Radiation Measuremtn~s, Vol. 23. Nos 213. pp. 519-522. 1994
Comricht (0 1994 Elsevier Science Ltd
Rinted~iiGrarBritein.All
rights resend
135&4487/94 s7.w + .oo
A COMPACT DESIGN FOR MONOCHROMATIC
OSL
MEASUREMENTS
IN THE WAVELENGTH
RANGE
380-1020 NM
L. J&TIER-JENSEN,N. R. J.
POOLTON, F.
W~UJMSENand H. CHRUIIANSEN
Rise National Laboratory, DK-4000 Roskilde, Denmark
Abstract-The
development and performance of a compact module is described that allows for the
monochromatic illumination of samples in the wavelength range 380-1020 nm, enabling the measurement
of energy-resolved optically stimulated luminescence. The unit is designed to couple directly to the existing
automated Riser TL/OSL dating apparatus, thus allowing for either routine scanning or more detailed
therm@optical investigations. The high throughput efficiency of the unit means that the existing 75 W
tungsten-halogen lamp can be directly used for such measurements on both quartz and feldspar samples.
The design allows for rapid spectral scanning with a choice of resolution of anywhere between 10 and
80nm: stray light levels are less than 0.01%. The unit can equally be used for recording waveiengthresolved emission spectra, whether photo-excited or thermally stimulated; the capabilities of the system
are demonstrated in the article.
1. INTRODUCTION
2. CONSTRUCTION
THE USE of optically
stimulated
luminescence
(OSL) in dating applications is now well established,
and a number of different stimulation sources are
now commonly used. These include the 514nm
argon-ion laser emission (Rhodes, 1988), infrared
emissions from light emitting diodes (BetterJensen et al., 1991) and broad band emitters such as
incandescent or arc lamps, in conjunction
with
carefully selected optical filters (Spooner and
Questiaux, 1990; Better-Jensen and Duller, 1992).
Ideally, however, the spectral emission and excitation
characteristics
of quartz and feldspar materials
prepared for dosimetric evaluation would be routinely scanned since, apart from giving valuable
information
on the mineralogy,
it would also
allow the possibility of choosing the most suitable
energy windows in which to carry out the
measurements.
We show in this article that high quality excitation
and emission spectra of quartz and feldspars can
easily be obtained using a simple unit that directly
attaches to the OSL unit of the automated Rise
TL/OSL reader (BPrtter-Jensen and Duller, 1992)
using as an excitation source the 75 W tungsten
halogen lamp already incorporated. As described, the
instrument is perhaps most closely related to the
rapid scanning interference filter monochromator
constructed by Bailiff et al. (1977).
NEW RIS0
AND DESIGN OF THE
MONOCHROMATOR
By using a variable interference iilter as the
wavelength
dispersive
element,
very compact
monochromator
systems can be designed having
high light throughputs and enabling very rapid
spectral scanning.
The instrument
we present
here can be used for a large range of different
experiments either as a stand alone unit, or attached
directly to the automated TL/OSL dating apparatus.
The design is based around the use of two filters
covering the spectral range of interest (the first
usable in the range 380-75Onm and the second
from 740-1020 nm). For UV/visible
measurements, the Schott filter VERIL S-60 was used which
gives spectral resolution of 1Onm across the entire
range, transmission factors of around 35% and
blocking to better than 0.01%. (The full characteristics are discussed in more detail below.) The
infrared filter was specially designed and built for
us by Barr Associates, and has a similarly high
performance (spectral resolution of 9nm, transmission 60%).
We have found that one advantage of splitting the
spectral range is that UV/visible emissionspectra can
be recorded at the same time as producing variable
wavelength, monochromatic excitation, thus giving a
complete luminescence
excitation/emission
spec
trometer that plugs straight onto the automated Rise
519
L. BIFITER-JENSEN
520
reader. A demonstration of the capabilities is given in
Section 3.
In Fig. l(a), we show a detailed cross-section of the
construction,
demonstrating
schematically
in
Fig. I(b) how it fits onto the existing dating apparatus; it adds only 4cm to the overall dimensions of
the system, since all the focusing and collimating
optics are placed in spaces already present. In the
excitation mode, collimated light from the 75 W
tungsten halogen lamp (or external source) is focused
onto a slit, behind which the filter is scanned; the
emerging monochromatic beam is then recollimated,
and finally refocused onto the sample. Our construction allows the slit width to be continuously adjusted
from 0 to IOmm, allowing flexibility in choice of
spectral resolution down to 80 nm: it should be
pointed out that with slit widths less than 1 mm, no
increase in resolution is achieved, since the limiting
factor then becomes the filter bandwidth (in our case,
around 10 nm). The filter is mounted on a precision
linear rail and is moved by means of a small stepper
motor: this configuration allows for high accuracy in
the positioning. In our system, the travel speed is
restricted to between 0 and 30 rim/s (at 0.14 nm per
step) for the visible system, or 50 rim/s (at 0.23 nm per
step) with the infrared filter, giving, in each case,
minimum scan times of around 15 s for the entire
spectral range.
In Fig. 2(a), we show the transmitted spectrum
from a high pressure mercury arc source, a He-Ne
laser and the output of a calibrated high resolution
spectrometer set at 880nm. We use this diagram to
highlight a number of points. Firstly, since the variation away from linearity of wavelength vs position
INTERFERENCE
SLIT
FILTER
I.R. 1
‘SAMPLE
EXTERNAL
SOURCE
DIODES
Fw. 1. Detailed cross-section of the Rise monochromator
(A) and a schematlc representation (B) of how it can be used
on the dating apparatus in either detection (mono 1) or
excitation
(mono
2) mode.
er al.
HeNe
400
600
H.Res.Mon.
8&I
1000
b
00400
00
400
600
800
1000
600
800
1000
WAVELENGTH
(nm)
FIG. 2. Technical aspects of the monochromator:
(a) resolution, linearity and stray tight rejection using a number of
sources; (b) transmission factors of the linear filters; (c)
demonstrating their use as an absorption spectrometer; (d)
the power throughput of the system when coupled to a 75 W
tungsten-halogen source.
along the filters is very small (certainly within the
spectral bandwidth), the wavelength setting can be
directly measured by counting the number of steps
advanced by the stepper motor (each step representing around 0.1 nm). Secondly, it will be noted that the
spectral bandwidth (at 10nm) is nearly constant
along both the filters. Thirdly, the laser line can be
used to determine the stray light levels of the system.
We have determined that at 450 nm, transmission of
the 633 nm laser line is lower than 0.005%, although
this increases to around 0.05% at 580 nm: similar
figures are measured for stray light throughputs of
the infrared filter.
The transmission characteristics for the filters are
shown in Fig. 2(b). Note that they are virtually
spectrally neutral (unlike, for example, diffraction
gratings), which makes correction to spectra for
instrumental response straightforward. For the “visible” filter, transmission changes from 31 to 38%
between 400 and 700 nm, respectively, and for the
infrared system, from 50 to 60% in the range
750-980 nm. These figures are measured using f;!
optics with two monochromators working in tandem,
A MONOCHROMATOR
the first acting as an absorption spectrometer and the
second as the “sample”. Using a similar configuration, we show in Fig. 2(c) the transmission characteristics of the two broad band interference filters
commonly used in OSL experiments.
Finally, in Fig. 2(d), the power throughput of the
system is shown, when linked to the 75 W
quartz-tungsten-halogen
lamp normally used for
“broad band” excitation of OSL. We have installed
a rear reflecting mirror behind the lamp, thereby
increasing the illumination intensity by around 90%.
and giving better filling of the monochromator slit.
The power ranges from 0.01 to 1.1 mW/cm’/nm in the
range 4QO-1000 run, following quite closely the blackbody curve for an emitter at 3400 K: the power
transmittance is not entirely smooth due to the use of
interference filters for heat rejection. Measurements
were made with a calibrated pyrometer. Whilst the
power throughput
may appear low, we have
never found that there is a shortage of excitation
intensity, and the fall off in power below 5OOnm is
in fact a great advantage, since the efficiency of
exciting trapped charge increases very rapidly at the
shorter wavelengths; the two effects therefore partly
compensate, making repeated scans possible whilst
only marginally affecting the trapped charge population. This effect is described in more detail in
Section 3.
3. EXPERIMENTAL
521
FOR OSL MEASUREMENTS
CAPABILI-ITES
3.1. Thermoluminescence emission spectra
As mentioned in Section 2, the particular construction we have chosen means that the spectral range
380-700 nm can be scanned in around 15 s, taking a
further 25 s to reset. This consequently limits the
heating rates usable if the TL emission spectra are to
be continuously recorded: for example, a ramp rate
of l”C/s would give a temperature resolution of 15°C
to the spectra with a repetition measurement every
40°C. Naturally,
better resolution, etc. can be
achieved either by slowing the ramp rate further, or
employing a faster motor. However, analysis of the
TL emission spectra previously published (Luff and
Townsend, 1993) shows that in general (at least for
silicate-type minerals), there are not usually rapid
changes of spectrum in the high temperature regions.
Useful thermoluminescence emission spectra can be
taken with our system, as demonstrated in Fig. 3(a)
discussed in Section 3.1. In this case, however, the
difficulties are slightly easier to overcome, since the
spectra are easily compensated if the time decay
characteristics of the OSL are known. Further, the
fall in intensity of the OSL signal can be partly
compensated by scanning the luminescence spectrum
from high to low wavelengths, thereby making use of
the rise of PM tube detection efficiency as the signal
decays. Erasing the rapidly decaying initial OSL
signal prior to measurement can also simplify correction procedures. Figure 3(b) shows a typical OSL
emission curve for the same sample as Section 3.1.
3.2.2. Time-stable PL. By illuminating
many
feldspar samples with deep-UV light, time-stable
photoluminescence can often be stimulated. We have
found the simplest method of exciting this type of
signal is to use the 75 W quartz-tungsten-halogen
lamp in conjunction with U-340 filters. Although the
UV output is very low, the luminescence e!Bciency is
often orders of magnitude higher than the OSL
signal. An example of such a spectrum is shown in
Fig. 3(c) for the same sample as used in Fig. 3(a): as
will be noted, many more luminescence transitions
are probed in this kind of measurement.
1
A
1
a
‘,! ,&lyI, ,
400
so0
600
loo0
C
0:.
I
400
8
I
600
.
I
800
.
1
loo0
for a museum specimen of Na feldspar.
3.2. Photoluminescence emission spectra
3.2.1. Time-decaying OSL. Comparison of the TL
and OSL emission spectra can be very important if
comparison of the dosimetric information coming
from the two techniques is to be made. Since the OSL
is obviously time dependent, the problems of scan
rate and repetition cycle are essentially the same as
WAVELENGTH
(nm)
FIG. 3. Demonstration of the capabilities of the monochromator system as applied to a sample of high albitc: (a) TL
emission spectrum; (b) OSL emission spectrum; (c) photoluminescence emission spectrum; (d) OSL excitation spectra.
Detection (D) or excitation (E) wavelengths are as indicated.
522
L. BmER-JENSEN
3.3. Luminescence excitation spectra
3.3.1. Time-decaying OSL. The original work of
Hutt et al. (1988) demonstrated how important it is
to analyse the photoexcitation
spectra of feldspar,
and our own work (Poolton et al., 1994) shows that
detailed knowledge of the excitation spectra of natural samples provides essential information on the
mineralogy. The monochromator
described in this
article is ideally suited for producing such spectra.
As the OSL signal decays under constant illumination, consideration of procedures for correcting the
excitation spectra produced must necessarily be
made. This is a major problem if high power excitation sources are used (e.g., tunable laser systems)
especially in multi-mineral samples, since the bleaching efficiency as a function of intensity and wavelength becomes complex. In our system, however, we
have been able to largely overcome these problems by
using a low power (75 W) tungsten-halogen
lamp as
the excitation source. Firstly, the output powers are
quite low (see Section 2) and the intensity falls off
rapidly, just as the photon capture cross-section for
the trapped charge is rising rapidly. As a consequence, during a typical wavelength-resolved OSL
excitation scan, only around 10% of the trapped
charge is evicted; in order to produce stimulation
spectra of the sample, therefore, only the intensity
spectrum of the exciting lamp is required, and is not
a running analysis of how much charge is lost. A
routine check of the validity of this method can be
made by repeatedly scanning the stimulation spectrum; the method is valid as long as the excitation
spectra remain the same from one cycle to the next.
An example of the OSL stimulation spectrum from
feldspar is shown in Fig. 3(d), where comparison can
be made with the emission from the same sample
(Fig. 3(a)-(c)). We have found that, in contrast to
feldspar, the stimulation efficiency for quartz is exponential with energy (Bnrtter-Jensen er al., 1994).
3.3.2. Time-stable PL. Whilst the energy-resolved
stimulation spectrum of the time-decaying OSL emission of the sample probed the energy levels of the
“donor” type defect (within a donor-(conduction
band)-acceptor
recombination
cycle), the energy
levels of the acceptor type defect can be probed by
scanning the stimulating wavelengths of the timestable, Stokes shifted photoluminescence. Whilst such
measurements are possible with our system, they
usually require highly photosensitive material.
er al.
4. CHOICE OF EXCITATION WAVELENGTH
AND INFLUENCE ON DOSlMETRIC
EVALUATION
Eventually, the main concern for dating is that the
most suitable choice of stimulation wavelength is
made in the OSL measurements. The instrument
described here allows the entire stimulation spectrum
to be scanned with very little loss of charge, so the
dose-dependent growth curves can be easily measured
for all stimulating wavelengths (400-1OOOnm). In
practice, we have found very little difference across
this range (Btiter-Jensen er al., 1994).
Acknowlcdgemcnrs-We are most grateful to Mr C. Karpof
Barr Associates Inc. for designing the infrared interference
filters and to Dr 0. Johnsen of Copenhagen University for
the loan of samples. This work is funded under CEC
contract SCl-CT92-0800and the Institute of Earth Studies,
Arhus University.
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