228
C. McCammon
5.2.4.1 Conventional and Point Source Comparison
Samples with diameters as small as 2 mm can be measured with a conventional
source if sufficient time is available for the measurement. Synthetic clinopyroxene
with composition (Fe 0.1 Mg 0.9 )(Ca 0.7 Li 0.3 )Si 2 O 6 was prepared from a glass starting
material at 3 GPa and 950 °C in a piston-cylinder apparatus for 18 h. Iron with
natural abundance of
57 Fe (2.14%) was used. A sample holder was constructed from
1 mm thick lead sheet with a 2 mm diameter hole into which 6 mg of powder sample
was placed, giving a dimensionless effective sample thickness of 2 (Sects. 5.4.2 and
5.4.4). The same sample mount was measured using a conventional source and point
source, where the source-sample distance was adjusted for each measurement to give
appropriate solid angles (Sect. 5.4.1). Data were collected for approximately 24 h
each.
The Mössbauer spectra of clinopyroxene from the conventional and point sources
show substantially different signal to noise ratios (SNR) (Fig. 5.3). Assuming Poisson
counting statistics, the SNR can be determined from:
SNR =
(N ∞ − N 0 )
√
N ∞
(5.2)
where N ∞ and N 0 are the baseline counts and counts at resonance (minimum counts
in spectrum), respectively, e.g., [1]. The main reason for the difference in SNR is
the much higher specific activity of the point source (154 GBq/cm
2 or 4.2 Ci/cm
2 )
compared to the conventional source (3.5 GBq/cm
2 or 0.1 Ci/cm
2 ) at the time of
measurement. While the overall count rate for the conventional source is lower, this
can be compensated for by extending the measuring time. The SNR of the point source
Fig. 5.3 Room temperature Mössbauer spectra of synthetic (Fe 0.1 Mg)(Ca 0.7 Li 0.3 )Si 2 O 6 clinopyroxene a collected with a conventional source and b point source. The same sample mount was
used for each measurement and the solid angles and measuring times were similar. The baseline
counts for the conventional and point source spectra are 2 × 10 5 and 2 × 10 6 counts, respectively,
while the respective SNRs are 10 and 58
C. McCammon
5.2.4.1 Conventional and Point Source Comparison
Samples with diameters as small as 2 mm can be measured with a conventional
source if sufficient time is available for the measurement. Synthetic clinopyroxene
with composition (Fe 0.1 Mg 0.9 )(Ca 0.7 Li 0.3 )Si 2 O 6 was prepared from a glass starting
material at 3 GPa and 950 °C in a piston-cylinder apparatus for 18 h. Iron with
natural abundance of
57 Fe (2.14%) was used. A sample holder was constructed from
1 mm thick lead sheet with a 2 mm diameter hole into which 6 mg of powder sample
was placed, giving a dimensionless effective sample thickness of 2 (Sects. 5.4.2 and
5.4.4). The same sample mount was measured using a conventional source and point
source, where the source-sample distance was adjusted for each measurement to give
appropriate solid angles (Sect. 5.4.1). Data were collected for approximately 24 h
each.
The Mössbauer spectra of clinopyroxene from the conventional and point sources
show substantially different signal to noise ratios (SNR) (Fig. 5.3). Assuming Poisson
counting statistics, the SNR can be determined from:
SNR =
(N ∞ − N 0 )
√
N ∞
(5.2)
where N ∞ and N 0 are the baseline counts and counts at resonance (minimum counts
in spectrum), respectively, e.g., [1]. The main reason for the difference in SNR is
the much higher specific activity of the point source (154 GBq/cm
2 or 4.2 Ci/cm
2 )
compared to the conventional source (3.5 GBq/cm
2 or 0.1 Ci/cm
2 ) at the time of
measurement. While the overall count rate for the conventional source is lower, this
can be compensated for by extending the measuring time. The SNR of the point source
Fig. 5.3 Room temperature Mössbauer spectra of synthetic (Fe 0.1 Mg)(Ca 0.7 Li 0.3 )Si 2 O 6 clinopyroxene a collected with a conventional source and b point source. The same sample mount was
used for each measurement and the solid angles and measuring times were similar. The baseline
counts for the conventional and point source spectra are 2 × 10 5 and 2 × 10 6 counts, respectively,
while the respective SNRs are 10 and 58