Ni Foils Irradiation

In preperation for VELO sensor irradiation tests at the Birmingham cyclotron, tests have been performed with the irradiation of thin sheets of natural nickel foil in a point to point pattern of varying irradiation times in order to approximate a 1/r^2 relationship from the z axis of the VELO detector, with the required fluences

The foils are scored into 10mmx10mm squares and marked with the respective coordinates. These are fixed to carbon fibre supports which can be securely mounted inside the target box.

Two types of irradiation runs will be performed. A single nickel sheet will be exposed to the beam with the same run pattern as the silicon sensor. This is to test desired fluences and relationship between points is achieved. Several points outside of the scan patter will be analysed to estimate beam overlap and halo effects.

The second type opf run involes two nickel foils in parallel, seperated by cardboard. As we will measure fluences from the real silicon irradiation runs by placing a nickel foil in front of the sensor, we can study energy loss effcts, possible scattering and beam alignment by comparing the two foils, with the rear foil being subject to the conditions the real sensor will experience.

Nominal paramters for proton beam run

  • Profile: 10mmx10mm square
  • Energy: 27MeV
  • Current: 10nA
  • Point to Point irradiation times: 59.16s - 1593s

Foil irradiation considerations

Irradiation times are calculated from beam current, conversion factor from neutron equivelent dose, and taking into account movement time of the stage between points. The fluences are estimated from measuring 1377KeV gamma yields from Ni57 disintegrations. In order to estimate fluences from post run activation measurements of the foil the following effects are considered:

  • Production rate of Ni57.
  • Ni57 lifetime effects during irradiation
  • Ni57 lifetime effects from irradiation to measurement
  • Detector efficiency
Ni57 production

The number of Ni57 atoms produced from the irradiation is calculated from the number of protons incident on the target integrated over the irradiation time.

Knowing the thickness (l) and density(ρ) of the natural nickel foil, this can be written as

where N_A is Avagadro's number, AW is the atomic weight, Np is integrated number of protons and the production cross section of nickel 57 from protons incident on natural nickel is found from data requested from the EXFOR database. The plot below shows these cross section measurements from various experiments as a function of proton energy. We see a maximum at ~27MeV

Values used:

  • NA = 6.022E-23
  • ρ = 8.908g/cm^3
  • l = 25μm
  • AW = 58.7
  • σ ~ 0.18b at 27MeV

Effect of finite lifetime during and post run

The lifetime of Ni57 is 2154 minutes. This results in ~1% decrease in activity every half hour. For long irradiation times, the decay of Ni57 must be taken into account during the actual run. The rate of change of Ni57 can be represented as the production rate - decay rate during the irradiation.

and the production rate is related to the instantaneous luminosity of the beam;

This gives the number of Ni atoms at the end of the irradiation to be:

which, for small irradiation times, t is simply

The number that remain after the cooldown period, between end of irradiation and measurement is

Detector Efficiency

A hyperpure germanium detector held at cryogenic temperatures is used for high resolution, low noise activity measurements of the foil. The sample sits a distance above the detector, and the crystal casing is surrounded by lead shielding for background supression.

The efficiency of the detector is calculated from the solid angle and sources of known activity. Conversion factors for moving weaker sources closer to the top of the crystal are calculated from ratios of counts of high activity sources from nominal height and closer distances.

The efficiency of the detector in cps/kBq (counts measured per 1,000 disintegrations) is found to be,

and in this particular case, gamma energy is 1377 keV.

The branching ratio of Ni57 -> 1377 keV gamma is taken into account, and is 81.2%.

Preliminary results

Expected relationship

Measured relationship:

System Setup

  • 20130620 FirstRun? OneFoil? ScreenShot? :

  • 20130620 SecondRun? TwoFoils? ScreenShot? :

-- SimoneBifani? - 20 Jun 2013

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jpgjpg 20130620_FirstRun_OneFoil_ScreenShot.jpg manage 1400.0 K 20 Jun 2013 - 14:12 SimoneBifani? 20130620 FirstRun? OneFoil? ScreenShot?
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Topic revision: r3 - 16 Jul 2013 - 16:09:24 - PeterGriffith
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