An actively shielded hard X-ray astronomical telescope has been flown on stratospheric balloons. An attempt is made to compare the measured spectral distribution of the background noise counting rates over the energy loss range 20-300 keV with the contributions estimated from a series of Monte Carlo and other computations. The relative contributions of individual particle interactions are assessed

Dean, A. J.

Dipper, N. A.

Lewis, R. A.

Perotti, F.

English

NASA Technical Reports Server

8 7 9 OG 9.3-7
A COMPARISON OF CALCULATED AND MEASURED BACKGROUND NOISE RATES IN
HARD X-RAY TELESCOPES AT BALLOON ALTITUDE
I i 1 2
A.J.Dean , N.A.Dipper , R.A.Lewis , F.Perotti
I. Department of Physics, University of Southampton, Southampton, UK
2. Istituto di Fisica Cosmica, Milano, Italy.
ABSTRACT
An actively shielded hard X-ray astronomical telescope has been
flown on stratospheric balloons. In this paper we compare the
measured spectral distribution of the background noise counting
rates over the energy loss range 20-300keV with the contributions
estimated from a series of Monte Carlo and other computations.
The relative contributions of individual particle interactions
ar_ assessed.
I. IntrodUction. The sensitivity of hard X-ray telescopes is dependent
to a first approximation upon a number of desirable characteristics:
a large sensitive area, long observation periods, high photon detection
efficiency and a low _ackground noise level. The first two factors
are very dependent u_n the platform from which the telescope must op-
erate, whilst the effective suppression of background noise requires
a detailed analysis of the physical processes involved. We present
in this paper the background levels observed in an actively shielded
collimated telescope during a balloon flight from Palestine, Texas in
1982. The experimental results are compared with theoretical and comp-
uter predictions of the background counting rates.
2. Confisuration of the Hard X-ray Telescope. The telescope was composed
of 8 NaI(TI) scintillation crystals (200x200x6mm) shielded over the
lower 2_ steradians by a 5cm thick NaI(TI) crystal of the same area.
A thin plastic scintillator designed to reject events due to charged
particles covered a large fraction of the upper 2_ steradians. The
aperture of the detectors was restricted to 3 degrees FWHM by a copper
collimator. A passive graded shield also surrounded the collimators
and the sides of the crystals. A schematic diagram of the telescope
is shown in Fig.l. In addition a Sodium Iodide detection crystal,
identical to the primary crystal in the telescope, was flown in a compl-
etely unshielded configuration in order to determine the efficiency of the
https://ntrs.nasa.gov/search.jsp?R=19850026649 2020-03-20T16:46:49+00:00Z
380 OG 9.3-7
shielding system. The telescope and unshielded crystal were flOWn On the
same stratospheric balloon gondola from Palestine, Texas.
--Pl_fic Scinfilla_
-- _olvmJltipl_
7 [ _CoU,mat_7
Grad_
Fig.l. A schematic view of the X-ray telescope detector units
' ' ' ..... I , ,
Fig.2. The counting rate in the
MIFRASO NaI(TI) detectors at 4.1mbs
__'-_LL residual pressure for four different
shielding configurations;
A a) counting rate in unshlelded det.I
I b) counting rate in unshielded det.
'_ _ with all active anticoincidence off.
c) counting rate in shielded det.
__m162 with NaI(TI) active shield on but with
plastic anticoincidence off.
d) counting rate in primary det. with
163 o
_ 100 200 300
Energy(KeV)
Figure 2 demonstrates the effectiveness of each type of shield by
showing the counting rates at balloon altitudes for different shielding
configurations. It clearly demonstrates that the majority of the reduction
381 OG 9.3-7
in background counting rate is achieved by the passive shield with both
sets of active anticoincidence causing roughly similar further reductions
in the unwanted background noise.
3. Comparison of measured and calculated background levels. The pre-
dicted background counting rate in the detectors of the MIFRASO telescope
was computed by a combination of theoretical and Monte Carlo techniques.
The photon induced background was determined from the results of an exten-
sive Monte Carlo model of the entire detection system. The program modelled
the processes of photoelectric absorption and Compton scattering, but
neglected electron transport due to the fact that the path lengths in-
volved for the energy range under consideration are generally small com-
pared to the dimensions of the elements of the telescope. The angular and
energy distribution of the gamma-ray fluxes incident over the detector
could be varied to suit the particular physical processes under investi-
gation° In each case the energy deposits in all the elements of the tele-
scope were recorded in order to provide an insight into the efficiency of
the various shielding systems.
Neutrons arriving at the primary crystal may generate background
noise by a variety of mechanisms but the most important are (n_) radi-
ative capture and inelastic scattering by the 1271 nuclei. Radiative
capture by 1271 leads to a variety of possible prompt gamma-rays by the
reaction 1271,( n,7 ) 1281 . The various gamma-ray energies are very ex-
tensive and extend from 60 keV up to 6.7 MeV. There is also a delayed
effect from the decay of 128 which has a half life of 25 min. The two
lowest levels to be excited by inelastic scattering at a 1271 nucleus lead
to the emission of gamma-rays at energies of 58 and 203 keV.
The neutron flux incident upon the detectors is made up of two comp-
onents, atmospheric neutrons and those produced within the payload by
"boil off" from cosmic ray interactions. The modified neutron flux will
depend upon the exact geometry and mass of the payload but may be estimated•
as described by Charalambous et al (1985) by assuming a uniform distr-
ibution of mass and integrating over the volume of the payload. This
neutron flux was folded with the cross sections for radiative capture and
inelastic scattering to yield an estimate for the background counting rate
of neutron induced events. The contribution from charged particles has
been estimated from the effect of the plastic scintillator shield. If
382 OG9.3-7
the shield is assumed to be 100% efficient over the solid angle that it
covers, it is possible by considering the effect of switching on and off
the shield, to estimate the number of events caused by particles that enter
through the solid angle that is not shielded.
High energy cosmic ray protons which are incident upon the material
of the detection plane give rise to background events which are derived
from the radioactive spallation products produced in the interaction.
In general, the veto pulse may be made as long as possible to suppress a
number of these types of events. However, since the timescales associated
with many of the decay schemes are considerably longer than practicable
veto pulse lengths the active shield does not eliminate this type of noise.
Calculations of spallation involved background are extremely complex
and subject to considerable error so that areliable estimate of this source
of noise is best obtained by experiment. Results of such tests have been
reported in the literature (Baker et al 1979) and have been used to yield
an estimate for the MIFRAS0 detectors.
The results of the various calculations are shown in Fig.3. The
agreement between calculation and experiment is quite good being within
a factor of two and having the same spectral distribution. The simi-
larity in spectral shapes is an encouraging sign that the relative con-
tributions of the various effects as determined by calculation are correct
' ' ''"I ' ' ' ''"I
References
i. Baker R.E.et aI.NIMS, 158.
[--] (1979) 595.
10"3 ['--r-I L-mp_t_nt 2. Charalambous,P.C.et al.
-_ NIMS (in press
_tat 3. Dean,A.J. and Dipper,N.A.
v_ MNRAS (1981), 194, 219.
_ L_ i I Ommed_tlcl_
10¢ i " i , ..j ..... Neuf_s
r_._.Ji _ _t__to
, b-.f
--.J
, , , ,,,,I , , lllll ilo.5 _ _ _ _ _
_gy(K_)
Fig.3. A comparison between observed and
calculated background noise events.


A comparison of calculated and measured background noise rates in hard X-ray telescopes at balloon altitude

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