Development of energy-dependent scaling for cosmic-ray neutron
intensities and for in-situ cosmogenic nuclide production rates
Marek Zreda - Arizona
Devendra Lal - Scripps
John Clem - Bartol
Darin Desilets -
Arizona
Neutron measurements
Neutron modeling
Artificial chemical targets
Kinetic energy (MeV)
10-8 10-7 10-6 10-5 10-4 100 101 102 103 104
Cum
ulat
ive
neut
ron
coun
ts (
%)
10
100
10
100
Bare counter
and
35Cl(n,)36Cl
NM-64 neutron monitor
39K(n,x)36Cl
40Ca(n,x)36Cl
16O(n,x)14C
16O(n,x)10Be
Cum
ulat
ive
nucl
ide
prod
uctio
n (%
)
32S(n,p)32P
Energy spectra for neutrons and nuclides
Vertical cutoff rigidity (GV) for Epoch 1980
Spatial variations of neutron intensity
Important discovery
Attenuation length in the air for neutrons depends on energy
Dorman function for neutrons depends on energy
NeutronMonitor
counter tubelead
polyethylene
John Clem - http://www.bartol.udel.edu/~clem/nm/display/intro.html
Arizona mobile neutron monitor
Sample results from Arizona neutron monitor
Atmospheric depth (g cm-2)
600 700 800 900 1000 1100
Nat
ural
log
of c
ount
rat
e -
Ariz
ona
3
4
5
6
7
3
4
5
6
7Carmichael survey
147.3 g cm-2
R2 = 0.9998
Arizona neutron monitor = 147.2 g cm-2
R2= 0.9998
Natural log of count rate/10 - C
armichael
03 2 14
Elevation (km)
Bare counter (BC) tubes?
Lower energy sensitivity
Thermal neutron detector (counter)
Specifications:
Diameter 2.6 cmLength 39.2 cmTotal pressure 1.03 MPaVoltage range 1100 - 1600 VWater resistance up to 8 mBackground counting rate 6 cts/hrCounting rate on Mt. Lemmon, AZ (2788 m) 3830 cts/hrCounting rate in Tucson, AZ (667 m) 900 cts/hr
Thermal neutron detector assembly
Sample results from our neutron detector
Atmospheric depth (g cm-2)
600 700 800 900 1000 1100
Nat
ural
log
of c
ount
rat
e (c
pm)
1.5
2.0
2.5
3.0
3.5
4.0
4.5
calculatedthermal neutron
profile
3 24
Elevation (km)
1 0
Atmospheric depth (g cm-2)
400 500 600 700 800 900 1000100
120
140
160
180
Atmospheric depth (g cm-2)
400 500 600 700 800 900 1000
Atm
osph
eric
atte
nuat
ion
leng
th (
g cm
-2)
140
160
180
200
Pth
dpl= 10o dpl= 60o
Lal
Dunai
Desiletsand Zreda
Psp
Pth - thermal neutron reactions
Psp - spallation reactions
Pth
Psp
Comparison with previous models (attenuation lengths for high-energy and thermal neutrons)
Current data
NM
• > 9 latitude surveys
• 2 comprehensive altitude/latitude surveys
• numerous NM stations
BC
• 1 latitude survey
• 1 comprehensive altitude/latitude survey
Possible outcomes:
(1) All nuclides scaled by NM
(2) All nuclides scaled by BC
(3) Low-E interactions scaled by BC, high-E scaled by NM
(4) Many scaling functions
Other results:
• Angular sensitivity• E sensitivity • Neutron spectrum from NM• Muon sensitivity
atmospheric depth (g cm-2)
640 680 720 760 800 840
natu
ral l
og o
f rel
ativ
e ag
e
9.6
9.8
10.0
10.2
10.4
10.6
10.8
Attenuation length for production of 36Cl(lava-flow samples, Mauna Kea, Hawaii)
Comparison of targets with geological samples
Geological samples Targets
Cosmic-ray intensity Estimated (from geological records) Known
Exposure duration Determined (by geological dating) Known
Exposure geometry Unknown (assessed, guessed) Known
Exposure time Long Short
Chemistry Complex Simple
Expected accuracy Poor Good
Expected precision Good Good
Other things ?????? ??????
Primary objectives
(1) To determine attenuation lengths for production of nuclides
- at known magnetic cutoff rigidity
- for low- and high energy reactions
(2) To relate these to measured attenuation lengths for neutrons
Primary objectives
(1) To determine attenuation lengths for production of nuclides
- at known magnetic cutoff rigidity
- for low- and high energy reactions
(2) To relate these to measured attenuation lengths for neutrons
Secondary objective
(3) To determine absolute production rates of selected other nuclides
Proposed target experiments
Experimental design matrixHawaii, 13 GV California, 4 GV Target type
O(n,x)10Be yes yes Primary
32S(n,p)32P yes yes Primary
K(n,x)36Cl yes yes Secondary
Ca(n,x) 36Cl yes yes Secondary
Experimental design matrixHawaii, 13 GV California, 4 GV Target type
O(n,x)10Be yes yes Primary
32S(n,p)32P yes yes Primary
K(n,x)36Cl yes yes Secondary
Ca(n,x) 36Cl yes yes Secondary
Proposed target experiments
Objectives
(1) To determine attenuation lengths for nuclide production
(2) To compare with neutron attenuation lengths
(3) To determine absolute production rates of selected other nuclides
High-energy reaction:
O(n,x)10Be
t½ = 1.5 My
Emed = 140 MeV
Target: H2O (water)
Concentration: 100%
Target size: 100-200 kg
Exposure time: >3 years
Repeated exposure: no
Measurement: AMS (Purdue)
High-energy reaction:
O(n,x)10Be
t½ = 1.5 My
Emed = 140 MeV
Target: H2O (water)
Concentration: 100%
Target size: 100-200 kg
Exposure time: >3 years
Repeated exposure: no
Measurement: AMS (Purdue)
Low-energy reaction:
32S(n,p)32P
t½ = 14.3 days
Emed = 7 MeV
Target: CS2
Concentration: 100%
Target size: 10-15 L
Exposure time: >2 months
Repeated exposure: yes
Measurement: Quantulus (Arizona)
Selected targets - primary
Low-energy reaction:
K(n,x)36Cl
Emed = 13 MeV
Target: KNO3
Concentration: 15%
Target size: 20 kg
Exposure time: >3 years
Repeated exposure: no
Measurement: AMS (Purdue)
Low-energy reaction:
K(n,x)36Cl
Emed = 13 MeV
Target: KNO3
Concentration: 15%
Target size: 20 kg
Exposure time: >3 years
Repeated exposure: no
Measurement: AMS (Purdue)
Selected targets - secondary
Mid-energy reaction:
Ca(n,x)36Cl
Emed = 55 MeV
Target: Ca(NO3)2
Concentration: 10%
Target size: 40 kg
Exposure time: >3 y
Repeated exposure: no
Measurement: AMS (Purdue)
Hawaii - 13 GV:
Mauna Kea & Mauna Loa:
several secure locations (government
facilities) between the summit (~4 km)
and the saddle (~1.6 km)
Why Hawaii:
high magnetic cutoff rigidity
large elevation span
much neutron/nuclide data exist
permanent neutron monitor (Maui)
Hawaii - 13 GV:
Mauna Kea & Mauna Loa:
several secure locations (government
facilities) between the summit (~4 km)
and the saddle (~1.6 km)
Why Hawaii:
high magnetic cutoff rigidity
large elevation span
much neutron/nuclide data exist
permanent neutron monitor (Maui)
Field locations
California - 4 GV:
White Mountains:
several secure locations (government
facilities) between the summit (~4 km)
and the Owens Valley (~1.6 km)
Why California:
low magnetic cutoff rigidity
large elevation span
close to Arizona and Scripps
permanent neutron monitor (Climax)