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Laser method for absolute velocity calibration of Mössbauer spectrometers
N U C L E A R I N S T R U M E N T S AND METHODS 75
(I969) I65-I66;
©
NORTH-HOLLAND
P U B L I S H I N G CO.
LASER METHOD FOR ABSOLUTE VELOCITY CALIBRATION OF MOSSBAUER SPECTROMETERS J . P . BISCAR, W. KI]NDIG, H. BOMMEL and R. S. H A R G R O V E
Department of Physics, University of California, Los Angeles, California 90024, U.S.A. Received 30 May 1969 The velocity of a Mrssbauer drive is converted into a beat frequency of Doppler shifted lines of a laser beam. The counting of this frequency allows a precise velocity calibration.
pattern" has been described3). Both of these techniques require sophisticated optical parts and precise alignments which probably impair the generalization of their use. In this paper an easy and convenient optical technique using a laser beam is described. A He-Ne laser (Electro-Nuclear Laboratories) having an optical cavity length of L = 33 cm and a linearly polarized output is used. The overall configuration is shown in fig. 1. A small mirror M is mounted beside the source on the MiSssbauer drive; it reflects the laser beam parallel to itself. The only alignment to be made is to place parallel the mirror M and the output mirror of the laser. This is achieved when the multiple beams reflected back and forth between the two mirrors coincide on the same spot on the mirror M. Visually one can identify up to 8 or 10 successive reflections which make the alignment trivial. A beam splitter attached to the laser reflects part of the light to a photodiode (S. G. D. 100 from EGG). Taking advantage of the coherence of the laser beam, it is through the Doppler shift of the light at each reflection on the mirror M that the velocity of the drive
Most M6ssbauer spectrometers are using an electromechanical transducer which moves the source relative to the absorber in any given velocity function; e.g. in the constant acceleration drive 1) the velocity function is a triangle or a sawtooth. The pulses from the p r a y detector are stored in a multichannel analyzer according to the velocity of the drive. One difficulty in this type fo spectrometer is the velocity calibration. Usually this is done by measuring spectra of various absorbers with known line positions. From the position of these lines and the assumed linearity of the drive the spectrometer is calibrated. For more accuracy one must use an optical technique. A fringe counting device, to measure directly the velocity in terms of the standards of length (wavelength of the green line of 198Hg ) and time, has been developed by the National Bureau of Standards2). Basically it is a Michelson interferometer using two corner cube reflectors and a K6ster's prism for a beam splitter. A198Hg lamp provides a known wavelength, the fringes are detected by a photomultiplier and their frequency is compared to a standard frequency. Another optical method, based on a "variable moir6
REAM SPLITTER
x/4
TRANSDUCER
-
-
-/
~
i'
~
H
e Ne LASER -
POLARIZER FEEDBACK AMPLIFIER TRIANGULAR WAVE GENERATOR
PULSE GENERATOR
IsD-';P'--I °
IF--
ADDRESS DATA [ ~+#2V PULSE 1 - ] REGISTER REGISTER--~ j, 1220#ZSHAPER ~ i
q
½
MULTICHANNEL ANALYZER
',
#'
L2NU06 ~"
,
J
Fig. 1. General configuration for an absolute velocity calibration of a constant acceleration M0ssbauer drive by means of cw laser beam.
165
166
J . P . BISCAR et al.
is converted into frequency. Each photon of frequency v undergoes a Doppler shift when reaching the mirror moving at the velocity v and again the same shift when it is re-emitted back by the mirror. At each reflection by M the total Doppler shift is (I)
Av = 2nvv[c,
where c is the velocity of light in vacuum and n the index of refraction of air. After the first reflection on the mirror M, part of the beam of frequency v + A v , is deflected to the photodiode, while the other part is reflected again to M by the output mirror of the laser and undergoes a new Doppler shift etc. The photodiode receives therefore the following frequencies: v,
v+Av,
v+2Av,
v + 3 A v ....
The Doppler shift is detected as a beat frequency f between two successive terms f = (2 Ivl n)/it.
(2)
Harmonic beat frequencies at 2f, 3f, ... are also possible. One way to eliminate the harmonics is to mix only the first two terms v and v + A v. For this one has to use a ¼ it plate and a polarizer between the beam splitter and the mirror M in the order shown in fig. 1. With respect to the polarization direction of the laser, the polarizer is perpendicular and the axis of the ¼ it plate is at 45 °. Another simple way to avoid this unwanted effect is to rapidly reduce the amplitude of the successive terms by introducing a neutral density filter between the laser and the beam splitter. A filter of 3 to 5 dB is sufficient to give a clean counting of the frequency f only. I f the laser output contains several longitudinal modes separated by ½ c/L, their lowest beat frequency will be, in our case, of the order of 500 M H z outside the band pass of the electronics used and finally each
longitudinal modes gives the same result as far as the Doppler shift beat frequency is concerned. A preamplifier, following the photodiode, gives a signal amplitude of about one volt before going to the pulse shaping unit. The - 10 V pulses from the shaping unit have a O. I psec risetime anad are directly counted b y the multichannel analyzer, having its address synchronized with the drive. The number N of address advance cycles is counted by another scaler. F r o m the counts C and the opening time At of each channel one gets the frequency f = C/(N . A t ) .
(3)
I f At is unknown, one has first to count a precise standard frequency to get At from the expression (3). Combining the expressions (2) and (3) one gets
Ivl = {2/(2 At n)} C/N.
(4)
For the He-Ne laser 2 = 6328.15 A. This expression gives an absolute velocity calibration for each channel. In contrast with t~ae earlier optical techniques, the laser method has some advantages: 1. The optics is very simple (a mirror and a coverglass beam splitter). 2. It is largely insensitive to the transverse motions of the transducer. 3. It is simple to align and operate. The described laser method is a very convenient and precise tool for the absolute velocity calibration of M/Sssbauer spectrometers. References 1) w. Kfindig, H. BOmmel, (3. Constabaris and R. H. Lindquist, Phys. Rev. 142 (1966) 327. 9) j.j. Spijkerman and F.C. Rucgg, Bull. Am. Phys. Soc. 9 (1964) 486; J. J. Spijkerman, D. K. Snediker, F. C. Ruegg and R. Devoe, Nat. Bur. Std. Misc. Publ. 260-13 (1967) 9. 3) H. de Waard, Rev. Sci. Instr. 36 (1965) 1728.
(I969) I65-I66;
©
NORTH-HOLLAND
P U B L I S H I N G CO.
LASER METHOD FOR ABSOLUTE VELOCITY CALIBRATION OF MOSSBAUER SPECTROMETERS J . P . BISCAR, W. KI]NDIG, H. BOMMEL and R. S. H A R G R O V E
Department of Physics, University of California, Los Angeles, California 90024, U.S.A. Received 30 May 1969 The velocity of a Mrssbauer drive is converted into a beat frequency of Doppler shifted lines of a laser beam. The counting of this frequency allows a precise velocity calibration.
pattern" has been described3). Both of these techniques require sophisticated optical parts and precise alignments which probably impair the generalization of their use. In this paper an easy and convenient optical technique using a laser beam is described. A He-Ne laser (Electro-Nuclear Laboratories) having an optical cavity length of L = 33 cm and a linearly polarized output is used. The overall configuration is shown in fig. 1. A small mirror M is mounted beside the source on the MiSssbauer drive; it reflects the laser beam parallel to itself. The only alignment to be made is to place parallel the mirror M and the output mirror of the laser. This is achieved when the multiple beams reflected back and forth between the two mirrors coincide on the same spot on the mirror M. Visually one can identify up to 8 or 10 successive reflections which make the alignment trivial. A beam splitter attached to the laser reflects part of the light to a photodiode (S. G. D. 100 from EGG). Taking advantage of the coherence of the laser beam, it is through the Doppler shift of the light at each reflection on the mirror M that the velocity of the drive
Most M6ssbauer spectrometers are using an electromechanical transducer which moves the source relative to the absorber in any given velocity function; e.g. in the constant acceleration drive 1) the velocity function is a triangle or a sawtooth. The pulses from the p r a y detector are stored in a multichannel analyzer according to the velocity of the drive. One difficulty in this type fo spectrometer is the velocity calibration. Usually this is done by measuring spectra of various absorbers with known line positions. From the position of these lines and the assumed linearity of the drive the spectrometer is calibrated. For more accuracy one must use an optical technique. A fringe counting device, to measure directly the velocity in terms of the standards of length (wavelength of the green line of 198Hg ) and time, has been developed by the National Bureau of Standards2). Basically it is a Michelson interferometer using two corner cube reflectors and a K6ster's prism for a beam splitter. A198Hg lamp provides a known wavelength, the fringes are detected by a photomultiplier and their frequency is compared to a standard frequency. Another optical method, based on a "variable moir6
REAM SPLITTER
x/4
TRANSDUCER
-
-
-/
~
i'
~
H
e Ne LASER -
POLARIZER FEEDBACK AMPLIFIER TRIANGULAR WAVE GENERATOR
PULSE GENERATOR
IsD-';P'--I °
IF--
ADDRESS DATA [ ~+#2V PULSE 1 - ] REGISTER REGISTER--~ j, 1220#ZSHAPER ~ i
q
½
MULTICHANNEL ANALYZER
',
#'
L2NU06 ~"
,
J
Fig. 1. General configuration for an absolute velocity calibration of a constant acceleration M0ssbauer drive by means of cw laser beam.
165
166
J . P . BISCAR et al.
is converted into frequency. Each photon of frequency v undergoes a Doppler shift when reaching the mirror moving at the velocity v and again the same shift when it is re-emitted back by the mirror. At each reflection by M the total Doppler shift is (I)
Av = 2nvv[c,
where c is the velocity of light in vacuum and n the index of refraction of air. After the first reflection on the mirror M, part of the beam of frequency v + A v , is deflected to the photodiode, while the other part is reflected again to M by the output mirror of the laser and undergoes a new Doppler shift etc. The photodiode receives therefore the following frequencies: v,
v+Av,
v+2Av,
v + 3 A v ....
The Doppler shift is detected as a beat frequency f between two successive terms f = (2 Ivl n)/it.
(2)
Harmonic beat frequencies at 2f, 3f, ... are also possible. One way to eliminate the harmonics is to mix only the first two terms v and v + A v. For this one has to use a ¼ it plate and a polarizer between the beam splitter and the mirror M in the order shown in fig. 1. With respect to the polarization direction of the laser, the polarizer is perpendicular and the axis of the ¼ it plate is at 45 °. Another simple way to avoid this unwanted effect is to rapidly reduce the amplitude of the successive terms by introducing a neutral density filter between the laser and the beam splitter. A filter of 3 to 5 dB is sufficient to give a clean counting of the frequency f only. I f the laser output contains several longitudinal modes separated by ½ c/L, their lowest beat frequency will be, in our case, of the order of 500 M H z outside the band pass of the electronics used and finally each
longitudinal modes gives the same result as far as the Doppler shift beat frequency is concerned. A preamplifier, following the photodiode, gives a signal amplitude of about one volt before going to the pulse shaping unit. The - 10 V pulses from the shaping unit have a O. I psec risetime anad are directly counted b y the multichannel analyzer, having its address synchronized with the drive. The number N of address advance cycles is counted by another scaler. F r o m the counts C and the opening time At of each channel one gets the frequency f = C/(N . A t ) .
(3)
I f At is unknown, one has first to count a precise standard frequency to get At from the expression (3). Combining the expressions (2) and (3) one gets
Ivl = {2/(2 At n)} C/N.
(4)
For the He-Ne laser 2 = 6328.15 A. This expression gives an absolute velocity calibration for each channel. In contrast with t~ae earlier optical techniques, the laser method has some advantages: 1. The optics is very simple (a mirror and a coverglass beam splitter). 2. It is largely insensitive to the transverse motions of the transducer. 3. It is simple to align and operate. The described laser method is a very convenient and precise tool for the absolute velocity calibration of M/Sssbauer spectrometers. References 1) w. Kfindig, H. BOmmel, (3. Constabaris and R. H. Lindquist, Phys. Rev. 142 (1966) 327. 9) j.j. Spijkerman and F.C. Rucgg, Bull. Am. Phys. Soc. 9 (1964) 486; J. J. Spijkerman, D. K. Snediker, F. C. Ruegg and R. Devoe, Nat. Bur. Std. Misc. Publ. 260-13 (1967) 9. 3) H. de Waard, Rev. Sci. Instr. 36 (1965) 1728.