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The Chandrayaan-2 Large Area Soft X-ray Spectrom-
eter (CLASS)
Conference Item
How to cite:
Radhakrishna , V.; Narendranath, S. ; Tyagi, A. ; Bug, M. ; Unnikrishnan, U. ; Kulkarni, R. ; Sreekantha, C.
V. ; Kumar, ; Balaji, G. ; Athiray, P.S. ; Sudhakar, M. ; Manoj, R. ; Chetty, S. V. ; Thyagaraj, M. R. ; Howe,
C. ; Gow, J. and Sreekumar, P. (2011). The Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS).
In: 42nd Lunar and Planetary Science Conference, 7-11 March 2011, Houston, TX, USA.
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THE CHANDRAYAAN-2 LARGE AREA SOFT X-RAY SPECTROMETER (CLASS). V.Radhakrishna
1
, S.
Narendranath
1
, A. Tyagi
1
, M. Bug
1
, U. Unnikrishnan
1,2
, R. Kulkarni
1
,
C.V. Sreekantha
1
, Kumar
1
, G. Balaji
1
, P.S.
Athiray
1
,
M. Sudhakar
1
, R. Manoj
1
, S. V. Chetty
1
, M. R. Thyagaraj
1
, C. Howe
3
, J. Gow
4
, P. Sreekumar
1
, (email:
rkrish@isac.gov.in)
1
ISRO Satellite Centre,
2
University of Calicut,
3
Rutherford Appleton Laboratory, UK,
4
Open
University, UK.
Introduction: The CLASS experiment on Chan-
drayaan-2, the second Indian lunar mission, aims to
map the abundance of the major rock forming elements
on the lunar surface using the technique of X-ray fluo-
rescence during solar flare events. CLASS is a continu-
ation of the successful C1XS [1] XRF experiment on
Chandrayaan-1. CLASS is designed to provide lunar
mapping of elemental abundances with a nominal spa-
tial resolution of 25 km (FWHM) from a 200 km polar,
circular orbit of Chandrayaan-2.
C1XS was developed at Rutherford Appleton La-
boratory, UK, in collaboration with ISRO and operated
for ~ 9 months in orbit. Although the instrument per-
formed very well [2], the lack of adequate solar activity
and a reduced mission life, prevented global coverage.
However, there are several observations that have
yielded good XRF data from C1XS.
Science objectives: The science objectives of
CLASS are to make global studies on the diversity and
distribution of lunar lithologies, quantitative estimate
of Mg abundance, essential for determining the distri-
bution of Mg suite rocks, bulk composition of the
crust, abundance patterns in the major crustal provinces
and mare basalt diversity. In addition to this regional
studies on the composition of central peaks of craters,
composition of large scale ejecta around basins, and to
study dark halo craters and possible associated crypto-
mare.
Results from C1XS indicate that the lunar highland
composition may contain more sodic plagioclase than
previously thought [3]. CLASS is hence designed to
operate down to 0.8 keV in order to also measure the
possible XRF signals from Na in addition to the majpr
elements Mg, Al and Si. Measurement of Ca, Ti and Fe
at the proposed spatial resolution can be achieved only
during strong flares; CLASS is expected to provide the
first distribution maps from direct chemical abundance
studies.
CLASS has a geometric area three times that of
C1XS which would enable it to measure lunar XRF
signals down to B flares promising better coverage
even if solar activity remains low.
CLASS Instrument: Our experience with C1XS
has proven swept charge devices as good choice for
experiments which demand large area (without imag-
ing) and require good spectral resolution even at tem-
peratures of -20 C (hence passive cooling is adequate).
SCDs of 1cm
2
area, CCD-54, manufactured by e2V
technologies, were used in C1XS. In CLASS, we pro-
pose to use large area SCDs (~20 x 20 mm
2
), sixteen of
which providing a total geometric area of 64 mm
2
.
From an orbital height of 200 kms, the collimators
placed over the detectors define a field of view of 14 x
14ÛNP):+0DWthe lunar surface. The collima-
tors over approximately one-fourth of the array are
designed to have a smaller field of view which would
yield a spatial resolution of 12 km on the lunar surface.
This would enable high resolution mapping during
strong solar flares. The CLASS payload is designed to
be operated in the range of -15 C to -25 C in the lunar
orbit using only passive cooling. A door is provided in
front of the detectors for protection of the SCDs from
high energy particles during transit through the radia-
tion belts. Thin aluminium foils were used as visible
light shields on C1XS. To prevent any contribution
from Particle Induced X-ray Emission (PIXE) from the
Al foil, we plan to use Be foils in CLASS. The sche-
matic of the CLASS payload is shown in Fig.1.
Figure 1. CLASS instrument showing the four qua-
drants with four SCDs each. The electronics is housed
in the box behind the detector units. An aluminum door
protects the detectors from radiation damage en-route
to the Moon. Passive radiators connected to heat pipes
provide the required low-temperature environment for
the detectors.
Large area SCDs, with larger pixel size and two
phase clocking are expected to give good spectral reso-
lution (< 200 eV @ 6 keV) and low energy threshold
(~ 0.8 keV). This has already been demostrated in the
laboratory with a CCD-236 device manufactured by
e2V Technologies. A comparison of spectra recorded
1708.pdf42nd Lunar and Planetary Science Conference (2011)
for CCD-54 used in C1XS and that from a large area
SCD, is shown in Fig.3. It is seen that the large number
of partially absorbed (split) events recorded for CCD-
54, is significantly reduced in the large area SCD. The
reduced fraction of split events in the large area SCD is
due to its larger pixel size. This is significant and one
can expect improved resolution and higher sensitivity
in CLASS.
Figure 2. CLASS electronics block diagram
Electronics: The electronics is packed behind the
detector unit into four PCB cards. The large area SCD,
is a single output, diagonally-read x-ray CCD with 2
phase clocks and is non-imaging. Charge generated by
an event is continuously clocked (~ 100 kHz) and
transported along the buried channels towards a node
to generate a voltage pulse. The output from an SCD is
similar to a conventional CCD and a correlated double
sampling (CDS) circuit is used for noise reduction be-
fore feeding the signal into ADC. The signal generation
and data processing is done using Actel’s RTAX-
1000S FPGA based system. The temperature interface
provides the continuous monitor of the SCD tempera-
ture for operation in favorable temperature range using
onboard logic. While spacecraft telecommand interface
provides different commands for ON / OFF and mode
of operation Telemetry interface is used to monitor
critical parameters of the payload. Base band data han-
dling interface is used to transfer the science data from
the spacecraft Solid State Recorder.
Performance: The large area SCDs on CLASS are
an improved version of that on C1XS. They operate
with 2 phase clocking and have larger pixels thus re-
ducing split events. Figure 3 shows spectra obtained
with C1XS SCDs compared to that with CLASS SCDs.
The fraction of events outside the photopeak is consi-
derably reduced and detection efficiency at the photo-
peak increased. Photopeak fraction for the recored
spectra increased by a factor of two for the large area
SCD compared to CCD-54.
Figure 3. Large area SCD(CCD-236) spectrum along
with C1XS SCD (CCD-54) spectrum showing lesser
contribution from split events to the photopeak.
Conclusions and future work: CLASS is cur-
rently under development at the Space Astronomy
Group, ISRO Satellite Centre, India. It is expected to
provide the first moderate resolution (25 km or better)
global map of lunar chemistry.
References:
[1] Grande, M. et al. (2007) PSS, 55, 494-502. [2]
Howe et al (2009) PSS, 753. [3] Narendranath et al
(2010) submitted to Icarus. [4] Tyagi, A. et al (2010)
Nat. Symp. Nucl.Instrum Proceedings, 593-595.
Split events
High
capacitive
load
driver
circuit for
clock and
Reset
Power Interface Module
DC-DC power supply
clock driver
voltages
SCD
voltages
Analog front end
and CDS voltages
CDS based ADC
Telemetry Telecommand Interface
Baseband Data
Handling Interface
FPGA system
Data
processing
and
reduction
Clock and
reset
generation
circuit
Handshaking
signal
generation for
CDS and ADC
SCD 16
SCD 1
Temperature sensor
interface with ADC
Wideband
Analog read out
circuit
FPGA
voltages
1708.pdf42nd Lunar and Planetary Science Conference (2011)
