. Groves, D.D.Warner, J.Billowes, M.P.Dancey and I.S. Grant, Phys. Lett. A, to be published.
OPTICS COMMUNICATIONS
1 April 1991
Position sensitive detection of photons in ultrasensitive fluorescent spectroscopy D.A. Eastham, A. Gilda, D.D. W a r n e r SERC DaresburyLaboratory, Daresbury, Warrington WA4 4AD, UK
D.E. Evans, J.A.R. Griffiths Department of Physics, Universityof Birmingham, Birmingham B I 5 2T~, UK
J. Billowes, M.P. D a n c e y a n d I.S. G r a n t Department of Physics, Universityof Manchester, ManchesterM I 3 9PL, UK
Received 9 October 1990
The use of a position sensitive detector for ultrasensitive laser spectroscopyis described. The detector has been used in conjunction with coincidencelaser spectroscopyof fast ionic/atomic beams to produce significantimprovementsin sensitivity.
In a previous communication [ 1 ] we reported on the use of a coincidence technique to improve the sensitivity of detection of fast radioactive beams. This method was a development of collinear laser spectroscopy [ 2 ] in which the background of non-resonant scattered radiation could be considerably reduced by recording only photons which were associated (coincident) with the ions/atoms in the beam. For favorable species, where the atomic transition is strong, this technique now allows measurements to be made with fewer than 100 ions per second [3 ]. The level of discrimination achieved is clearly determined by the coincidence timing resolution since the main cause of background is random coincidences between photons and ions. These may be either ions which did not give a photon count in the detector or they may be ions of the same mass but a different atomic number than the desired one. (The latter is the most important source of background when studying weak radioactive beams from a hot source which often produces a much stronger beam of a stable isotope with the same atomic mass. ) The main physical factor determining the coincident time resolution is the differing path length of the ions from the point of emission of the photon to
the final detector. This has two contributions, stemming from the differing trajectories of ions and the differing points of emission along the intersection region for the photons. The latter is by far the largest contribution. It cannot be reduced by decreasing the interaction region since to do so would reduce prorata the overall coincidence rate and hence preserve the same signal-to-noise ratio. However, if the position from which the photon is emitted is measured using an imaging photomultiplier, then an appropriate time correction can be applied to compensate for the ion path length. Fig. 1 shows the general layout of the system for use with an ion beam. The method for atomic beams is given in the earlier paper. Part of the overlap of the laser and the ion beam is imaged onto the photocathode of the multiplier using Fresnel lenses and a retro-reflector (comer cube) as shown. With this arrangement there is unit magnification and rays collected in the reflector are each projected back through their point of origin so that the light collection efficiency is increased by about 50%, after losses in the optics and solid angles are considered. The proximity-imaging photon detector (manufactured by ITL, St-Leonards-on-Sea, East Sussex, U K ) con-
0030-4018/91/$03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )
23
Volume 82, number 1,2
OPTICS COMMUNICATIONS
1 April 1991
__-II. . . . . . A .,¢" .;:" " [ I I I
Corner cube Ions
Single Fresnel lens
_~__
Doublet Fresne[ lens
~ h
Position sensitive
PM
f--~
, , L 90n~ J
Deflector
Laser ~"~ ' t
f~ old cathode
--j
Channel plate detector
Coinc
Tac
Ddny
I X,Y ~
90nS
t Fig. 1. Experimental layout for position sensitive detection of ions in coUineargeometry.
sisted of a series of channel electron multipliers placed behind a photocathode. Electrons from the photocathode are amplified in the channel plated before being collected on a resistive anode. Four conducting strips in the form of a square are embedded into the outside of this anode with each strip being connected to a separate charge sensitive amplifier. The charge measured on each strip can be used to determine the position of the incident photon in the x, y plane in the normal manner. For fast timing a signal was coupled via a capacitor from the last anode of the channel plate assembly into a charge-sensitive preamplifier. The system was tested using a stable beam of 134Ba accelerated to 28.573 keV ( _+ 1 eV) using the Daresbury on-line isotope separator DOLIS [4]. The ions were made to interact with a counter propagating laser beam tuned to resonate with the 2=455.4 nm (6s 2S1/2-6p =P3/2) line in the singly ionized BalI spectrum. The frequency of the dye laser output was locked to an absorption line in tellurium and the resonance of Ba ions was achieved by altering the voltage on the light collector region (Doppler tuning). 24
Delayed coincidence events were recorded using a TAC (time-to-amplitude converter) whose start pulse was taken from the photomultiplier (suitably delayed to account for the ion transit time) and whose stop pulse was taken from the channel plate ion detector (fig. l ). Each TAC output pulse representing the time difference of each "coincidence" was associated with signals representing the x, y position. An "event" consisting of four numbers x, y, t and V (the voltage on the light collector) was recorded on disc via a CAMAC based data acquisition system for each output of the TAC. To measure the improvement in the time resolution events were recorded with the voltage set at the peak of the fluorescence resonance. The x, y distribution consists of a background of counts spread uniformly over the circular photocathode area superimposed with a strip along the x direction for the resonantly scattered photons. By selecting only events within this strip the dark counts and non-resonant scattering could be kept to a minimum. The x, t plot of events in this strip is shown in fig. 2a and consists of a line whose slope represents the velocity of the
Volume 82, number 1,2
OPTICS COMMUNICATIONS
1 April 1991
/,000
(b}
3000
2~
90nS
1
OV-
oo
?~. %
0
~0
80
120
160 200
CHANNELNUMBER
O
m ~0 W
(el
z
3~
2000
-
J
20nS
I000i 'o
(a)
'~s
'3=
:w
'6,
~0
'~
'~la
TIME
0
/.0
80
120
160 200
CHANNEL NUMBER
Fig. 2. (a) The position versus time plot where the slope represents the velocity of the ions. (b) The projection of the plot on the time axis. (c) The projection when the time has been corrected for position. In all cases one channel along the time axis represents 2 ns.
1S4Ba ions (2 × 10s m s - ~). The photocathode is 18 m m in diameter and the projection on the t-axis of the events gives an uncorrected time resolution of 90 ns as shown in fig. 2b. This corresponds to the transit time of the Ba ions across the face of the photocathode. To correct for the differing transit times, the time coordinate of each event is modified using its measured position, viz tc = t - x / s where s is the slope dx/ dt determined from the x, t plot of fig. 2a. The resuiting corrected time spectrum is shown in fig. 2c for comparison. The full width at half height is reduced to 20 ns so that the improvement in peak to background is about a factor of 5. The corrected time resolution here is still significantly larger than what could be achieved with these optics. The limit to time resolution is determined principally by the finite
depth of field needed to image the laser beam. For a beam of about 1 m m this should be around 5 ns so the limiting resolution will be somewhat larger than this. Ultimately the system should be capable of attaining sensitivities of a few ions (or atoms ) per second in collinear laser spectroscopy. In the meantime, however, the system has already been used to measure the hyperfine structure of a previously unknown nuclear isomer in '27Ba [5].
References [ 1 ] D.A. Eastham, P.M. Walker, J.R.H. Smith, J.A.R.Griffith, D.E.Evans, S.A. Wells, M.J. Fawcett and I.S. Grant, Optics Comm. 60 (1986) 293.
25
Volume 82, number 1,2
OPTICS COMMUNICATIONS
[2] S.L. Kaufmann, Optics Comm. 17 (1976) 309. [3]D.A. Eastham, P.M.Walker, J.R.H.Smith, D.D.Warner, J.A.R.Griffith, D.E.Evans, S.A. Wells, M.J.Fawcett and I.S. Grant, Phys. Rev. C 36 (1987) 1583.
26
1 April 1991
[4]I.S.Grant, D.A. Eastman, J.Groves, D.W.L. Tolfree, P.M.Walker, V.R.Green, J.Rikovska, N.J.Stone and W.D. Hamilton, Nucl. Instr. and Meth. B 26 (1987) 95. [5] J.A.R. Griffith, D.E. Evans, D.A. Eastham, J. Groves, D.D.Warner, J.Billowes, M.P.Dancey and I.S. Grant, Phys. Lett. A, to be published.