Reduction of cosmogenic activation of Ge by means of movable iron shielding

I. Barabanov, S. Belogurov et al. INR/ITEP, Moscow Dubna GERDA meeting 27-29.06.05

Reduction of cosmogenic activation of Ge by means of movable iron shielding

I. Barabanov, S. Belogurov, L. Bezrukov, A. Denisov, V. Kornoukhov, and N. Sobolevsky

Outline

• Entry conditions for simulations• The method• Principal results• Analysis• Prospects and open questions• Conclusions


Entry conditions for simulations

Nuclear disintegrations at the sea level are mostly due to N-component of CR (98%) and induced fast nucleons (~2%), [Cocconi, 1951].

Our goal is to suppress N-component.

Flux density of nucleons at the sea level,[Ziegler, 1981]

Angular distribution: ~cos3.5(θ)

Compare to February, inconsistence in spectra is found and corrected.

neutrons

protons


The method

Our tool for hadron transport simulations is the SHIELD code, - why?

• There is a lot of criticism about hadron transport simulation in GEANT 3,4

• We have an expert in nuclear interactions models and their software realizations – Prof. Sobolevsky, the head of the SHIELD team, so we do not deal with a “black box”

Details of the SHIELD simulations will be reported by Andrey Denisov to TG10.


Ground (5),Depth=4 m

Container Fe (1)

Air gap (6) Normalizing sphere (4),R=150 cm

Air (7)

OUT (8)

Cavity (2)

Ge(3)

120 cm

Container: R1=70 cm, H1=126.5 cm Bottom depth 15 cmCavity: R2=27 cm, H2=40 cmGe-shipment: R3=21 cm, H3=27 cm

Ground (5)Depth= 4m

Simulation geometry


Principal results

Simulation of a complete configuration

– Absolute isotope production rates

– Reduction coefficients

Step by step analysis for comparison with literature and optimization of shielding

– Spectra of nucleons inside the cavity

– Excitation functions (cross section) for isotope production


Production rates of 60Co and 68Ge

Target Total By sea level protons

No shield Shield No shield Shield

70Ge 281.4 (0.5%) 33.0 (2%) 17.17 (1.1%) 4.90 (1.5%)

72Ge 55.34 (1.4%) 6.20 (4%) 4.78 (2%) 0.96 (3%)

73Ge 28.0 (1.3%) 2.94 (7%) 2.54 (3%) 0.45 (6%)

74Ge 14.53 (2%) 1.46 (8%) 1.48 (4%) 0.24 (6%)

76Ge 4.22 (4%) 0.4 (8%) 0.54 (6%) 0.06 (12%)

68Ge production rates (per day, per 1 kg )


Production rates of 60Co and 68Ge

Inside the container, sea level protons produce 15% of 68Ge and 20% of 60Co , while their initial flux is only 3-4% of all the nucleons.It is due to hardness of proton spectrum.

Target Total By sea level protons

No shield Shield No shield Shield

70Ge 1.73 (7%) 0.118 (33%) 0.170 (11%) 0.028 (19%)

72Ge 2.88 (6%) 0.256 (19%) 0.285 (9%) 0.046 (14%)

73Ge 3.14 (4.0%) 0.265 (24%) 0.335 (8%) 0.035 (21%)

74Ge 3.35 (4%) 0.23 (21%) 0.380 (8%) 0.050 (14%)

76Ge 3.31 (4%) 0.156 (13%) 0.455 (7.0%) 0.036 (15%)

Table 3: 60Co production rates (per day, per 1 kg)


Attenuation factors

For 68Ge production by N-component 10

For 60Co production by N-component 15-20

Taking into account contribution from

For 68Ge 8

For 60Co 12-15


Analysis and comparison with literatureAttenuation of neutron flux in Iron (through the upper plane of the cavity with and without

container) - published attenuation length ~200 g/cm2

* -neutrons in the cavity

-Sea level neutrons-Sea level protons

- protons in the cavity

Spectra are not in equilibrium

max ~ 240 g/cm2 (for neutrons)


Neutron fluxes from different surfaces

Container Fe

Cavity


Neutron fluxes from different surfaces

Container Fe

Cavity

Container Fe

Cavity

20% improvement of shielding


Nucleon flux density inside the cavity- total neutrons

- total protons

-protons from Sea level neutrons only

* - neutrons from sea

level neutrons only

Rate =

pnpnpn dEnVEEJ

, 0

,, )()(


Excitation functions (cross sections) for isotope production

10 100 10000,1

1

10

100

Target: Ge70 Ge72 Ge73 Ge74 Ge76

Cro

ss s

ect

ion

, mb

Energy, MeV

Excitation Functions of Production of the Ge68 at Interactionof Protons with Nuclei-Targets

100 1000

0,1

1

10

100

Targets: Ge70 Ge72 Ge73 Ge74 Ge76

ISABEL code: Ge76

Excitation Functions of Production of the Ge68 at Interactionof Neutrons with Nuclei-Targets

Cro

ss s

ect

ion

, mb

Energy, MeV


Excitation functions (cross sections) for isotope production

100 1000 10000

0,01

0,1

1

Excitation Functions of Production of the Co60 at Interactionof Neutrons with Nuclei-Targets


Cro

ss s

ectio

n, m

b

Energy, MeV

100 10001E-3

0,01

0,1

1

Excitation Functions of Production of the Co60 at Interactionof Protons with Nuclei-Targets


Exp. points: Ge70 Ge76

Cro

ss s

ectio

n, m

b

Energy, MeV

60Co production cross section increaseswith increase of Ge mass number !


Prospects and open questions Applied task: Shape optimization within fixed mass

0 20 40 60 80 100 1200,0

0,1

0,2

0,3

0,4

0,5

0,6

Number of particlesentered the Germanium from specified zone of the container

Pa

rtic

les,

1/c

m2

Height, cm

Side Distribution of Effective Penetration Points for Neutrons of Cosmic Rays over the Surface of Containerwithout Bottom (Statistics 10 mln initial neutrons)

0 10 20 30 40 50 60 700,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8Distribution of Effective Penetration Pointsfor Neutrons of Cosmic Rays over the Top Surface of Container


Par

ticle

s, 1

/cm

2

Radius, cm

Container Fe

Cavity


Prospects and open questions Applied task: Shape optimization within fixed mass

0 20 40 60 80 100 1200,0

0,1

0,2

0,3

0,4

0,5

0,6


Pa

rtic

les,

1/c

m2

Height, cm

Side Distribution of Effective Penetration Points for Neutrons of Cosmic Rays over the Surface of Containerwithout Bottom (Statistics 10 mln initial neutrons)

0 10 20 30 40 50 60 700,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8Distribution of Effective Penetration Pointsfor Neutrons of Cosmic Rays over the Top Surface of Container


Par

ticle

s, 1

/cm

2

Radius, cm

Container Fe

Cavity


Prospects and open questions

Methodical task:Validation of a method by

simulating the classical work

of Cocconi


Prospects and open questions

Scientific tasks:

To take into account muon induced fast nucleons a lot of data for less energetic neutrons, a lot of doubts

subject for discussion at TG10 and common work with Tuebingen

a review paper “muon-nuiclear interactions: theory, experiment, simulations”

is wanted

How to measure contents of 60Co in the detector?76Ge detector is not a low background one – 22 decay smears the 60Co

spectrum. Measurements with natural, or better depleted detector with known activation history may help, however RELATIVE production

cross sections should be checked – it is a new task for accelerator activation experiment.


Conclusions• The container provides activation reduction factor about one order of

magnitude

• Absolute rates are known within factor 2-3

• Shape optimization within fixed mass is possible

• 60Co production rate increases with increase of Ge mass number

• 68Ge production rate decreases with increase of Ge mass number

• Contribution of muon induced fast nucleons should be studied better

• Transportation is not a bottle neck any more. Next step? 0.5-1 m iron shielding above technological equipment at every stage of detector manufacturing seems feasible.

Documents

Reduction of cosmogenic activation of Ge by means of movable iron shielding