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)