On the low-energy background gamma radiation of the continuous
spectrum in our laboratories, and everywhere else
Analyzing the low-energy background spectrum of an HPGe detector
situated both in a ground based and in a shallow underground laboratory we
determined the intensity and the origin of the low-energy background
gamma radiation of the continuous spectrum, which permeates not only our
laboratories but our entire environment as well. The HPGe detector is of the
vertically oriented true coaxial type, of an efficiency of 35%, in the radiopure magnesium housing. It was shielded completely from low-energy
radiations coming from the lower hemisphere by a 10 cm thick lead castle,
while it was completely open to all the radiations coming from the upper
hemisphere. The deconvolution of the continuous part of the low-energy
background spectrum of the thus shielded detector is helped by measuring
the background spectra with a set of thin lead absorbers (0.04 mm to 4.5
mm) placed so as to block the way to the radiations arriving from the upper
hemisphere, and by the corresponding MC simulation (GEANT4) of the
situation, which takes into account the absorption of this radiation in the
front dead layer of the detector and in the 1 mm thick Mg window. It has
been found that the major part of the low-energy continuous background
spectrum of the unshielded detector, arriving from the upper hemisphere,
both on the ground level and underground, is due to the presence of the
radiation of a continuous spectrum that start at lowest energies and stretches
up to about 2 MeV, with an intensity sharply decreasing towards higher
energies (Figs.000). To determine what part of this radiation is due to offthe-air and off-the-walls scattered environmental radiations (known as the
skyshine radiation) and what part is due to the soft cosmic radiation we
Monte Carlo simulated (Dejo-GEANT4, and Dime-CORSIKA+GEANT4,
respectively) both possible situations. We conclude that most of this
omnipresent low-energy radiation is of the cosmic-ray origin, while only its
practically negligible part may be attributed to the skyshine radiation
(J.Swarup, NIM 172(1980)559). We also estimate that the intensity of this
low energy radiation, of an average energy of about 100 keV, in the surface
based laboratory is about 400 Photons/m2.s.2 srad and about 300
Photons/m2.s.2 srad in the underground laboratory, what gives the dose rate
of about …
The low-energy parts of the background spectra in the described geometry,
with lead absorbers of increasing thickness, normalized to the same
measurement time of 100 000 seconds, and in two different energy scales,
are presented in Fig.1, first in the ground level and then in the underground
laboratory.
Fig.1 Experimental background spectra of a HPGe detector with a set of lead
absorbers of different thickness positioned so as to block the radiations
arriving to the detector from the upper hemisphere
A number of features of these spectra are here of interest. First, there is a
striking similarity between the spectra at the ground level and in the
underground.
Second, with increasing absorber thickness the intensity maximum of the
continuous spectrum shifts to higher energies, suggesting that this part of the
spectrum is produced by radiations of the same energy at which they appear
in the spectrum, which is thus progressively more depleted at lower energies,
where photoelectric absorption is greater (typical hardening of the
continuous spectrum)
Third, the step in the height of the spectrum at K lead X rays, at the
position of the K-absorption edge (88 keV), reflects the fact that the
instrumental spectrum is indeed mostly the direct spectrum of radiations
impinging the detector, and not the result of escaped Compton scattered
radiations of higher energies within the detector.
Fourth, the initial increase of the intensity of fluorescence X-rays of lead
with the increasing absorber thickness, also suggests that photoelectric
absorption of energies that appear in the spectrum is responsible for the
overall decrease of the spectral intensity with the increasing absorber
thickness, at least at energies below some 200 keV, where cross sections for
the Compton effect can in the first approximation be neglected as compared
with the photoelectric cross sections.
.
Fig.2. The counts in the channel which corresponds to 89 keV from the
spectra taken with lead absorbers of different thickness
To further check the conjecture which stems from the four remarks above,
that in the low-energy continuous spectrum the energies as registered by the
spectrometer equal the true photon energies, we plot the count in a given
channel as the function of the corresponding absorber thickness. The results
for some characteristic energies (81 and 89 keV, below and above the Kabsorption edge), are presented in Figs. 2 and 3. At 89 keV (but on all other
energies as well) it is seen that there are two distinctive absorption curves –
the steep one corresponds satisfactorily, minding the wide angular
distribution of the impinging radiation, to the energy of 89 keV (d1/2~0.13(2)
mm, instead of 0.08 mm, and at 1 mm, what amounts to about 10 halfthicknesses, it practically dies out completely), and the flat one corresponds
to much higher energies, witnessing that it is due to the higher energy
gamma rays (of an average energy around 1 MeV) Compton scattered within
the detector, and then escaping detection (if so, this component should
behave similarly at all energies, as indeed seems to be the case, see Fig.3).
The results of such analysis underground are in close agreement with the one
in the ground laboratory.
Fig.3. Where it can be seen that the penetration component behaves more or
less in the same way at all energies up to about 250 keV, where the Compton
electrons of scattered higher energy gamma rays are found
Fig.000a
Fig.000b
Figs.000c