ANITA neutron source
Characteristic measurements with activation detectors
Measurement requests
• The homogeneity of the neutron beam
• Low-energy neutrons in the neutron beam
• Protons in the neutron beam
• Positions “user2” and “user3” (positions 6 and 12 m from the target)
Activation method
• Monoisotopic materials– Al, Au, Bi, Ta, natCu– ca. 1g of material – 10^8 neutrons
• Irradiation (8h of 350 nA beam)
• Gamma spectra measurements (days after)
• Analysis of spectra
Detectors positions
user2, user3 positions
(6m, 12m)
concrete wall
collimator
cross of detectors +
Cu detector
for LE neutrons
Irradiation
Measurements
Results
• Neutron beam uniformity:– No non-homogeneity out of the limits of statistical
error ~ 5%• Low-energy neutrons:
– Present in all foils, less near the wall ?– Spectrum cannot be determined from our data
• Extra positions:– “user2” - 6 m : 4x less LE neutrons than at “user1”, 7x
less 196Au, 3x less 194Au (196Au, 194Au not reliable, close to detectable limits)
– “user3” - 12 m : 8x less LE neutrons, others not detected, upper limit for 196Au ~ 7x less
Protons in neutron beam I
• Protons are monitored by the reactions in natCu:– 63Cu(p,n)63Zn– 63Cu(p,2n)62Zn
• No traces of Zn isotopes were found in the Cu detector
• The amount of 63Zn and 62Zn ca. 10x lower than the amount of 61Cu would be detected– 61Cu was well detected– 63Zn and 62Zn lines should have more that
3*resolution counts ~ 200 counts– 200 counts corresponds to 10x lower amount of Zn
comparing to 61Cu
Calculations• Neutron (and proton) spectra can
be simulated:– MCNPX– FLUKA (can simulate also in
magnetic field)• Simplified target was used in
simulations• Cross-sections are taken from
TALYS and verified against values from EXFOR (agreement is good enough for our estimations)
• The spectrum multiplied with appropriate cross-sections = the amount of produced radioisotope
• Absolute and relative comparison
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
1E+1
1 10 100 1000
Energy [MeV]
Flue
nce[
MeV
-1 p
-1 c
m-1
], XS
[bar
n]
neutron spectrum
(n,2n)
(n,4n)
(n,5n)
Simulated spectra
• Shapes (FLUKA, MCNPX and preliminary spectrum – same shape, but different absolute values, because of simplified target, distance-detector target)
• Relations between n and p spectra (there is in total 1000x less p than n)• Magnetic field - no noticeable difference
1E-12
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
0 20 40 60 80 100 120 140 160 180
Energy [MeV]
Flu
en
ce
[M
eV
-1 p
-1 c
m-2
]
n (MCNPX) p (MCNPX)n (FLUKA) p (FLUKA)n (FLUKA+B=1T) p (FLUKA+B=1T)Preliminary
Protons in neutron beam II• Simulated p and n spectra (MCNPX) are convoluted with xs for 62Zn
and 61Cu• Result: 100x less 62Zn than 61Cu, this is 10x lower than our detection
limit (10x, slide Protons – part 1)• We experimentally set the upper limit for the proton spectrum to 10x
higher values than was simulated (1 proton per 100 neutrons):
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1 10 100 1000
Energy [MeV]
Flu
en
ce
[M
eV
-1 p
-1 c
m-2
] neutronsprotons
Experiment / calculation
0
1
2
3
4
5
6
0 20 40 60 80 100 120
Energy at xs maximum [MeV]
Exp/
sim
- no
rmal
ized
Energy [MeV} Isotope exp/sim
9 Mg-27 0.6
14 Na-24 1.2
15 Au-196 1.0
31 Cu-61 1.3
34 Bi-206 1.3
35 Au-194 1.8
43 Ta-177 2.5
45 Au-193 1.8
45 Bi-205 2.7
48 Cu-60 1.4
53 Ta-176 2.4
55 Bi-204 1.7
56 Au-192 2.1
63 Ta-175 5.5
67 Au-191 1.3
67 Bi-203 1.6
77 Bi-202 2.1
78 Au-190 2.2
84 Ta-173 1.9
87 Bi-201 1.3
99 Bi-200 0.8
•Average value of exp/sim : 1.64
•Causes of discrepancies are:
•Simplified simulation
•Cross-sections
•Systematic errors
Conclusion• Measurements with activation detectors showed that
ANITA neutron source meets all requirements for a good neutron source:– Homogenous neutron beam– LE neutron background measured (simulations for
understanding)– Low proton background (at least 100x less protons than
neutrons)• Relative neutron intensities in positions at 6 m and 12 m
were measured (LE neutrons measure reliably, for 196Au and 194Au some upper estimations were set)
• Calculated spectra convoluted with cross-sections give acceptable production rates – the shape of the real spectrum (10-100 MeV) is determined
• Magnetic field in simulation does not change the results (codes MCNPX and FLUKA are both usable)