The IBEX background monitor
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Allegrini, F. et al. “The IBEX Background Monitor.” Space
Science Reviews 146.1-4 (2008) : 105-115. © 2008 Springer,
Part of Springer Science+Business Media
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http://dx.doi.org/10.1007/s11214-008-9439-8
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The IBEX Background Monitor
F. Allegrini1, G. B. Crew2, D. Demkee1, H. O. Funsten3, D. J. McComas1, B.
Randol1, B. Rodriguez1, N. A. Schwadron4, P. Valek1, S. Weidner1
1
Southwest Research Institute, San Antonio, Texas
2
MIT Kavli Institute for Astrophysics and Space Research, Cambridge, Massachusetts
3
Los Alamos National Laboratory, Los Alamos, New Mexico
4
Boston University, Boston, Massachusetts
Abstract
The IBEX Background Monitor (IBaM) provides a small and lightweight method for independently measuring IBEX’s high-energy proton background, by integrating the flux of > ~ 14 keV protons over a ~7º conical FOV. The IBaM is part
of the IBEX-Hi sensor and has a co-aligned look direction. This paper describes
the principle of the IBaM and details its design and responses. In particular, we
show the response of major components to both ions and ultraviolet (UV) light
background. We also provide the geometric factor and calculate expected count
rates.
1 Introduction
The Background Monitor (IBaM) for the IBEX mission (see McComas et al.,
this volume) quantifies the background environment by making an integral measurement of >~14 keV protons, just above the positive 10 kV potential of the Hi
and Lo collimator/entrance subsystems. The IBaM is part of the IBEX-Hi sensor
(Funsten et al., this volume) and is located close to the detector section. The IBaM
and the IBEX-Hi detector section use identical detectors (channel-electron multiplier, CEM) and share a similar electronics design. The look direction of the IBaM
is aligned with that of the IBEX-Hi sensor, with a comparable field-of-view (~7º
FWHM).
Due to their extreme sensitivity the IBEX sensors will also see numerous
sources of noise and background that are described in Wurz et al. (this volume).
Among these sources, energetic neutral atoms (ENAs) can be created by 1) charge
exchange of plasma ions with outgassing species from the spacecraft or 2) by scattering of high energy ions off the edges of a collimator grid; such ions are a particular concern in the magnetosheath and foreshock regions. The foreshock region
2
refers to the region beyond but magnetically connected to the Earth’s bow shock.
Locally produced ENAs could then masquerade as real ENAs from the termination shock. For both of these sources, the neutral yield is a function of the ambient
ion flux, which can be measured with the IBaM.
By design, the IBEX team sought to minimize the effect of the outgassing species from the spacecraft by venting the internal volumes of the spacecraft only
along the directions of the spin axis. We will also segregate the observations when
the IBEX spacecraft look direction is in the solar wind, magnetosheath, and foreshock regions (see Schwadron et al., this volume). The IBaM will help identify
and quantify time periods with potentially high backgrounds from the local energetic protons. This information is valuable for understanding IBEX backgrounds
by directly measuring ions with energies in the range that can penetrate through
the +10 kV potential on the collimator plates.
Sanderson et al. (1996) show an example of an energetic particle event upstream of the Earth’s bow shock. This particular event was observed by instruments on the Wind spacecraft when it was in the solar wind. The spectrum of upstream events usually follows a power law with power index around -4. Such
events will be identified by the IBaM, making it straightforward to remove them
from the IBEX observations.
2 Background Monitor Principle
The IBaM makes an integral measurement of protons above ~14 keV in a 7º
FWHM field-of-view aligned with the IBEX-Hi viewing direction. The principle
is illustrated schematically in Fig. 1. With energies above ~14 keV (in red), protons with the proper incident direction will go through the collimator and cross
two thick carbon foils (21 and 22 g/cm2 for CF1 and CF2, respectively). The protons will be detected with a channel electron multiplier (CEM). The CEM is biased so that the top of the funnel is at about -1700 V and the anode, where the
electron avalanche is collected, is at ground. This bias design allows us to use an
almost identical design for the electronics as is used on the IBEX-Hi detector (see
Funsten et al., this volume). The grid is attached to the CEM funnel to ensure that
the secondary electrons (in green) created in the funnel are accelerated toward the
throat of the channel rather than back toward the carbon foils, which are at ground.
Most of the protons with less than ~14 keV (in light blue) will not be able to cross
both carbon foils because of the energy straggling in the foils. The foils are also
thick enough to significantly attenuate the UV radiation (in purple), to which
CEM detectors are moderately sensitive. Carbon foils can have pinholes and
thickness variations (Funsten et al, 1992a, 1992b). Because of the potential UV
background from a pinhole in a single carbon foil, we chose to use two. The likelihood that pinholes in two foils are aligned is extremely low. Thus, the impact of
pinholes in the carbon foils is mitigated.
3
Fig. 2 shows a cross section (bottom right) of the IBaM with detailed pictures
of the main components. The CEM grid (bottom left) is affixed onto the MCP
with conductive epoxy EPO-TEK H21D, which is compatible with CEMs’ operation and lifetime (McComas and Bame, 1984). The CEM is attached to a ceramic
board where the high voltage and signal pickup connections are made.
Light tightness, required to ensure that stray light does not produce a background in the IBaM, is achieved at the level of the collimator and carbon foils and
at the bottom of the IBaM housing. A spacer is placed between the carbon foils
with grooves and holes to provide venting for the small volume between them but
a tortuous path for UV photons. The bottom part of the IBaM has a removable baffle (dark green) also for venting through the channel on the right of the IBaM
housing. This channel also provides a venting path for the electrostatic analyzer
and detector subsystems of IBEX-Hi. The channel, removable baffle, and spacer
between the carbon foils are blackened using the Ebonal-C process. The most direct access for UV photons to the CEM is through the collimator and the carbon
foils.
The carbon foils (dark area on the top left picture) are mounted on electroformed nickel grids that have 70 lines per inch for a transmission of 90%. The grids are formed with a 0.005-inch-thick rim, which gives them enough stiffness to
be handled without an additional fixture. The carbon foils were manufactured by
Arizona Carbon Foils.
The collimator (yellow part in the cross section image) consists of a 2.5-mmthick aluminum plate with drilled holes in a close packed pattern. The hole diameter is 0.343 mm, and the spacing between the hole centers is 0.394 mm. There are
2593 holes on a 13-mm x 26-mm area. A detailed electron microscope picture of
the collimator holes is shown in the upper right of Fig. 2. The collimator was electro-polished to remove burrs from machining. The collimator, the grids supporting
the carbon foils, and the front of the CEM funnel all have the same aperture area:
13 mm x 26 mm.
3 Background Monitor Response
3.1 Collimator field-of-view
Measurement of the IBaM field-of-view (FOV) was performed with a 2-mm
diameter, 30 keV H+ beam aimed at the center of the collimator. The IBaM was
oriented using two orthogonal rotational actuators (providing rotation angles alpha
and beta), and the rate was recorded for each orientation. The result of the angular
scan with a resolution (pixel size) of 1° x 1° is illustrated in Fig. 3 where the rate
is color-coded and scaled to a maximum of 100. The maximum intensity or the
4
center of the FOV corresponds to normal incidence on the collimator. This direction is found by calculating the weighted average of the rate as a function of angles. By 1) reporting the rate as a function of angular distance between the center
of the FOV and each pixel (angle theta) and 2) binning these data into 1° bins, we
find the transmission function depicted in Fig. 4. In other words, the data from
Figure 3 of the 3-D angular scan is collapsed and displayed in the form of a histogram in Figure 4.
The orange curve in Fig. 4 is a fit to the data using the following function
T   
2 

2
h
 h
h
 
arccos tan    tan  1   tan   


d
 d
d
 


(1)
which models the transmission of a cylindrical collimator. The FWHM derived
from the fit in Figure 4 corresponds to a height-to-diameter ratio h/d=6.67178 of
the holes in the collimator. The maximum angle for transmission from the center
of this FOV is found when the square root in Eq. (1) is equal to 0:
 max  arctand h   8.52  . The FOV of the collimator is defined by
d  T  d , where     2 1  cos  is the solid angle of a cone of apex angle 2. Using the fact that d  2 sin d we find the FOV of the collimator
 max

 T  2 sin d  0.0175 sr
0
3.2 Energy response
We measured the energy response using an 8-mm-diameter proton beam aimed
at the center of the IBaM aperture. An absolute beam monitor, based on the principle described in Funsten et al. (2005) was used to infer the beam flux impinging
on the IBaM. Hence, we were able to determine the absolute detection efficiency,
, as a function of energy shown in Fig. 5. Both plots show the same data in linear
(on the left) or logarithmic (on the right) representations. The data points and their
error bars are in black.
The red curve is a fit to the data points defined by
0
for E  9 keV


 E    0.125 tanh 0.65E  16   0.1251
(2)
for 9  E  14.214 keV
0.23 arctan0.155E  18.023  0.1451 for E  14.214 keV

where E is in keV. This empirically derived function fits the data quite well.
The efficiency asymptotically tends to a maximum of 0.506. This efficiency curve
includes the grid and carbon foil transmissions (CF1, CF2), the collimator transmission, and the CEM detection efficiency up to ~50 keV, but it does not include
5
the CEM detection efficiency at higher energies. The CEM detection efficiency
for ions above 50 keV decreases very slowly as a function of energy, so neglecting
the variation of the CEM detection efficiency has a small impact on the IBaM
measurement. At high energies, the flux of protons decreases very rapidly as
shown in Sanderson et al. (1996). The contribution of protons with energies larger
than 50 keV for a power law distribution with power index of -4 is significantly
less than between 10 and 50 keV.
3.3 Active area
The active area is defined as the surface area of the sensor aperture where particles can be detected. There can be variations of the efficiency across the active
area, but it is usually independent of the energy of the incoming particles. In the
case of the background monitor, the efficiency across the area depends on the
energy of the protons. There are at least two reasons for this energy dependence.
First, the protons scatter when they pass through the carbon foils. Those that scatter significantly might miss the CEM funnel and remain undetected. The scattering
half-width is inversely proportional to the incident energy of the protons so that
scattering increases with decreasing energy. As a consequence, protons entering
the IBaM at the center of the active area with a given energy will be more efficiently detected than the ones that enter the IBaM at the edge of the active area.
Second, the detection efficiency of the CEM not only depends on the energy of the
protons but also on the direction of incidence of the protons and the spatial location where they hit the funnel (Seah and Smith, 1991). These characteristics are
not quantitatively known for the IBaM CEM.
In order to quantify the spatial dependence of the active area, we measured the
relative efficiency variations across the active area at 30 keV with a 2-mmdiameter beam. Fig. 6 shows a color-coded map of the IBaM with 1x1 mm pixels.
We calculate the active area with the following equation
Ri
A  1.3  2.6
cm2
(3)
R
0
i

where 1.3 cm and 2.6 cm are the dimensions of the collimator aperture, R0 is
the average rate in the center of the area for a ~8-mm-diameter beam, Ri is the rate
for pixel i (from the 2-mm-diameter beam), and the sum is done over all the pixels. Equation 3 assumes that all the protons at the center of the active area are detected. Even though this assumption is incorrect because of the scattering, if we
combine this definition of active area with the energy response of the IBaM and its
FOV, the overall result is consistent and leads to the correct estimate of the geometric factor given below. In summary, Equation 3 does not give an area per se,
but accounts for the detection efficiency variations across the active area. Applying Equation 3 to our measurements, we find A=1.25±0.10 cm2 at 30 keV.
6
We do not have measurements at other energies to determine the energy dependence of the active area defined here. To overcome the lack of measurements
and still predict the efficiency variations across the active area, we developed a
simple model of the effect of angular scattering of the proton in the two thick carbon foils. In particular, we looked at the fraction of protons that would hit the
CEM funnel aperture after being scattered in the foils. This very simple model,
based solely on geometrical considerations, lets us understand the basic relationship between the active area and the energy. Fig. 7 shows the fraction of protons
hitting the CEM funnel as a function of energy (left) and the same function scaled
such that the model curve passes through the data point at 30 keV (right). This
choice is arguable because we do not have enough data points to verify our predictions. However, an error in the estimate of the active area at low energies (around
10 keV) or at high energies (more than ~50 keV) has a minor impact on the IBaM
measurement. At low energies, the driver of the IBaM response (geometric factor)
is the cutoff due to the carbon foils as shown in Fig. 5 with the efficiency curve.
At high energies, we can use the same argument as for the detection efficiency described above: the proton flux decreases very rapidly with energy and its contribution to the IBaM measurement is significantly less than that from the proton flux
between 10 and 50 keV. Therefore, we believe that the assumptions made for the
model of the active area are acceptable. The equation of the green curve in Fig. 7
is
AE   0.71arctan0.13E  4  0.35
(4)
2
where the energy E is in keV and the area A is in cm , with an asymptotic value
of 1.465 cm2.
3.4 Geometric factor
The IBaM reduced geometric factor, G´, is defined as the product of the solid
angle, , with efficiency, , and active area, A, defined above. Since both the efficiency and the active area depend on the energy, the reduced geometric factor also
depends on the energy. Thus, G E    E AE  . Plugging in (2) and (4) for 
and A and the numerical value of , we obtain an analytical expression for the reduced geometric factor:
G E   1.75 10 2  0.71arctan0.13E  4  0.35 
0
for E  9 keV or E  1000 keV








0
.
125
tanh
0
.
65
E
16
0
.
1251
for 9  E  14.214 keV

0.23 arctan0.155E  18.023  0.1451 for 14.214 keV  E  1000 keV

(5)
where E is in keV, and G´ is in cm2 sr. The upper limit of 1000 keV is simply
set for convenience. It does not influence the expected rate calculations since the
flux at these energies is much lower than around 10-50 keV. A more general ex-
7
pression for the geometric factor, G, can be derived such that the units of G are
cm2 sr eV/eV.
G E   G  T E 
(6)
1000
where G=12.34 cm2 sr eV/eV and
 T E dE  1
9
T E  
2
1.75 10
 0.71arctan0.13E  4   0.35 
12.34 E
0
for E  9 keV or E  1000 keV


for 9  E  14.214 keV
 0.125 tanh 0.65E  16   0.1251
0.23 arctan0.155E  18.023  0.1451 for 14.214 keV  E  1000 keV

(7)
3.5 Typical Response and Count Rate Estimate
In order to estimate the count rate that the IBaM is likely to observe in orbit, we
use the spectrum given in Sanderson et al. (1996). The count rate is given by
E max
R
 GE  jE dE
(8)
E min
where Emin and Emax are the limits of the integration (for example, Emin=9 keV
and Emax=1000 keV), G´(E) is given by Eq. (5), j(E) is the differential intensity
and the energy is expressed in keV. Fig. 8 shows the term G´(E)j(E), which
represents the IBaM response for a typical spectrum, as a function of energy. The
response peaks between 20 and 30 keV with a width of ~60 keV, which confirms
that most of the counts that the IBaM will record fall between ~10 to ~50 keV.
The discontinuities in the curve come directly from the spectrum.
The result of the integration is a rate of ~120 Hz, which needs to be compared
with the “intrinsic” background noise of the IBaM. We measured the background
noise of the IBaM over an accumulated period of 19.1 hours during the IBEX
payload cross calibrations and found a rate of 11.5±0.4 mHz. The background
noise is many orders of magnitude smaller than the expected signal.
As described below, the signal from the IBaM will be integrated over ~3 minutes and divided into sixty 6-degree pixels in the sky. If the signal hits the same
pixel at the estimated rate of 120 Hz, we can expect a total number of counts of
the order of 360 events in each of the 60 angular pixels. Clearly these measurements are more than adequate to identify high background intervals for removal
from the IBEX ENA observations.
8
3.6 UV Sensitivity
We tested the response of the IBaM to ultraviolet (UV) light using a krypton
line lamp (Resonance Ltd., model KrLM-L) with two main lines at wavelengths of
116.5 and 123.6 nm and a typical intensity F≥3x1015 photons/(s sr). Taking into
account the intensity of the lamp, the distance between the lamp, the different grids and collimator transmissions, and the IBaM aperture, we calculated a rate of
R=1.2x1012 photons/s that would reach the CEM if there was no carbon foil. The
IBaM measured a rate of ~650 Hz when the UV lamp was shining straight into the
IBaM aperture (also maximum rate). Thus, the attenuation factor (including CEM
detection efficiency) is roughly 650/1.2x1012≈5.4x10-10. Assuming a CEM detection efficiency of about 0.02 at the wavelength of the UV source (Timothy and
Bybee, 1975; Paresce, 1975), we obtain a carbon foil attenuation factor of 5.4x1010
/0.02≈2.7x10-8.
Hsieh et al. (1980) measured the transmittance of UV at wavelengths of 121.6
nm and 58.4 nm through carbon foils between 2 and 14 g/cm2 thick. At 121.6 nm
they found that the transmittance is 0.26 exp(-0.37x) where x is the carbon foil
thickness in g/cm2. By extrapolating to the IBaM total carbon foil thickness of
21+22=43 g/cm2, the predicted attenuation factor is 3.2x10-8. The estimate based
on the extrapolation is fairly close to our measurement given the fact that it is well
beyond the thickness range of their measurements. Since the IBaM FOV is coaligned with the IBEX-Hi FOV, it looks out essentially perpendicular to the
spacecraft’s Sun-pointing spin axis. In this orientation the IBaM never views toward the Sun and will only see minimal counts when it sweeps past particularly
bright UV stars.
4 Operations and data products
Upon consideration of the likely background sources detectable by the background monitor, it was deemed prudent to collect its data in phase with the IBEXHi direct events. This results in a flight telemetry product approximately every
three minutes, which is a reasonable timescale for magnetospheric phenomena.
As with IBEX-Hi, the counts are separated in sixty 6-degree azimuthal bins, which
may provide adequate discrimination among possible background sources.
These count histograms are then compressed and sent to the ground along with
the IBEX-Hi singles rates which are collected on an approximately 12-minute
cycle. Thus, 4 cycles of background monitor data may be directly compared with
the IBEX-Hi singles rates for any 6-degree patch in the sky. Since we do not expect count rates above ~1 kHz, the compression scheme saturates at approximately
1 kHz. However, above this level the saturated compression still identifies the
counts in that pixel as very large (>1 kHz), which is the information needed to cull
the interval.
9
After the telemetry is delivered to the IBEX Science Operations Center (ISOC)
(see Schwadron et al., this volume), it is processed to identify periods of high
background from which IBEX-Hi and IBEX-Lo data should be excluded from
consideration. This identification process will be calibrated on orbit based on the
response to known sources, as well as any unanticipated ones as they are discovered. This processing will routinely produce a background monitor intensity curve
(counting rate with time and angle) and a list of background time intervals. Additional products are planned on an as-requested basis through a web interface. This
would include counting rates with time from selected regions of the sky and sky
maps of counting rates over multiple orbits.
Acknowledgments
We are grateful to many individuals who contributed at different stages of the
design, development, testing, and implementation of the background monitor. In
alphabetical order they are Josh Alquiza, Tom Broiles, Rob Ebert, Danny Everett,
Mike Gruntman, Ron Harper, Paul Janzen, Chris Kofoed, Roberto Livi, Stefano
Livi, Matt Maple, Susan Pope, Mike Young, and the IBEX team in general. We
would like to thank the Goddard Space Flight Center for supporting acoustic testing of the flight carbon foils for the background monitor.
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Fig. 1. Principle of the Background Monitor.
11
Fig. 2 Engineering cross section of the background monitor and detailed pictures of the major
components.
Fig. 3 Collimator transmission as a function of two orthogonal angles.
12
Fig. 4 Collimator transmission as a function of angle with respect to the look direction.
Fig. 5 Detection efficiency as a function of energy in linear (left) and logarithmic (right) representations.
Fig. 6 Map of the IBaM response across the aperture at 30 keV.
13
Fig. 7 Fraction of protons hitting the CEM funnel after being scattered in the thick carbon foils.
Fig. 8 Background monitor response for the upstream event shown in Fig. 1.