June, 7, 2007 Cf rate measurements using Photonics PMT

The 60 [math]\mu[/math]Ci Cf-252 source was placed 16.5 cm away from the face of the BC420 scintilator which is attached to the Photonics PMT whose base was set to -1200 Volts. The PMT output signal was descriminated by a Lecroy Model 821 descriminator set to pass any voltage below -0.5 mV. The descriminated output was then sent to a counter Model TC 531.

Configuration Definitions

1: Placed the Cf source 16.5 cm from face of Scintillator. neutrons passing through scintillator would also pass through PMT

2: Inserted a 2.5 cm thick steel shielding wall between Scintillator and Cf source. Right up against Scintillator

3: Replaced steel wall with 5 cm thick lead shielding brick

4: Replaced lead brick with 6.7 cm thick paraffin block ( Paraffin N.F. , Household wax made by Parowax. 4 cackes, total net weight 16 oz (454 g))

5: Increased thickness of paraffin block to 13.4 cm

6: No source backround

Measurements

Using Photonics PMT and BC420

Configuration	Rate (counts/min)	Statistical Error in Rate (Counts/min)
1	2.7e3	0.09e3
2	2.2e3	0.01e3
3	0.8e3	0.02e3
4	2.2e3	0.08e3
5	1.8e3	0.02e3
6	0.07e3	0.005e3

Analysis

Properties of Materials used

Steel

Zeff = 26 and [math]\rho[/math]=8.02 [math]g/cm^3[/math]

Atomic composition for AISI 304) in %

Element	%
Si	1
Cr	19
Mn	2
Fe	68
Ni	10

Lead

Z =92 density = 11.35 [math]\frac{g}{cm^3}[/math]

Paraffin

Zeff=

according to http://ptcl.chem.ox.ac.uk/MSDS/PA/paraffin.html
density = 0.9 [math]\frac{g}{cm^3}[/math]

[math]C_8 H_{18}[/math]

Photon Mass Attenuation Lengh [math]\lambda = \frac{\rho}{\mu} (\frac{g}{cm^2})[/math]

[math]\mu[/math]= attenuation length [math]\rho[/math] material density

Steel Calculation

(i) A 1 MeV photon has [math]\lambda \approx 18 \frac{g}{cm^2}[/math]

The photon intensity is attenuated such that [math]I = I_o e^{-\mu x}[/math]

The half length [math]X_{\frac{1}{2}}[/math] is the distance needed to reduce the photon intensity in half

[math]I=\frac{I_o}{2}=I_o e^{-\mu X_{\frac{1}{2}}}[/math]

Therefore [math]X_{\frac{1}{2}} = -\frac{\ln(\frac{1}{2})}{\mu} = -\frac{\lambda \ln(\frac{1}{2})}{\rho} [/math]

In the case of steel: [math]X_{\frac{1}{2}} = -\frac{18 \ln(\frac{1}{2})}{8} (cm) =1.5 cm = 0.61 inches [/math]

So if the Californium source were just 1 MeV photons then I would expect a 2.5 cm thick steel wall to drop the rate by at least a factor of 2. In reality the photons from Cf-252 are a distribution of energies so we would need to use a montecarlo to make a more accurate prediction. All that can be concluded at this time is that a drop in rate from 2700 to 2200 counts per minute corresponds to a 19% drop in rate which indicates that the particles causing light in the scintillator are not just 1 MeV photons. One can conjecture that because the photon distribution from CF-252 contains more photons at energies below 1 MeV than above that there must be other particles besides photons (probably neutrons) making a signal in the scintillator.

(ii) Simulation of half-length value in the case of 1 MeV [math]\gamma[/math]-rays passing thru the steel. GEANT4 was used, 0.5 cm cut was applied for [math]e^-[/math], [math]e^+[/math] and [math]\gamma[/math]

The difference between simulated value of half-length and calculated one is about 32.74% (??)

Lead Calculation

(i) A 1 MeV photon has [math]\lambda \approx 14.01 \frac{g}{cm^2}[/math]

Compare to: http://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z82.html

[math]X_{\frac{1}{2}} = -\frac{\ln(\frac{1}{2})}{\mu} = -\frac{\lambda \ln(\frac{1}{2})}{\rho} [/math]

In the case of lead: [math]X_{\frac{1}{2}} = -\frac{14.01 \ln(\frac{1}{2})}{11.35} (cm) =0.855 cm = 0.3 inches [/math]

(ii) Simulated attenuation half-length of 1 MeV [math]\gamma[/math]-rays via lead (GEANT4, default cuts).

The difference between simulated [math]X_{1/2}^{sim}[/math] and calculated [math]X_{1/2}^{calc}[/math] values of half-length in the case of lead is about 30% (?)

(iii) The percentage of particles passed thru the lead of thickness 0.855 cm in the case new cuts values:

Particle	Cut	%	Cut	%	Cut	%	Cut		Cut	%
[math]\gamma[/math]-rays	0.5 [math]\mu[/math]m		0.5 nm		1cm		10 cm		1 m
[math]e^-[/math]	0.5 [math]\mu[/math]m		0.5 nm		1 cm		10 cm		1 m
[math]e^+[/math]	0.7 [math]\mu[/math]m	60.74	0.7 nm	60.96	1 cm	60.78	10 cm	60.04	1 m	60.32

Paraffin Calculation

(i) Assume a 1 MeV photon has [math]\lambda \approx 13.72 \frac{g}{cm^2}[/math]

Then

[math]X_{\frac{1}{2}} = -\frac{13.72 \ln(\frac{1}{2})}{0.93} (cm) = 10.26 cm = 4 inches [/math]

(ii) Simulation of half-length value in the case of 1 MeV [math]\gamma[/math]-rays passing thru paraffin (C - 85.1395%, H - 14.8605%).

GEANT4 was used, 0.5 cm cut was applied for [math]e^-[/math], [math]e^+[/math] and [math]\gamma[/math]

The difference between simulated value of half-length and calculated one is about 35.87% (??)

Neutron Attenuation

Simulation of neutron attenuation

1. Neutron spectrum from Cf-252 source (http://hep.uchicago.edu/atlas/electr/Rad_testing/Glink_radtest.pdf)

This figure was taken from paper "Neutron Irradiation Test of an S_LINK-over-G-link System" by K. Anderson, J. Pilcher, H. Wu, E. van der Bij, Z. Meggyesi, J. Adams.

2. Spectrum of neutrons emitted according to the formula [math]N = exp(-0.88E)sinh[(2E)^{0.5}][/math]

3. Spectrum of neutrons simulated using GEANT4 code.

The number of events processed is [math]2*10^5[/math].

4. Simulated spectrum of neutrons attenuated by paraffin of thickness 6.3 cm and lead of thickness 5 cm.

The number of events processed is [math]2*10^5[/math]. Wit.h only 3.5e4 events remaining the simulation would predict that the 6 cm thick paraffin blocks 90% of the nuetrons

5. Simulated spectrum of neutrons attenuated by paraffin of thickness 2X6.3 cm and lead of thickness 5 cm.

The number of events processed is [math]2*10^5[/math].

References

W. Manhart, Evaluation of the CF-252 fission neutron spectrum for between 0 and 20 MeV., Report IAEA-TECDOC-410, IAEA, Vienna, 1987, pg 158.
Long, A. B.; Wehring, B. W.; Wyman, M. E., Physical Review, vol. 188, Issue 4, pp. 1948-1957