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Radiation Studies for Mu2e Experiment. V. Pronskikh Fermilab August 21, 2012. Basics of Mu2e experiment. muon converts to electron in the presence of a nucleus, coherent conversion: 1) neutrinos are not emitted 2) nucleus remains intact 3) signature – 105 MeV monoenergetic electron.

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radiation studies for mu2e experiment

Radiation Studies forMu2e Experiment

V. Pronskikh


August 21, 2012

basics of mu2e experiment
Basics of Mu2e experiment

muon converts to electron in the presence of a nucleus, coherent conversion:

1) neutrinos are not emitted 2) nucleus remains intact 3) signature – 105 MeV

monoenergetic electron

Beam power 8 kW, two batches of 4E12 protons from the booster every 1.33 second

Data-taking 3 years, apparatus lifetime ~5 years at 2×107 s/yr

Best limit : (90% C.L.) from SINDRUM II

Search for Charged Lepton Flavor Violation, rate in SM

Explanation: SUSY, second Higgs doublet, large extra dimensions, leptoquarks, etc.

mu2e apparatus
Mu2e apparatus

MARS15 model developed










m-, p-











(not shown: Cosmic Ray Veto, Proton Dump, Muon Dump, Proton/Neutron absorbers,

Extinction Monitor, Stopping Monitor)

main issues
Main Issues


  • Primarily, Mu2e apparatus with 8-GeV proton beam, also applicable to COMET, MuonCollider, Project X, Neutrino Factory and other superconducting setups in high radiation fields

Main goal:

  • Maximize useful particle production, minimize background particle yields


  • Quench: power density and dynamic heat load of superconducting (SC) coils.
  • Integrity and lifetime of critical components: integrated dose in organic materials, i.e. epoxy, insulator.
  • Radiation damage to superconducting and stabilizing materials: atomic displacements (DPA), integrated particle flux
  • Damage to electronics (single event upsets (SEU), lifetime)
  • Safety aspects: shielding, nuclide production, residual dose, etc.
requirements to mu2e heat and radiation shield
Requirements to Mu2e Heat and Radiation Shield
  • Absorber (heat and radiation

shield) is intended to prevent

radiation damage to the magnet

coil material and ensure quench

protection and acceptable heat

loads for the lifetime of the


    • Total dynamic heat load on the coils
    • Peak power density in the coils
    • Peak radiation dose to the insulation and epoxy
    • Displacements Per Atom (DPA) to describe how radiation affects the electrical conductivity of metals in the superconducting cable
displacement per atom dpa
Displacement per atom (DPA)
  • DPA (displacement per atom). Radiation damage in metals, displacement of atoms from their equilibrium positions in a crystalline lattice due to radiation with formation of interstitial atoms and vacancies in the lattice.
  • A primary knock-on (PKA) atom is formed in elastic particle-nucleus collisions, generates a cascade of atomic displacements.
  • A PKA displaces neighboring atoms, this results in an atomic displacement cascade. Point defects are formed as well as defect clusters of vacancies and interstitial atoms (time scale=ps).
    • Residual Resistivity Ratio degradation (RRR, ratio of the electric resistance of a conductor at room temperature to that at the liquid He one), the loss of superconducting properties due to change of conditions of electron transport in metals.
mu2e limits
Mu2e Limits
  • DPA limit: RRR degrades from ~1000 to 100. After this RRR reduction we must warm-up and anneal Al (once a year).
  • Definite cooling requirements lead to limits on peak power density calculated based on the heat map
  • Dynamic heat load limit depends on cooling system
requirements peak power density
Requirements: Peak power density

Volume temperature, K

Power density, µW/g


4.8 K

17.9 µW/g


T plot for T0 =4.6K (liquid He temp)

Tc= 6.5K; (supercond+field)

Tpeak= Tc-1.5K = 5.0K.

Peak coil temperature starts to violate allowable value based on 1.5 K thermal margin and 5 T field after 30 µW/g


requirements absorbed dose to organic materials
Requirements: Absorbed dose to organic materials

Ultimate tensile strength degradation

7 MGy before 10% degradation

of ultimate tensile strength

(shear modulus).

Mu2e apparatus lifetime is 5 years

Current LHC limit 25-50 MGy over

the lifetime

e-, γ, n

also Radiation Hard Coils, A. Zeller et al, 2003, http://supercon.lbl.gov/WAAM


requirements rrr vs dpa
Requirements: RRR vs DPA


NRT model of DPA

– from KEK measurement

Range 4-6E-5

Broeders, Konobeyev, 2004

; 0.357 – 0.535

scales NRT to experiment

dpa modeling status

DPA Modeling Status

Codes using NRT model (MARS15, FLUKA, PHITS) agree quite well (10-20%).

Industry standard NRT and state-of-the-art models (BCA-MD) differ by a factor of 2 to 3 in some cases

More experiments at high-energy neutron spectrum are necessary to benchmark models

More data on non-annealable (irreversible, transmuted DPA) are important for experiments with spallation targets (Mu2e, COMET, Project X etc.)

experiments at jinr dubna december 2011
Experiments at JINR, DubnaDecember 2011

d beam

d beam

Synergy with ADS program at JINR

0.8-2 AGeV deuterons, total fluence~ 4E13 d

Secondary neutron fluence ~ 1-3E7 n/cm^2/s

“On proposal to measure irreversible DPA”,

Mu2e-doc-1996-v1, October 2011

Cryogenic measurements are needed

The samples were placed at the depth of 10 cm inside

the target

secondary neutron spectra of mu2e ps coils kvinta and gamma3 at 3 gev a reactor
Secondary neutron spectra of Mu2e PS coils, KVINTA and GAMMA3 at 3 GeV, a Reactor

GAMMA3 d at 3 GeV

Reactor spectrum

arb. units

Mu2e coils

KVINTA d at 3 GeV

30 MeV


mars15 dpa model development
MARS15 DPA model development


Broeders, Konobeyev, 2004

  • Based on ENDF/B-VII, calculated for 393 nuclides
  • NRT (industry standard) corrected for experimental η
  • η – ratio of number of single interstitial atom vacancy pairs (Frenkel
  • pairs) produced in a material to the number of defects calculated using
  • NRT model
dpa for 8 kw beam power baseline
DPA for 8-kW beam power baseline

Limit 4-6*10-5

3.2*10-5 yr^-1

neutron flux 100 kev
Neutron flux >100 keV
  • 3.1x1021 n/m2 over lifetime

3.1*109 cm^-2s^-1

power density mw g
Power density, mW/g

Limit ~30 µW/g

18 µW/g

limits and design values
Limits and design values
  • Radiation damage is a key issue for experiments at the Intensity Frontier
  • Models are developed and experiments are proposed to understand and address the issue
  • Current Mu2e design solutions are safe during the lifetime of the experiment but more work is needed for fine tuning, value engineering and upgrade (Project X)
alternative hrs designs
Alternative HRS designs

W, 5cm

Fe (Cu, WC), 20cm

BCH2, 12 cm

Fe (Cu, Cd), 3cm

5 multilayer cases

Tungsten, WC, U-238

Cases #1-#10











Mu2e @ PX preliminary design




L(W)=235 cm, L(Cu1)=365 cm, L(Cu2)=560 cm

Ep= 3 GeV @1 MW, C target: ~190 tonnes of W/130 tonnes of Cu

accidental mode
Accidental mode

Peak values for 1 ms: 1E-14 DPA, 0.1 µJ/g

radiation quantities at ts1
Radiation quantities at TS1

DPA=2.2E-6/yr, Power density= 0.5E-3 mW/g,

Absorbed dose = 1.1E4 Gy/yr,

radiation quantities at ts3
Radiation quantities at TS3

Q(coll, up) = 20.5 W; Q(coll, down) = 0.41 W

Peak DPA 1E-6/yr, power density 5E-4 mW/g, absorbed dose 1E4 Gy/yr

Material: bobbins and flanges: (5083): Si 0.4%, Fe 0.4%, Cu 0.1%, Mn 0.4%, Mg 4%, Zn 0.25%,Ti 0.15%, Cr 0.05%, Al 94.25%

coils: NbTi8.27%, Cu 9.51%, G10 15.53%, Al 66.69%

Residual activation of PS parts

Also calculated for walls, beam dump, end cap, cryostat, many parts of the PS hall

residual dose from mu2e target
Residual dose from Mu2e target

1-st method: MARS15 for contact dose,

scaling factor for the target size, scaling factor

for distance, correction for finite target size

2-nd method: residual nuclei from MARS15,

Activities (DeTra), activities to doses using

specific gamma-ray constants

On contact 20 kSv/hr

  • Excellent agreement between
  • the two methods.
  • High dose on contact but
  • not very high at a distance
  • Typical shielding should be enough
  • Precise methods for residual dose
  • determination are developed.
  • First method should be more
  • precise for extended targets.
decay heat of target
Decay heat of target

~1400 nuclides

Decay heat of the target was determined using MARS15+DeTra codes to be 11.3 W (1 year of irradiation), which is negligible compared to the dynamic heat load (~800 W)

beam absorber dump
Beam absorber (dump)

The proposed beam dump represents itself a Fe or Al box with dimensions 150x150x200 cm, surrounded by 100 cm thick concrete walls from each side, with the albedo trap towards the beam entrance window with sizes 250x250x100 cm, and the beam entrance window 150x150x100 cm.

~1E8 n/cm2/s

airflow activation
Airflow activation

7.8 Ci – made in fins w/o transit time of airborne activity, (depends on vent rate and release point to outdoors distance,

21 Ci – released in the target hall, Max 28.8 Ci a year

If we assume 500 cfm of air to target hall and release near P-bar, annual activated air <21 Ci

surface ground water and air
Surface/ground water and air

Based on MARS15 simulations of the hadron flux and star density, using the Fermilab standard Concentration Model at the design intensity, the average concentration of radionuclides in the sump pump discharge will be 24 pCi/ml due to tritium and 2 pCi/ml due to sodium-22.

This is 2% of the total surface water limit if the pumping is performed once a month (conservative scenario). Build-up of tritium and sodium-22 in ground water at 1.2E20 protons per year will be as low as 6.2E-8 % of the total limit over 3 years of operation.

Air activation and flow estimations show that at 500 cfm, for the configuration without a pipe connecting the target region to the beam dump (average hadron flux over the whole hall volume is 5.5E6 cm-2s-1), the maximum annual activity released from the target hall is less than 29 Curies.~ 7% of the Lab’s air release limit.

ds mars15 model
DSMARS15 model

Stopping target

Monitor (HPGe


(U)p (L)eft (R)ight D(own) (T)op (B)ottom

stopping target monitor ge crystal radiation damage
Stopping target monitor (Ge crystal) radiation damage


Neutron flux total:

5600 n/cm2/s (MARS15)

Neutron flux:

En>100 keV ~5400 n/cm2/s

Gamma flux:

3.4E4 photons/cm2/s

from V.Borrel, NIM A 430 (1999) 348, n-type HPGe detectors

  • Degradation of detector resolution (asymmetry of gamma
  • peaks) by atomic displacements (DPA) in Ge (peak 1E-8/yr).
  • For this HPGe, FWTM increase is ~25% in 5 days (study is
  • needed for a particular neutron spectrum and detector type)
  • Fermilab has an 241Am-Be neutron source (En=4.5 MeV)
  • 6E7 n/s -> 5500 n/cm2/s at 30 cm from source
to do
To do
  • Heat and Radiation Shield optimization
    • second groove for FNAL extinction monitor
    • material reduction in the upstream part
    • Considering alternative designs
  • Hot Cell and PS hall shielding studies
  • Cask optimization
  • TS coils neutron internal shielding studies
  • CRV shielding optimization
  • Cryogenic measurements at Dubna (?)
  • Implementation and benchmark of a new

DPA model in MARS15

  • Publishing results

... … work in progress

mu2e hall mars15 model
Mu2e hall MARS15 model

Production Solenoid

Transport Solenoid

Detector Solenoid

V. Pronskikh, Radiation damage, NuFact’12, July 23-28, Williamsburg

mu2e requirements rrr vs dpa
Mu2e Requirements. RRR vs DPA


= 2.7E-6 Ω*cm

= (Mu2e requirement)

– from KEK measurement

(RRR degradation from 457 to 245)

- DPA using NRT model with

correction for defect production


Range 4-6E-5

; 0.357 – 0.535

Broeders, Konobeyev, 2004

V. Pronskikh, Radiation damage, NuFact’12, July 23-28, Williamsburg

dpa model in mars15
DPA Model in MARS15
  • Norgett, Robinson, Torrens (NRT) model for atomic displacements per target atom (DPA) caused by primary knock-on atoms (PKA), created in elastic particle-nucleus collisions, with sequent cascades of atomic displacements (via modified Kinchin-Pease damage function n(T)), displacement energy Td (irregular function of atomic number) and displacement efficiency K(T), Ed – energy to nuclear collision.

Td in Si


M. Robinson (1970)

R. Stoller (2000), G. Smirnov

All products of elastic and inelastic nuclear interactions as well as Coulomb elastic scattering of transported charged particles (hadrons, electrons, muons and heavy ions) from 1 keV to 10 TeV. Coulomb scattering: Rutherford cross-section with Mott corrections and nuclear form factors for projectile and target. 10% agreement with FLUKA, PHITS

V. Pronskikh, Radiation damage, NuFact’12, July 23-28, Williamsburg