Dynamic Nuclear Polarization by Electron Spin in the Photoexcited Triplet State: I. Attainment of Proton Polarization of 0.7 at 105K in Naphthalene

April 22, 2011

GoLuckyRyan papers reading , , , Leave a comment

DOI: 10.1143/JPSJ.73.2313

This paper reported an excellent detail and considerations on polarization of nuclear spin by Dynamic Nuclear Polarization (DNP) method. the polarizing sample is naphthalene with 0.018mol% of pentacene. Pentacene has a paramagnetic triplet state with population independence of external magnetic field and temperature. This paramagnetic triplet state is very suitable on DNP, since the paramagnetic state like a switch to turn on the nuclear spin relaxation, due to the state will decay to diamagnetic state.

There are 5 conditions for sample:

  1. Highly polarization of electron triplet state  ( since it is the source of polarization )
  2. High concentration of guest molecule ( another aspect for polarization source ) ( but too high, guest molecule cluster forms and this reduce the polarization )
  3. High Inter-System-Crossing rate, which is the rate from higher excited state to the triplet state, so faster the polarization build up time.
  4. Long nuclear spin-lattice relaxation time ( the nuclear spin lost rate )
  5. Suitable triplet state life time and electron spin-lattice relaxation time. long enough for transfer the electron spin to nuclear spin, but not so long that nuclear spin can use the paramagnetic triplet state as a channel for spin relaxation.
On the laser:
  1. the pulse width should longer than the lifetime from excited state to the triplet state, but shorter than the triplet lifetime.
  2. the intensity should be very high, so, the triplet state excitation depth increase. but it is not so high to increase the stimulated emission.
  3. high intensity may melt the sample.
about the microwave and field sweep:
  1. the time interval should be  long and not excess the adiabatic limit.
  2. the sweeping range should be cover the ESR line width.
  3. the field sweep should be within the lifetime of the triplet state.
they used water as a reference for the polarization measurement. the enhancement Q is defined to be:
Q = \left( \frac { T E}{N g} \right) / \left( \frac{ T_w E_w}{ N_w g_w } \right)
where T is temperature, N is number of proton spin, g is receiver gain and  E is the recorded signal amplitude.
at the end of this paper, it talks about some applications.

First experiment of 6He with a polarized proton target

February 4, 2011

GoLuckyRyan experimental, papers reading , , , , , , , , , , , , , , Leave a comment

DOI : 10.1140/epjad/i2005-06-110-5

this paper reported a first spin polarized proton solid target under low magnetic field ( 0.08 T ) and hight temperature ( 100K )

the introduction overview the motivation of a solid target.

  • a polarized gas target is ready on many nuclear experiment.
  • on the radioactive beam ( IR beam ), the flux of a typical IR beam is small, since it is produced by 2nd scattering.
  • a solid target has highest density of solid.
    • most solid target can only be polarized on low temperature ( to avoid environmental interaction to reduced the polarization )
      • increase the experimental difficult, since a low temperature should be applied by a cold buffer gas.
    • high field ( the low gyromagnetic  ratio ).
      • high magnetic field make low energy scattered proton cannot get out from the magnetic field and not able to detect.
  • a solid target can be polarized at high temperature and low magnetic field is very useful

the material on use is a crystal of naphthalene doped with pentacene.

the procedure of polarizing the proton is :

  1. use optical pumping the polarize the electron of pentacene
    • the population of the energy states are independent of temperature and magnetic field.
  2. by Dynamic Nuclear Polarization (DNP) method  to transfer  the polarization of the electron to the proton.
    • if the polarization transfer is 100% and the relaxation time is very long. the expected polarization of proton will be 72.8%

The DNP method is archived under a constant microwave frequency with a sweeping magnetic field. when the magnetic field and  microwave frequency is coupled. the polarization transfer will take place.

the next paragraph talks about the apparatus’s size and dimension, in order to fit the scattering experiment requirements.

the polarization measurement is on a scattering experiment with 6He at 71 MeV per nucleons. By measuring the polarization asymmetry \epsilon , which is related to the yield. and it also equal to the polarization of the target P_t  times the analyzing power A_y .

\epsilon = P_t \times A_y

with a reasonable guess of the target polarization. the analyzing power of  6He was found.

the reason why the polarization-asymmetry is not equal to the analyzing power is that, the target is not 100% polarized, where the analyzing power is defined. when the polarization of the target is 100%, both are the same.

in the analysis part. it used optical model and Wood-Saxon central potential to simulate the result. And compare the result from 6He to 6Li at same energy. the root mean square of 6Li is larger then 6He. it suggest the d-α core of 6Li may responsible for that.

they cannot go further discussion due to the uncertainly on the polarization of the target.