Download gamma - radiation connected to atmospheric precipitations.

32nd International Cosmic Ray Conference, Beijing 2011
STUDY of RADIATION RELATED with
ATMOSPHERIC PRECIPITATIONS
E.V. Vashenyuk, Yu.V. Balabin,
A.V. Germanenko, B.B. Gvozdevsky
Polar Geophysical Institute, Apatity, Russia.
ABSTRACT
Continuous measurements (monitoring) by X-ray spectrometer
in the atmospheric surface layer of the arctic (Spitsbergen
archipelago)
and
subarctic
(Apatity)
regions
discovered
systematic relationships between increases in the low-energy
gamma (X-ray) background and precipitations as rains and
snowfalls at a low and dense cloudiness. It is shown that these
increases are not connected with any radioactivity.
As the reason of X-ray increases the bremsstrahlung X-ray
radiation produced by the secondary cosmic ray electrons,
accelerated in electric fields inside rain clouds is suggested.
The calculated X-ray spectrum obtained in the model
assumptions,
measurements.
are
in
satisfactory
agreement
with
OUTLINE
1. Instrumentation for study of X-rays during precipitations
2. Examples of X-radiation during precipitations
3. Additional experiments clearing up a nature of observed
effects
4. Proofs of absence of influence of a radioactivity on
discussed effects
5. A model of generation of X-radiation by CR electrons
accelerated in an electric field of clouds
Gamma-spectrometer for monitoring of x-ray background in
the lower atmosphere
Fig. 1. The detecting block of the
scintillation
gamma
spectrometer
including
crystal
NaI(Tl),
the
photomultiplier, a high-voltage supply and
the amplifier. The detector is made by the
instrument used for balloon research of
auroral X-rays [Lazutin, 1982].
Fig.2. Block-diagram of scintillation x-ray
spectrometer
Energy resolution of gamma-spectrometer
The spectrometers were tested
with the help of a reference
gamma source of Am241, having
a weak line of 26 KeV and a
basic one of 60 KeV.
The relative energy resolution
(which is also known as pulse
height resolution and energy
resolution), defined as the fullwidth
at
half-maximum
(FWHM) divided by the value of
peak centroid.
The
measured
energy
resolution of the instruments
makes ~20 %.
Spectrum of reference gamma source (Am241)
Detector of charged particles
Charged particles are detected by
Geiger-Mueller counters (2 rows by
8 tubes). The upper and lower
counter layers are sensitive to
electrons >0.2 MeV, protons >5
MeV.
The coincidence signal detects
electrons with energies >5 MeV,
protons >30 MeV and muons with
energies >20 MeV.
G-M counter has the very low
effectiveness (~1%) of detecting γrays and ~ 100% of detecting
charged particles
Precipitation detector
Infrared (IR)
The instrument is not calibrated and allows identifying only the presence of
precipitation and qualitative estimating its intensity. Therefore, the precipitation
intensity is given in arbitrary units.
X-ray increases related to precipitations,
Apatity station
Event of 26 July, 2009
Event of 30 August, 2009
The characteristic increase profiles of gamma (X-rays) in the channel > 20 KeV
during a rain for station Apatity. Clear correlation between rain and gamma
(X-rays) increases at ground level is seen.
Fig. 4. Examples of X-ray events related to precipitations in Apatity for the 4 seasons.
X-ray increases related to precipitations,
Barentsburg station (Spitsbergen)
Event of 12 November, 2009
Event of 10 December, 2009
The characteristic lateral views of increase of a gamma (X-rays) > 20 KeV and
precipitations (snowfall) for station Barentsburg. Data on precipitates (3 hour
averages) are obtained from the meteorological observatory of Barentsburg.
Characteristic spectrum of X-ray increases
Fig.8. The X-ray > 20 KeV increase event in
Apatity on 8.03.2010
Fig.9. X-ray energetic spectrum of the increase
X-ray spectrum in a maximum of increase of 8.03.2010 and the background spectrum
measured after event, accordingly red and dark lines. Spectra were obtained with the
4096-channel pulse height analyzer. The spectra do not show gamma lines, characteristic
for radionuclides.
Other proofs of absence of connection of a radioactivity with observable
increases are obtained from the analysis of precipitations (rain water and
snow).
Upper limit of energy
Figure 7. The open X-ray detector
shows an increase during
precipitations. The second detector
covered by lead bricks does not show
an increase. The lead shield cuts off
photons of energies <5 MeV.
There is no radiation in rainwater
Figure 8. The rainwater had been collected and placed over the
covered detector The detector did not show an increase. This
is a clear evidence of the lack of any radionuclide in rainwater.
Radiochemical control of precipitations
In several cases of considerable X-ray increases the samples of
precipitations in the form of rain and snow have been collected and
analyzed with radiochemical methods in the regional laboratory of
radiochemical control. This laboratory carries out regular measurements of
all radionuclides, both natural, and anthropogenic origin for the last few
years. For instance, see the results of the test of the collected rain-water in
the event on October10, 2010, when increase of about 25 % was observed.
The gamma-spectrometric analysis of 2 liters collected rain water has
shown usual presence of the natural and induced radio nuclides. Namely:
the trace amount is registered of natural radionuclides of series Th-232 and
U-238, daughter products of their decay (Ra-226, Pb-212,214, Bi-212,214,
etc.), K-40, Ве-7 and anthropogenic radionuclide Cs-137. By ‘trace’ we
mean that it is at the limit of sensitivity of the method. And it is much less,
than normalized value for potable water. And it is quite unlikely that it could
cause the X-ray increases registered.
The detected total specific alpha-activity, 0.0002 Bk/l, is more than 2
orders of magnitude lower than the normalized value for potable water (0.2
Bk/l). The total specific beta-activity is an order of magnitude lower than the
normalized value for potable water (1 Bk/l).
Another hypothesis about nature of the X-ray increases was related with electric
field existing inside clouds. The capability of thunderstorm clouds to accelerate
particles with their strong electric fields is well known (Lidvansky&Khaerdinov,
Makhmutov et al., 2010, Chilingaryan et al, 2010). In the Arctic (Spitsbergen)
and subarctic (Apatity) regions thunderstorms are very rare. But we observe
Increases of X-rays during each intense precipitations, with dense and low
altitude cloudiness (200-600 meter the cloud base ).
Such type of clouds has a title nimbostratus. Data on cloudiness and their
base altitudes we take at [http://rp5.ru/1122/ru].
The significant electric fields were discovered inside the nimbostratus clouds:
Rust, W.D. and Trapp, R.J, Initial balloon soundings of the electric field in winter
nimbostratus clouds in the USA. Geophys. Res. Lett. V.29, No 20, 1959, 2002.
Two examples of electric field measurements from the paper ( Rust & Trapp, 2002)
Vertical (blue) and Horizontal (red) components of electric field in nimbostratus
clouds measured by balloons ( Rust & Trapp, 2002)
Left: the maximal vertical field is 10 kV/m and horizontal one 27 kV/m
Right: volume charge
Thus, in the rain (snow) clouds the electric field gradient is rather high and
may reach even tens of kV/m. But this is not enough due to the strong
ionizing energy losses to accelerate thermal electrons to necessary energies
for generating observable fluxes of X-rays. However, secondary cosmic ray
electrons having energies of several units/tens of MeV after additional
acceleration in the electric field of nimbostratus clouds are capable to
generate such fluxes of X-rays. As our observations show the X-ray increase
events are observed only for clouds with a base not higher than 300-600 m.
It probably is a consequence of strong absorption of Х-rays in air, that also
proves to be true by accounts.
The major contribution to the spectrum of X-rays gives the low energy (< 1
MeV) part of electronic spectrum. It can be approximated by the exponential
law:
dN ( E )
 N 0  e  E E0 ,
dE
In its motion in the atmosphere accelerated electrons generate X-ray
bremsstrahlung (XRB). Efficiency of this process is spotted by differential cross
section XRB Q (hν, E), depending on energy of electrons E and X-ray quanta hν.
Completely expression for Q, is:
 f  i
d (h , E )
8e 2 h i f 1  e 2i
Q( E , h ) 
 2 2 3

 ln
 2 f
d (h )
3 m c h 1  e
 f  i
where
mc2
i  az
zE
mc2
 f  az
z ( E  h )
1
a
137
Due to the strong absorption of electrons in the air, we shall consider only those
X-rays, which were generated at the lower edge of a cloud (l meters above the
earth's surface).
Based on the calculations of (Lazutin, 1982) as well as assumptions about the
linearity of the absorption coefficient of X-rays in the energy range 100-1000 KeV,
we obtained an expression that describes a spectrum of X-ray radiation at the
ground base after accounting for losses due to absorption of both electrons and
gamma-rays:
   (h ) * (l  x)  
  E  (k  x)  
dE dx
N (h )   exp 
   Q( E , h )  exp 
10
E0

 h

 
0 
l
where
l is altitude of generation of accelerated electrons, reaching the base of clouds
μ is the linear attenuation factor of gamma-radiation
Q (E, hν) is differential cross-section of a braking radiation
k is energy losses of electrons in air (220 KeV/m)
Comparison of modeling spectra (color lines) with the observed spectrum (dark)
Figure shows the results of X-ray spectra modeling under the formula given above.
Spectra of bremsstrahlung quanta at a surface of the earth, born by the accelerated
electrons in an atmosphere layer at the altitude l for various values of this parameter
from 450 to 650 m. Thick curve is the spectrum of quanta measured in event on March,
8, 2010. It is seen that the measured spectrum in the best way fits the modeling, gained
for height of generation of 550 m. The results of modeling have rather weak
dependence on the form of electron energy spectrum.
Results
The continuous measuring (monitoring) by X-ray spectrometers in a ground
stratum of atmosphere of arctic (Spitsbergen) and subarctic (Apatity) regions
has found out systematic relationships of increases of low-energy gamma
(X-rays) background with atmospheric precipitations as rain and snow at a
low and dense cloudiness.
It is shown that these increases are not
connected with any radioactivity, including radon.
As the reason of increases the bremsstrahlung X-rays yielded by cosmic ray
electrons, accelerated in electric fields in the nimbostratus clouds is
suggested. The calculated X-ray spectrum obtained in the model
assumptions, are in satisfactory agreement with measurements.
Thank you for Attention!