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)