Absorption of Light

by PEN SamplesMEGAN PAGE

CHARLIE FORSHAW

DR HELEN O’KEEFE

DR DANIEL MUENSTERMANN

1

Light Transmission

When light is shone through a sample, a fraction of the light is

Reflected, 𝑅, off the sample interfaces

Transmitted, 𝑇, through the sample

Absorbed, 𝐴, by the sample

Assuming a negligible amount of light is lost due to scattering, we can assume:

1 = 𝑇 + 𝐴 + 𝑅𝑡𝑜𝑡𝑎𝑙

We are interested in the fraction of light Absorbed by the samples of

PEN, 𝐴.

2

(1)

Cary 60 UV-Vis

Light calibrated by deviceTransmitted

light through sample

Sample

Light transmitted through the PEN samples can be determined using a Cary60 UV device

3

Transmitted Light The fraction of light that is transmitted through a sample can be

determined in terms of an initial intensity, 𝐼0 and a transmitted

intensity, 𝐼𝑡𝑟 before and after passing through the sample:

𝑇 =𝐼𝑡𝑟𝐼0

The Cary 60 UV-Vis produces data for the Measured Absorbance,

𝐴𝑏𝑠𝑚, of a sample:

𝐴𝑏𝑠𝑚 = − log10𝐼𝑡𝑟𝐼0

= − log10 𝑇

Using the data given by the machine for the absorbance of a sample, the fraction of transmitted light can be determined:

𝑇 = 10−𝐴𝑏𝑠𝑚

4

(2)

(3)

(4)

Reflected Light If light hits any surface interface there will be a fraction of it reflected.

The fraction of reflected light is dependent on the different surfaces’ refractive indexes.

The fraction of reflective light at air:PEN interface is given by Fresnel’s equation:

𝑅 =𝑛air − 𝑛PEN𝑛air + 𝑛PEN

2

Using a refractometer, the refractive index of the PEN samples can be approximated to 𝑛PEN = 1.65 ± 0.1 which is in agreement with Nakamura published values [1].

Therefore 𝑅 = 0.0602

When analysing a PEN sample, the front and back surfaces must be considered when determining the total fraction of reflected light

𝑅𝑡𝑜𝑡 = 2 𝑅

Note: For simplicity, the inner interface reflections are ignored (for now)

𝑛2𝑛1

Surface 1 Surface 2

5

(5)

Absorbed Light From 1 = 𝑇 + 𝐴 + 2𝑅, an expression for the fraction of light absorbed per mm of the

sample can be determined for each sample:

𝐴 per mm =1 − 10−𝐴𝑏𝑠𝑚 − 2 0.0602

𝐿 in mm

Using this, the fractions of absorbed per mm of all the PEN samples can be

determined and compared.

Note: For BC408 sample, 𝑛2 = 1.58 was used to determine 𝑅

6

(6)

Different set up Configurations The PEN samples were investigated under several different set ups.

We measured the Absorbance when the samples were:

Vertical

Horizontal

Stacked with another sample (of the same parameter set)

Stacked with 2 other samples (of the same parameter set)

Greased together with another sample (of the same parameter set)

Greased together with 2 other samples (of the same parameter set)

In theory, the different configurations of the PEN samples should give the same results for Absorption per mm 𝐴 per mm .

The Optical Grease used was National Enterprises NE586 but no information can be found on its refractive index.

Nakamura has previously found the peak emission wavelength of PEN to be 425nm [2]

7

Horizontal Measurements

When setting up the horizontal measurements, it was difficult to

reduce the scattering at the sample’s surface due to the reduced

target area of the sample:

vs

Therefore, the results for the fractions of light absorbed per mm are

not accurate for the horizontal measurements as you can no longer

consider the fraction of light lost due to scattering as negligible

(seen later)

8

9

10

11

Individual Vertical Samples 12

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Absorption per mm for Individual Samples

426nm 424nm

Individual Horizontal Samples 13

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Absorption per mm for Horizontal Samples

426nm 424nm

2 Stacked Samples 14

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Absorption per mm for 2 Stacked Samples

426nm 424nm

3 Stacked Samples 15

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Exp2 Exp3 Exp6 Exp9 Exp10 Exp11

Absorption per mm for 3 Stacked Samples

426nm 424nm

2 Greased Samples 16

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Exp1.2 Exp1.12 Exp1.13 Exp2.1 Exp3.3 Exp3.4 Exp7 Exp2 Exp9 Exp10

Absorption per mm for 2 Greased Samples

426nm 424nm

3 Greased Samples 17

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

D2 D9 D10

Absorption per mm for 3 Greased Samples

426nm 424nm

Comparing Configurations 18

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Exp1.2 Exp1.12 Exp1.13 Exp2.1 Exp3.3 Exp3.4 Exp7 Exp2 Exp9 Exp10

Fraction of Light Absorbed per mm at 424nm

Individual Horizontal Stacked 2 Greased 2 Stacked 3 Greased 3

Comparing Configurations 19

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Exp1.2 Exp1.12 Exp1.13 Exp2.1 Exp3.3 Exp3.4 Exp7 Exp2 Exp9 Exp10

Fraction of Light Absorbed per mm at 424nm

Individual Stacked 2 Greased 2 Stacked 3 Greased 3

Comparing Configurations 20

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Absorption per mm at 424nm

Individual 2Stacked

Conclusions

Fraunhofer Exp 1.13 and Dortmund Exp 6 have the lowest

Absorptions per mm.

Fraunhofer Exp 2.1 and Dortmund Exps 9 & 11 have the highest Absorptions per mm.

It can be seen on slide 20 that there is a increase in the fraction of

light absorbed per mm for when the samples are stacked

compared to them individually.

Although the application of optical grease decreases this

difference, it shows that the reflections at the inner surface

interfaces must also be considered.

21

UV Irradiation

Dortmund samples Exp1 and Exp7, samples which produced

respectively low and high absorbance were placed in the Cary60

UV device and their absorbance measured again.

Then, they were irradiated under UV light for 1 hour.

~10 mins after irradiation, their absorbances were measured again

to investigate the effect of UV irradiation on the samples’ ability to

transmit light.

22

UV Irradiation – D.Exp 1.1 (~13% dif.)23

UV Irradiation – D.Exp 7.2 (~15% dif.)24

Future Work The intensity of light decreases exponentially as it passes through a sample:

𝐼

𝐼0= 𝑒−𝐿/𝜆

Where 𝜆 is the absorption length and 𝐿 is the thickness of the sample

Using the fraction of light absorbed per mm, the absorption length of the PEN samples can be determined:

𝜆 =𝐿

ln 𝐿 in mm ∗ 𝐴 per mm

With current calculations, the absorption length at 424nm of Fraunhofer’sExp1.13:

𝜆Exp1.13 = 0.0018𝑚

This seems extremely small so will be investigated in more detail

Determine the light yield of the highest and lowest light absorbing samples

Determine the light yield of our Teijin and BC408 samples

25

(7)

(8)

References

[1] Nakamura, H., Shirakawa, Y. and Kitamura, H. (2013). Blended

polyethylene terephthalate and polyethylene naphthalate

polymers for scintillation base substrates. Radiation Measurements,

59.

[2] Nakamura, H., Shirakawa, Y., Takahashi, S., Shimizu, H., 2011.

Evidence of deep-blue photon emission at high efficiency by

common plastic. EPL (Europhysics Letters) 95 (22001).

26

UV Irradiation – D.Exp 1.127

UV Irradiation – D.Exp 7.228