Download Determination of activity of Cr51 artificial neutrino sourse

BNO INR RAS
Determination of activity of 51Cr source on
gamma radiation measurements
V.V.Gorbachev, V.N.Gavrin, T.V.Ibragimova,
A.V.Kalikhov, Yu.M.Malyshkin,A.A.Shikhin
Institute for Nuclear Research, RAS
The Intrnational workshop on Prospects of Particle Physics:
Neutrino Physycs and Astrophysics
Valday, Russia, Feb 1-8 2015
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Neutrino experiments of new generation
on search of thin effects, accuracy <5%
Experiments with artificial sources of a neutrino
Transitions to sterile states
Magnetic moment, …
Experiment BEST (The Baksan experiment on sterile transitions)
Radiation of the Ga target (50 t) of the solar neutrino detector of
SAGE
Neutrino source 51Cr, 3 МCi
Total systematic uncertainty ~2.6%
The expected error of measurement of activity of the source <1 %
How to measure the activity of 51Cr:
1) On thermal emission – calorimetric method (36.7 keV/decay)
2) On gamma radiation
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BEST: 2-zone gallium experiment on
search for short baseline oscillations
(Δm2 ~ 1 eV2)
• measurements on two bases
•experiment on nuclear capture
71Ga+ν
e
Target – 50 t of metallic Ga
Masses of target in zones: 8 t and 42 t
Path length in each zone <L>=55 cm
51Cr source: T =27.7 d, 3 MCi
1/2
Eν = 750 keV (90%), >95 % captures
430 keV (10%)
→ 71Ge + e–
Evidence for oscillations:
1) different capture rates in zones of target
2) suppression of capture rates in both zones
Gamma radiation 51Сr
1. 320 keV, 10% decays
2. IB, <430 keV, 1.2·10-4 x 0.0983
3. IB, <750 keV, 3.8·10-4 x 0.902
Internal Bremsstrahlung:
y( x)  A  (1  x)  x
2
x
E
Q
Radiation spectrum beyond the
shield
Activity is distributed unevenly on volume
Radiation spectrum in the source
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The method of measurement
We divide (mentally) the source on N parts
(for example, with uniform activity):
Source: Cr, 7.2 g/cm3, cylinder ø8.6 x 9.5 cm
Shield: W alloy, 16.6 g/сm3, thickness 2.5 сm
In the detector we register a signal –
the sum of signals from all parts :
N
S (E)   Si (E)
Details are in the paper of V. Gorbachev, Yu.M.Malyshkin, to be published in
Instruments and Experimental Techniques,(2015)
i 1
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N
Signal in detector:
S ( E )   A( E , xi )  ( E , xi )
i 1
А(Е,xi) – activity of i-th part of the source on photons with energy Е
xi
– i-th part of the source coordinate
ε(E,xi) – efficiency of detection of photons with energy Е from part xi
or entering replacement
 i ' ( E )   ( E , xi )  f ( E )
1
Bi  A( E , xi ) 
f (E)
и
Here Вi – absolute activity of i-th part and it does not depend on Е;
f(E) – release of photon with energy Е per 1 decay
N
Then signal is:
S ( E )   Bi  i ' ( E )
i 1
And total activity:
ATot 
N
B
i 1
i
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One forms a system of linear equations for Nj
energy intervals {Еj} :
or in matrix form:
N
S j    ij 'Bi
i 1
S   'B
In case Nj=N, the solution will be :
B  ( ' ) 1  S
Condition for solution:
det( ij ' )  0
It is convenient to solve the system of equations using method χ2:
1) more information is used (Nj can be more than N)
2) the errors are estimated directly

N j  S j    ij 'Bi
i 1
 2  

j
j 1


N
For uncorrelated errors:
The errors σ(Bi) are determined on the
boundaries of areas:






2
2
 2   2   min
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Calculation of the Compton scattering
Spectrum of photons, which leave the shielding of the source, is distorted:
monochromatic photons with energy Е have continuous spectrum with
energies Е’ ≤ Е
Δj – signal from photons, registered in
the energy range Ej, but emitted
with energy E > Ej:
Result:
где
 ij " 
Nj

j ' j
ijj '
'
εijj’ – exit from the i-th part of the source of photons with energy Ej,
generated with energy Ej’ > Ej
I.e. the notation of the signal does not change and all described operations
(solutions) remain the same after the change εij’ to εij’’
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On detector resolution
N parts of the source requires Nj ≥ N energy ranges
In case the source is separated in 3 parts in each coordinate, N=33=27
I.e, it is necessary to separate the spectrum of 51Cr from 300 to 750 keV in
N~27 ranges
The width of one range Δ ~ (750-300)/27 = 15 keV
Detector resolution~ 15 keV/ 750 keV= 2%
I.e. one can use the Ge SCD
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Collimator
Why we need the collimator:
1) To limit the regions with equal efficiency in the detector
2) To remove the reflected (from the walls) radiation
3) To separate the registration efficiency on directions, to fit the
condition
det( ij ' )  0
Uniformity of efficiency: simulations for ø=1cm and ø 2 cm
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Expected counting rate
From Monte Carlo (Geant4)
Exit beyond the limits of the primary shielding:
320 keV – 2.8·10-6
IB 430 keV – 1.5·10-6
IB 750 keV – 5.1·10-4
The total number of photons outside the shielding will be 5.1·1010 s-1 per
decay
Of them 320 keV – 61%, IB 750 keV – 39%, IB 430 keV– 4·10-3 %
At a distance of 10 m with collimator 1 сm one can expect 3000 photons/sec
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On detector response
x=E – current energy in spectrum
x1
– the energy of photon line
y0(x) – spectrum before the detector
y1(x1,x) – detector response function
y2(x) – spectrum, registered in
detector
Forming of signal:
For photon line:
y0 ( x )  {
y01, _ x  x1
0, __ x  x1
Spectrum in detector will be:
y2 ( x)  y01  y1 ( x1 , x)
For arbitrary signal:
y 2i   y1 ji  y 0 j
j i
In the assumption that xj > xi by j > i
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Function of the response y1(x1,x)
Reconstruction of the spectrum y0(x) using
monochromatic lines on y2(x)
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For continuous spectrum the function y2(x) is
determined using only one point x = xk :
y2 k 
y0 k 
N
y
j  k 1
1 jk
 y0 j
y1kk
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How to take into account the radiation of impurities
The activity of impurities for 51Cr 3 MCi, obtained on results of the
1994 yr source experiment:
59Fe 6∙109 Bk,
182Ta 9∙109 Bk, 60Co 2∙1010 Bk, 46Sc 3∙1011 Bk
We can write our system of equations:
Signal from impurities:
k
i
c
 ijk
K
N
k 1
i 1
П j   (  c ik  ijk )
− the activity of k-th impurity from the i-th part of the source
− the efficiency εij for k-th impurity
Assuming the dependence of the activity of
impurities on the source activity :
The signal in detector in presence of impurities:
The number of unknowns: N+K: {Bi} + {pk}
cik  Bi pk
N
K
i 1
k 1
S j   Bi ( ij "  pk  ijk )
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Uncertainties of the method
1) Geometry of source and of detector
(size, density of medium for absorption, relative location of
the source and detector, direction of collimator,…)
2) Knowledge of spectrum of the source irradiation
(accuracy of the available calibration of γ-sources > 3%, the IB
spectrum is known only in the first approximation, …)
3) Accuracy of calculation of efficiencies and response functions
4) Statistics of events:
The number of events on the «average» part of the spectrum:
in case the efficiencies of the signal recorded from the «near»
and «far» parts of the source differ on the order, then
(1  0.1) N  0.1N
I.e. the minimal statistics N > 1100 for one Е interval
And the summarized statistics > 27∙1100 = 3∙104.
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Absolute measurements of statistics and spectrum of IB of 51Cr
We shell use 2 point sources of 51Cr with activities:
1) ~ 20 mCi (109 Bq) and 2) ~ 1 kBk
Source №1
1) Measurement of the IB spectrum above 320 keV through collimator
2) The same measurements with collimator covered with Pb 2.6 cm: to
control of the quality of measurements spectrum because of the
pulses imposition
Source №2
1) Measurement of the total counting rate in 4π-geometry with a couple
of NaI detectors
2) Measurement of the rate of 320 keV line in Ge detector through
collimator
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Summary
The following methods have been developed:
1) Measurements of activity of intense source on
measurement of continuous spectrum of γ-radiation
2) Reconstruction of spectra on measurements in SCD
3) Absolute calibration of the activity and of the
measurement of the IB spectrum of the 51Cr source
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Thank you
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